Aliso: A Journal of Systematic and Floristic Botany Aliso: A Journal of Systematic and Floristic Botany Wood Anatomy of Acanthaceae: A Survey Wood Anatomy of Acanthaceae: A Survey

Qualitative and quantitative wood features are reported for 38 species representing 22 genera, including the scandent genera Mendoncia and Thunbergia. Woods of Acanthaceae are characterized by relatively narrow vessels with simple perforation plates and alternate lateral wall pitting, septate libriform fibers, scanty vasicentric axial parenchyma, rays both multiseriate and uniseriate, erect ray cells abundant in rays (some species rayless or near-rayless), numerous small crystals or cystoliths in ray cells in a few genera (first documented reports of both characters in woods of Acanthaceae), and nonstoried structure. This constellation of features is very closely matched by woods ofGesneriaceae, Scrophulariaceae, Pedaliaceae, Martyniaceae, Bignoniaceae, and Myoporaceae (families listed in order ofdecreasing resemblance). Narrowness ofvessels in tropical Acanthaceae appears related to understory ecology. A fewspecies in warm and seasonally dry areas have narrow, short vessel elements numerous per unit transection. Vasicentric tracheids occur in two nonscandent genera in dry areas. Vesselgrouping is roughly proportional to dryness of habitat. Thunbergia alata, T. laurifolia, and all collections of Mendoncia have interxylary phloem (first report for Mendoncia). That feature, plus presence of occasional acicular crystals in rays and axial parenchyma and presence of large gelatinous fibers in phloem ally Mendoncia closely with Thunbergia, and Mendonciaceae is not justified for this and other reasons. Species of Thunbergia differ among themselves, and T. erecta and T. holstii resemble shrubby Acanthaceae more than they do Mendoncia in wood features. Thunbergia thus should not be segregated from Acanthaceae.


tomy.

The pr
dominantly herbaceous or moderately woody nature of such a high proportion of Acanthaceae makes the family potentially interesting with respect to wood anatomy, because patterns in such a group are different from those in predominantly woody families.The families Gesneriaceae (Carlquist and Hoekman 1986a), Myoporaceae (Carlquist and Hoekman 1986b) and Plantaginaceae (Carlquist 1970) are instructive in this regard .At another extreme with respect to habit is Mendoncia, a genus of tropical lian as, and Thunbergia, which consists of variously woody vines or (T.erecta) shrubs.These scandent genera present anatomical patterns that involve abnormal cambial configuration; the histology of these patterns has been in need of study.The xylary peculiarities of Mendoncia and Thunbergia are also pertinent in connection with systematics, because some accounts have recognized Mendonciaceae as a separate family (Bremekamp 1953;Dahlgren 1980;Cronquist 1981;Takhtajan 1987), and some have considered Thunbergiaceae a worthy segregate family (e.g., Bremekamp 1953;Dalhgren 1980;Takhtajan 1987).

Although the majority of Acanthaceae are characteristic of the humid tropics, some of the Acanthaceae studied here are characteristic ofdrier or cooler regimes.Anisacanth s thurberi occurs along gravelly washes in northwestern Mexico and adjacent Arizona and New Mexico (Johnston 1924;Kearney and Peebles 1960).Ruellia peninsularis grows on sea bluffs near La Paz, Baja California (Johnston 1924).Justicia californica (formerly considered a species of Beloperonei may be found in washes at the northern and western edges of the Colorado Desert (Munz 1974).Diapedium assurgens (sometimes included in Dicliptera) occurs in disturbed places in shallow coral soils of exposed sites of the florida Keys and adjacent peninsular f orida (Long and Lakela 1971).Jacobinia is an understory plant that ranges from Mexico to Bolivia; it does not occur in moist understory sites, but in shrubby or dry forests, as beneath Cercidium (Gentry 1942).Detailed ecological data on Tanzanian Acanthaceae are not at hand, although the data in Dale and Greenway (1961) suggest that genera such as Himantochilus and Pseudoblepharis characterize relatively dry sites in eastern Africa.

The remaining Acanthaceae studied here are from areas of relatively heavy rainfall.These species are of interest with respect to ecological wood anatomy because dicotyledons with mesomorphic woods have been assumed to be the norm instead of diverse adaptations to various kinds of humid and moist conditions.Metcalfe and Chalk (1950) report on the wood features of 15 genera (species not given) of Acanthaceae and the information they give appears accurate.The present study amplifies that account both in qualitative and quantitative aspects.No survey of wood anatomy in Acanthaceae other than their account has appeared elsewhere.


MATERIALS AND METHODS

The ma ority of specimens studied were available in dried form.Most of these were derived from the SJRwand MADw collection of the Forest Products Laboratory, Madison, Wisconsin (see Table I).Dried samples were boiled in water and stored in aqueous 50% ethyl alcohol.Other samples from plants in cultivation (Table I) were preserved in aqueous 50% ethyl alcohol.Sections of woods of most collections were prepared by means of the sliding microtome.However, vining and lianoid species (Mendoncia.Thunbergia) cannot be sectioned readily in this way because of the sectioning problems presented by large-diameter vessels and by pockets of thin-walled parenchyma (some of which proved to contain phloem) intercalated among the fibers.For these species, an alternative method involving softening in ethylene diamine, infiltration, embedding in paraffin, and sectioning on a rotary microtome (Carlquist 1982) was used.Sections were stained with a safranin-fast green combination.Macerations were prepared by means of Jeffrey's Fluid and stained with safranin.

Means for wood features were obtained by averaging 25 measurements except for vessel wall thickness, libriform fiber dia eter at widest point, and libriform fiber wall thickness.For these latter features, a few typical conditions were measured and averaged.Vessel diameter is taken at the widest point and excludes wall thickness; although this is a less common way of measuring vessel diameter, this method has the merit of representing lumen diameter, which is a dimension more pertinent where physiological and ecological features are being discussed than inclusion of wall thickness (although inclusion of vessel wall thickness does not alter data significantly except in species with very narrow vessels).Vessels per group is based on averaging counts w

re a solitary vessel =
I, a pair of vessels in contact = 2, etc. Vessels per mm? represents a count of all vessels (pores, not vessel groups) within a field.Terminology in wood features follows that of the IAWA Committee on Nomenclature (1964).Growth-ring types are according to Carlquist (1988), which represents essentially the same system presented earlier (Carlquist 1980).Preparation of quantitative data, some of the wood sections and macerations, and some observations on qualitative features are the work of the second author.Preparation of the text, photomicrography, and some observations of qualitative features represent the work of the first author.David A. Hoekman prepared some wood sections and some quantitative data.

Provenance of the specimens is as follows (except for specimens taken from cultivation by the first author): Acanthus ebracteatus (New Britain); Anisacanthus thurberi (5 miles southwest of Tucson, Arizona); Aphelandra pulcherrima (Guyana) ; A. scabra (Guamitas, Venezuela); A. tetragona (Colombia); Asystasia zambiana (Shaba, West Africa); Barleria ristata (cult.Kamehameha School, Hono-


Growth Rings

Only a minority of Acanthaceae possesses growth rings, and even these would probably not be designated as ring-porous in the ordinary sense of that term.In Anisacanthus thurberi (Fig. I), vessels are wider in earlywood or somewhat after the initiation of earlywood (the rather unusual Type 10 ofCarlquist 1988).In this growth-ring type, onset of temperature suitable for growth and beginning of the rainy season are not synchronous.This is observable in the habitat for this species, southern Arizona, in which temperatures suitable for growth occur in early spring months, but the majority of rain does not come until summertime.

Another distinctive type of growth ring is demonstrated by Whitfieldia colorata (Fig. 3), in which little change in vessel diameter occurs in growth rings, but in which latewood has libriform fibers thicker walled than in the earlywood (Type IE of Carlquist 1988) .Type 1E is also present in Bravaisia integerrima and Graptophyllum insularum (Fig. 9).

Thunbergia holstii (Fig. 21) has the unusual Type 12 (Carlquist 1988) , in which the wood is diffuse porous and only presence of a parenchyma band defines the beginning ofa growth ring.A terminal parenchyma band is present in some growth rings of Justicia californica, in which moderate fluctuation of vessel diameter occurs and thus Type 1C (Carlquist 1988) is present.

Diffuse-porous wood with no appreciable evidence ofgrowth-ring activity characterizes the r mainder of Acanthaceae studied here, as illustrated for Beloperone crenata (Fig. 5), Sanchezia williamsii (Fig. 13), Trichanthera gigantea (Fig. 24), Thunbergia laurifolia (Fig. 25), T. alata (Fig. 26), and Mendoncia gigas (Fig. 29).


Vessel Elements

Vessel grouping (Table I, column 1) is moderate in Acanthaceae compared to that in certain other families (e.g., Asteraceae): it is reported here to exceed 2.5 vessels per group in Graptophyllum insularum (Fig. 9), Jacobinia ghiesbrechtiana, Mendoncia sp., Razisea spicata, Thunbergia alata (Fig. 26), T. laurifolia (Fig. 25), and Wh itfieldia colorata (Fig. 3).However, the resemblance in transectional view of very narrow vessels and vasicentric tracheids to libriforrn fibers results in undermeasurement of vessels per group in Anisacanthus thurberi (Fig. I) and Justicia californica, which, therefore, should be added to this listing.Vessel grouping occurs as short radial multiples or as pore clusters.Species with vessels predominantly solitary (1.5 vessels per group or fewer) include Bravaisia floribunda, B. integerrima, Sanchezia decora, Thunbergia erecta , T. holstii (Fig. 21), and Trichanthera gigantea (Fig. 24).More than half of the family averages between 1.5 and 2.5 vessels pe

re for a fami
y in which the libriform fiber is the type of tracheary element present (Carlquist 1984).

Vessel density and vessel diameter-figures that tend within limits to be reciprocals of each other in a given group with notable exceptions (e.g., vines and lianas)-are given for Acanthaceae in Table I (columns 2 and 3).Metcalfe and Chalk (1950) have commented on the narrowness of vessels in most Acanthaceae.The data of Table I in general show that vessels are narrower in the mesic species than one might expect on the basis of other dicotyledonous families (e.g., Whitfieldia colorata, Fig. 3).Vessel diameter is probably less in Anisacant us thurberi :

and Justicia californica than the figures in Table 1 indicate, because narrow vessels grade into vasicentric tracheids in these species, and these cell types are so difficult to distinguish from libriforrn fibers in transections that overestimating vessel diamter and underestimating vessel density is almost inevitable.Vessel diameter in reases with age in stems of Asystasia zambiana, Thunbergia crispa, and T. holstii.The low figures for vessel density in Acanthaceae can be appreciated by comparison with xylem in species ofthe woody flora ofsouthern California, where relatively high vessel density is to be expected on account of the predominant dryness of this area.In the southern California flora, the ean number of vessels per mm-is 257 (Carlquist and Hoekman 1985).Only three of the Acanthaceae studied here have vessel density higher than that figure.

Vessel element l

Mendoncia and in Ruspolia hypoc
ateriformis and Thunbergia alata.Vasicentric tracheids were previously reported for the family in Justicia calijornica (Carlquist 1985a).The "tracheids" reported by Solereder (1908) for Thunbergia are doubtless what would now be termed vasicentric tracheids.

Axial Parenchyma Metcalfe and Chalk (1950) have characterized the axial parenchyma of Acanthaceae as vasicentric scanty, and the present study confirms this.Axial parenchyma forms a complete or partial sheath one cell thick around vessels or vessel groups in all collections studied; such parenchyma is illustrated for Sanchezia williamsii (Fig. 13).Vasicentricparenchyma was observed to be very scarce in Barleria cristata, Diapedium assurgens, and Pseudoblepharis glischrocalyx.Record and Hess (1943) report terminal and diffuse parenchyma in Anisacanthus.Our material of A. thurberi reveals that in the growth rings, parenchyma is probably initial rather than terminal.In this species, the vessels are narrow at first in growth rings, with wider vessels appearing somewhat later in the earlywood; latewood may be defined by the libriform fibers of narrowest radial diameter.Similar short bands of thin-walled parenchyma were observed in Justicia calijornica.The report by Record and Hess of diffuse parench ma in Anisacanthus is very likely a result of the fact that so many narrow vessels and vasicentric tracheids, cell types that resemble libriform fibers in transection, occur in Ani- sacanthus.Parenchyma scattered among narrow vessels and vasicentric tracheids should be called vasicentric parenchyma in accord with current usage.

The wood of Thunbergia holstii (Fig. 21, center) possesses parenchyma in a band that may be at the end ofa growth ring , o fluctuation in vessel diameter can be used to define such a ring.This parenchyma is thin walled and nonseptate, and may represent a kind of fiber dimorphism (Carlquist 1958(Carlquist , 1980(Carlquist , 1988)).In other species of Acanthaceae, axial parenchyma is generally in strands of two to five (often three) cells.

M endoncia and two species of Thunbergia, T. alata and T. laurifolia, have large pockets of thin-walled apotracheal axial parenchyma (Fig. 25-27, 29), but these contain interxylary phloem and are considered below under that heading.In T. crispa , the earlier-formed secondary xylem begins without apotracheal parenchyma, b t later-formed wood contains strands of apotracheal parenchyma like those of T. alata and T .laurifolia but somewhat smaller and with no phloem.


Vascular Rays

Acanthaceae show a wide range of expressions in presence of uniseriate or multiseriate rays.Multiseriate rays much more common than uniseriate rays were observed in Anisacanthus thurberi (Fig. 2), Razisea spicata, Sanchezia nobilis, S. rubriflora, and S. williamsii.Multiseriate rays somewhat more common than uniseriate rays were seen in Acanthus ebracteatus, Aphelandra tetragona, Bravaisia jloribunda, B. integerrima, Justicia californica, Sanchezia decora, Trichanth era gigantea, and Whitfieldia colorata (Fig. 4).

Multiseriate rays about as numerous as uniseriate rays occur in Aphelandra pulcherrima, A. scabra , Calycacanthus sp., Graptophyllurn insularum (Fig. 10), Himantochilus marginatus, Megaskepasma erythrochlamys, Mendoncia retusa, Pachystachys lutea, P. spicata, Pseuderanthemum laxiflorum, Pseudoblepharis glischrocalyx, Ruellia peninsularis, Sanchezia stenomacia, and Thunbergia erecta.Multiseriate rays fewer than uniseriate rays characterize Barleria cristata, Mendoncia gigas (Fig. 30), M .erythrochlamys, and M .sp.

Various degrees of raylessness characterize some Acanthaceae.Species with few rays (and those all uniseriate) include Jacobinia ghiesbrechtiana, Salpinxantha coccinea, and Thunbergia holstii.The ontogenetic development of a rayless condition can be seen in Diapedium assurgens (Fig. 8).Woods observed to be entirely rayless include Jacobinia carnea, Justicia magnifica (Fig. 5, 6), and Thunbergia laurifolia (Fig .25).

In species of Acanthaceae in which rays are present, a large proportion have abundant erect cells in the multiseriate portions of multiseriate rays.Erect cells only were observed in Thunbergia erecta and T. holstii.Erect cells almost exclusively, with a few square cells, characterize multiseriate portions of multiseri

lls) in multis
riate portions of multiseriate rays: Bravaisia floribunda, B. integerrima, Himantochilus marginatus, Psilanthele jamaicensis, and Trichanthera gigantea.Erect cells characterize uniseriate rays and uniseriate portions of multiseriate rays except in the species listed above in which procumbent cells are common in multiseriate portions of multiseriate rays ; in these latter species, square or procumbent cells are occasional but the majority of cells are erect in uniseriate rays and uniseriate portions of multiseriate ra s.Thus, upright cells bulk very large in rays of all but a few Acanthaceae.The species with wider rays (Table I, column 10) tend to have a higher proportion of procumbent cells in multiseriate portions of multiseriate rays: Anisacanthus thurberi, Aphelandra deppeana , Bravaisia floribunda, B. integerrima, and Trichanthera gigantea could be cited in support of this trend.Rays are characteristically narrow in Acanthaceae: the mean width of multiseriate rays at their widest point is only 2.6 cells for the family at larg (Table I, column 12), and in only seven species does the figure exceed 3.0 cells.

Ray cell walls are only moderately thick in the family (Fig. 2,4,10,14,19,20,24,30), but all ray cells were lignified except in Thunbergia alata, in which rays are very few.Pits interconnecting ray cells were simple in all species studied with the exception of a few pits of ray cells in Aphelandra pulcherrima.

Ray heights are shown in T ble I, columns 10 and II.In general, ray heights parallel length of vessel elements or libriform fibers.The genera Sanchezia and Trichanthera have long vessel elements and also exceptionally tall multiseriate rays.


Cystoliths

Cystoliths have been reported for various Acanthaceae by Solereder (1908), who has even provided (p.616-617) a key to the various types of cystoliths in the family.However, the organographic distribution of cystoliths in the family is not given by Solereder (1908) nor by Metcalfe and Chalk (1950), and no account to date has indicated in which genera in the family cystoliths occur in wood.

In the present study, cystoliths were observed in some ray cells of Bravaisia floribunda (Fig. 20), B. integerrima (Fig. 19), Sanchezia decora (Fig. 18), S. nobilis, S. rubriflora, S. williamsii, and Trichanthera gigantea.All ofthese cystoliths could be referred to the same basic type: they are oval to elongate structures that tend to conform in their shape to the shape of the cells in which they are contained.Hyaline outer layers or tips (Fig. 18) can be seen on some, while others exhibit a knobby appearance (Fig. 19, center).Cystolith lamellae that stain brightly with fast green, and which therefore probably represents nonlignified cellulosic layers, can be seen in all cystoliths observed in Acanthaceae.These layers are illustrated in Fig. 20.


Crystals

Calcium oxalate crystals have not hitherto been reported in Acanthaceae.In the present study such crystals were uncovered only in three species.In Graptophyllum insularum (Fig. 12), crystals are small, rhomboidal, and numerous per ray or axial parenchyma cell.Although crystals are rather uniformly small in G. insularum, there are a few larger rhomboidal crystals in ray or axial parenchyma cells of this species.Small and relatively short acicular crystals were in ray cells ofAphelandra tetragona.Acicular crystals of this nature were observed in ray cells and apotracheal parenchyma of Thunbergia crispa, and in axial parenchyma associated with interxylary phloem in T. alata (Fig. 28, bottom; Fig. 32) and T. laurifolia as well as all four collections of Mendoncia.


Amorphous Deposits

Irregularly shaped accumulations of tanninlike materials were observed in ray cells of Aphelandra pulcherrima, A. tetragona, Pseudoblepharis glischrocalyx, Ruellia peninsularis, and Salpinxantha coccinea.Such deposits are to be expected in other species of the family, because in various dicotyledonous woods, amorphous deposits seem to occur without reference to taxonomic groupings.


Interxylary Phloem and Other Cambial Variants

Although "fissured xylem" has been reported fo Mendoncia and Thunbergia (Metcalfe and Chalk 1950), interxylary phloem has been reported in Acanthaceae only for Thunbergia (Mullenders 1947).Interxylary phloem is present in Thunbergia alata (Fig. 26, 27), T. laurifolia (Fig. 25), Mendoncia g

as (Fig. 29
, M .microchlamys, M .retusa, and M .sp.In Thunbergia alata and T. laurifolia, the phloem strands are sheathed in parenchyma cells that have a smaller diameter than the sieve tube elements and thus are easily distinguished from them.The remaining xylem areas consist of vessels , libriform fibers, and vasicentric tracheids, cell types that have thick, lignified walls, and thus are quite disti ct from the parenchyma-sheathed interxylary phloem zones (Fig. 25-27).

The interxylary phloem of Thunbergia alata and T. laurifolia lacks fibers, although large scattered gelatinous fibers do characterize the extraxylary phloem of these species (Fig. 25,top) .In Mendoncia gigas and other species of Mendoncia, gelatinous fibers identical to those of Thunbergia occur not only in extraxylary phloem (Fig. 29, top and left) but also in some of the interxylary phloem strands.

In Thunbergia holstii , a small cambial anomaly was noted; although the xylem cylinder was otherwise normal, a patch of vessels embedded in parenchyma (Fig. 22, top) outside of the xylem cylinder was observed.The cambium presumably is external to the patch of vessels at this point.T

apotrach
al parenchyma strands in later-formed wood of Thunbergia crispa represent a conditional transition between the absence of such parenchyma bands (T.erecta) and the strands of parenchyma that contain interxylary phloem in T. alata and T. laurifolia.


SYSTEMATIC CONCLUSIONS

The features that characterize Acanthaceae include: vessels with simple perforation plates and lateral-wall pitting of medium-sized alternate bordered pits ; imperforate tracheary elements with simple pits and therefore libriform fibers (vasicentric tracheids also present in a few genera); libriform fibers septate in most genera; axial parenchyma vasicentric scanty (also initial or terminal in a few genera); rays both multiseriate and uniseriate, the former mostly biseriate (some species rayless or nearly

yless); multiseriat
portions of multiseriate rays with procumbent cells in a few genera, rays otherwise paedomorphic (see Carlquist 1962Carlquist , 1980Carlquist , 1988) by of predominance of erect cells; cystoliths present in ray cells in a few genera; small rhomboidal or acicular crystals present in axial and ray parenchyma of a few genera; wood nonstoried.

All of the features listed in the above pa

graph are matched by those in wood of Gesneria
eae, except for cystolith presence (Carlquist and Hoekman 1986a).The presence of septate libriform fibers in both Gesneriaceae and Acanthaceae provides a striking similarity, and the two families are also alike in presence of paedomorphosis and raylessness in a number of genera.Raylessness could be claimed to form an instance of parallel evolution in the two families, indicative of a similar range of herbaceousness in both families.The entire family Plantaginaceae is rayless or nearly so (Carlquist 1970), but is probably not as close to either Gesneriaceae or Acanthaceae as it is to Scrophulariaceae, a family which also has some rayless species in genera such as Hebe.One genus of Gesneriaceae, Coronanthera, has fiber-tracheids (Carlquist and Hoekman 1986a), but the r maining genera have libriform fibers.Fiber-tracheids characterize some Scrophulariaceae, although many have septate libriform fibers and a few (e.g., Hebe) have tracheids (Metcalfe and Chalk 1950).In Pedaliaceae and Martyniaceae, libriform fibers are present; they are septate in Sesamothamnus of the Pedaliaceae (Carlquist 1987).

Bignoniaceae have aliform-confluent axial parenchyma, and thus represent a slightly greater departure from Acanthaceae than the above families; imperforate tracheary elements in Bignoniaceae are fiber-tracheids or libriform fibers, the latter septate in vining genera (Metcalfe and Chalk 1950).Myoporaceae share vasicentric scanty parenchyma with Acanthaceae, but Myoporaceae differ in having fibertracheids rather than libriform fibers and a predominance of procumbent rather than erect cells in rays (Carlquist and Hoekman 1986b).Occasional bands of parenchyma, probably the result of fiber dimorphism, such as those obs

ved in Thunbergia holst
i, may be found in some Myoporaceae (Carlquist and Hoekman 1986b), Scrophulariaceae (Michener 1986) and Pedaliaceae (Carlquist 1987), but these may not be indicative of relationship because they appear to have originated independently in a number of dicotyledon families.Storying is present in some Scrophulariaceae (Michener 1983(Michener , 1986)), Drymonia of the Gesneriaceae (Carlquist and Hoekman 1986a), some Myoporaceae (Carlquist and Hoekman 1986b), and many Bignoniaceae (Metcalfe and Chalk 1950).The lack of storying in Acanthaceae may be significant because the number of collections of Acanthaceae surveyed was large in comparison to collections studied in the other families.Presence of inconspicuous forms ofhelical sculpture in a scattering ofAcanthaceae should probably be interpreted more as an ecological adaptation to dry conditions (Carlquist 1966) than as an in icator of systematic relationship.

Although the families of Scrophulariales (Bignoniales of some authors) do deviate from one another with respect to wood anatomy, the order is remarkably uniform with respect to wood features compared with other dicotyledonous orders in which wood heterogeneity is much greater.This may aid in defining Scrophulariales and in determining the degree of relationship among the component families.

The present study offers significant evidence about the status of families sometimes segregated from Acanthaceae.Bremekamp (1953) and Cronquist (1981), who recognize Mendonciaceae as a monogeneric family, cite absence of cystoliths and a specialized seed dispersal mechanism as reasons for segregating However, cystoliths are not present in all Acanthaceae (Metcalfe and Chalk 1950).In addition, three highly distinctive wood features link Mendoncia with the scandent Thunbergia species (T.alata, T. laurifoliay: presence of large, scattered gelatinous fibers in extraxylary phloem; presence ofinterxylary phloem; and presence of acicular crystals in the parenchyma associated with interxylary phloem.With such a constellation of unusual features linking the two genera, segr gation of Mendoncia from Acanthaceae seems inadvisable unless Thunbergia is also excluded from Acanthaceae.Thunbergia has been segregated from Acanthaceae by some workers (e.g., Dahlgren 1980).However, wood oftwo species of Thunbergia, T. erecta and T. holstii, does not share the features cited for Mendoncia but is, instead, compatible with the wood of remaining Acanthaceae.Thus, segregation of Thunbergia from Acanthaceae is not supported here.

Specific and generic characters probably exist in Acanthaceae (e.g., cystolith presence or absence), but the sampling of the family in the present study is insufficient to establish such features with any degree of validity.Study of wood anatomy in genera with diversity among species (e.g., Justiciai in habit and ecology is likely to reveal some wood features that correspond with species limits.


ECOLOGICAL CONCLUSIONS

Although the majority of Acanthaceae occur in moist tropical habitats, some range into dry tropical and even desert (Anisacanthus, Justiciat localities, although the two genera named occur in desert washes , where moisture availability is very likely greater than on desert flats.In viewing woods of Acanthaceae, therefore, one is likely to find mesomorphic adaptations.

The vessels of Acanthaceae from the wet tropics are much narrower than one would expect on the basis of observations on wood of tropical trees.Mean vessel diameter for the world flora (admittedly a sample that favors tree species) is 94 J.Lm (Metcalfe andChalk 1950, p. 1360), whereas the mean vessel diameter for Acanthac ae is 70 J.Lm (Table I, column 3).Moreover, the presence in several genera (e.g., Bravaisia, Sanchezia) of perforation plates appreciably narrower than the vessels in which they occur, suggests a suboptimal conformation of vessels where flow of volume per unit time is concerned.If one views the characteristic habitats of genera such as Sanchezia, the nature of quantitative vessel features in he family becomes evident.Tropical Acanthaceae are typically understory plants.Plants in this habitat would transpire lower volumes per unit time than plants with canopy status, so a conductive system adapted to moderate transpiration is not surprising.

The genus Jacobinia occupies understory habitats where seasonal drought occurs (see Introduction), and thus the canelike shoots of limited duration in that genus are understandable when one compares Jacobinia to genera with single trunks, such as Sanchezia.We may take the Mesomorphy ratio value of about 50, well below the family mean of 1361 (Table I, column 13), as the threshhold below which woody dicotyledons are xeromorphic (Carlquist and Hoekman 1985).Jacobinia falls close to that threshhold, as does Justicia.Notably low Mesomorphy values are represented by Anisacanthus thurberi (30), Diapedium assurgens ( 19) , and Ruellia peninsularis (13).Desert shrubs of southern California have a Mesomorphy value (20.9) in about the same range (Carlquist and Hoekman 1985), and correlated with the habitats these three species (see Introduction).The specimen of Ruellia peninsularis was obtained from a cultivated plant receiving regular watering, demonstrating that xeromorphic wood fe tures may be heritable with less phenotypic modification than a scattering of comments in the literature on wood anatomy might lead one to expect.

The range of Mesomorphy-ratio values in Acanthaceae is relatively great, and the figures obtained do appear to be reliable indicators of ecology.However, a species with a value of 10,000 probably should not be considered ten times as mesomorphic as one

ith a value of 1000: va
ues in this range may indicate differences in ability to conduct volumes per unit time, but values at the low end of the Mesomorphy scale are probably more significant with respect to order of magnitude of numerical values, because they indicate conductive safety (ability to maintain water columns intact through a dry season).

Vessel element length, one of the dimensio s on which the Mesomorphy ratio is based, very clearly correlates with ecology in Acanthaceae.If one notes which species in Table I have a vessel element length greater than the world flora mean, 649 ~m (Metcalfe and Chalk 1950), one finds that they are all broadleaved shrubs of moist tropical forest understory habitats.Degree of vessel grouping is held to be proportional to xeromorphy in woody dicotyledons that have libriform fibers or fiber-tracheids (Carlquist 1984).With respect to this criterion, species of Acanthaceae appear mostly mesomorphic (Table 1, column 1).In this connection, one should note that the vessel per group figures for Anisacanthus thurberi and Justicia californica are not accurate because narrow vessels and vasicentric tracheids are difficult to distinguish from Iibriform fibers, and if the former cell types could have been counted accurately, a much higher vessel per group figure would have been obtained for those species.

In addition to f atures involving vessel dimensions, one should note that presence of vasicentric tracheids in Anisacanthus thurberi and Justicia califarnica is of great importance as a probable mechanism for survival of drought, as noted earlier (Carlquist 1985a).Growth-ring presence and presence of helical sculpture in vessels, although both of these are present in relatively moderate forms in the Acanthaceae that possess them, seem to correlate with relative dryness of habitat.

One must remember that features of the vegetative apparatus other than wood may be of overriding importance to survival.For example, if Jacobinia stems are moderately succulent, leaves tend to be drought-deciduous, and shoots are of short duration and can die back to the woody caudex without causing any loss in ability to produce flowers the following year.


CONCLUSIONS RELATIVE TO HABIT

Paedomorphosis in dicotyledon wood (Carlquist 1962(Carlquist , 1980(Carlquist , 1988) ) is indicated by a series of structural features characteristic ofwood of herbs and herblike plants (particularly their metaxylem and early secondary xylem) that are protracted indefinitely int the secondary xylem .One of these tendencies is predominance of erect cells in rays , especially in the multiseriate part of multiseriate rays , in which procumbent cells are normally abundant in wood of typically woody dicotyledons.This tendency is remarkably common in Acanthaceae; erect ray cells predominate in multiseriate rays ofall but the woodiest ofthe Acanthaceae studied here.

Raylessness is a form of paedomorphosis in which ray cells not only so erect that they resemble imperforate tracheary elements in length, they also are like libriform fibers rather than ray cells in morphology (cells a e fusiform rather than blunt).Raylessness has been considered as an indicator of secondary woodiness (Carlquist 1970) or an increase in strength involving relatively slender but tall stems in herbs or herblike plants, as in certain species of Phacelia (Carlquist and Eckhart 1984).The slender stems of Plantago princeps Cham.(Carlquist 1970) and ofJacobinia, both good examples of rayless ness , may be representative of both tendencies.

The genera Mendoncia and Thunbergia show adaptation in wood structure to the vining habit.In the genus Thunbergia, T. alata, T. crispa, and T. laurifolia show the most pronounced expressions of these modifications; T. erecta and T. holstii are more nearly shrubs than vines in habit and in wood anatomy.Thunbergia alata, T. laurifolia, and the various species of Mendoncia have vasicentric tracheids in addition to very narrow vessels.Thus, they illustrate the dimorphism in vessel diameter that is characteristic of scandent woody dicotyledons (Carlquist 1981), and t ey also exemplify the tendency for vines and Hanas with libriform fibers (or fiber-tracheids) to have vasicentric tracheids (Carlquist 1985b).Acanthaceae should be added to the list of families with vasicentric tracheids given in Carlquist (1985b) .Vasicentric tracheids are theorized to offer a form of safety in vines and lianas, because they could be more resistant to embolism spread than vessels, and thereby could continue to conduct ifthe wide vessels were temporarily disabled.
sheath of starch-rich parenchyma around vessels in vines and Iianas is also theorized to be a potential conductive safety mechanism (Carlquist 1985b), because hydrolysis ofstarch into sugar, followed by transfer of sugar into the vessels , could increase volume ofwater within the vessels by osmotic pressure.Thunbergia alata, T. laurifolia, and the four colle

ions of Mendoncia studied here
all show vasicentric parenchyma unusually abundant for the family (sheaths around vessels usually two cells thick).Dobbins and Fisher (1986) support the idea that presence of parenchyma may help regeneration of conductive tissue following wounding, as might happen when a liana falls from its host tree.While their studies do show callus formation and cambial renewal after artificial wounding, they do not show that this process helps in survival of damaged lianas in the wild.One might expect that evolutionary mechanisms that prevent damage to the conductive system would have greater selective value than those that repair damage; disruption to conductive tissue of a liana could be fatal.

The role of parenchyma in providing flexibility and thereby potentially reducing damage to liana stems has been stressed by various authors (see Carlquist 1985b).The parenchyma sheaths around phloem in Thunbergia alata and T. laurifolia are quite prominent and suggest a function in protecting the integrity of the interxylary phloem.This function might be in those species related to the abundant acicular crystals in those cells, potentially an herbivore deterrence mechanism rather than a flexibility mechanism.Another possible function of parenchyma abundance in stems oflianas and vines is providing sites for fissuring of the xylem i to separate units.Fissuring of the xylem in Mendoncia has been mentioned by Metcalfe and Chalk (1950) and by Obaton (1960).When fissuring is active, interxylary phloem provides a mechanism for close phloem association with the fragmented xylem strands.

Wide vessel diameter has long been as a characteristic of scandent dicotyledons (for a review, see Carlquist 1985b).Vessels in Thunbergia alata , T. laurifolia, and the four collections of Mendoncia do not appear exceptionally wide according to the data of Table I, column 3.However, the presence of numerous narrow vessels in those species undoubtedly masks, in averaging vessel diameters, the existence of exceptionally wide vessels revealed in preparations such as those illustrated in Figure 25, 26, and 29.The presence of numerous narrow vessels in a lianoid species, as in Asystasia zambiana, may have a function in improving conductive safety much like the safety conferred by vasicentric tracheids, for very narrow vessels may resist embolisms nearly to the degree that vasicentric tracheids do so see Carlquist 1988).

The exceptionally thick walls of vessels in Thunbergia crispa and T. holstii are unusual in Acanthaceae.Walls of this nature might represent a mechanism for maintaining the integrity of water columns in vessels .This possibility should be investigated, and additional instances of notably thick walls in lianas should be reported.As a generalization one may say that wider vessels in dicotyledons have thicker walls.The degree to which th is is true deserves study; if wider vessels do tend to be notably thick walled , the function of this thickness may be in maintaining integrity of the water column, because mechanical strength of the stem itself could be achieved much more readily by a moderate increase in thickness of libriform fibers.Thicker vessel walls might also characterize species in which imperforate tracheary elements also tend to be thicker walled, so that an increase in wall thickness of one type of cell tends to be applied, for genetic reasons, to other cell types as well.

The rayless or nearly rayless wood of Thunbergia alata and T. laurifolia nd the relatively ray-poor wood of T. crispa may seem curious in view of the prominence of wide rays in scandent families such as Aristolochiaceae and Vitaceae.However, raylessness has been reported in another woody vine, Cobaea (Carlquist, Eckhart and Michener 1984).The abundant parenchyma of Cobaea and the two Thunbergia species may serve not only the functions of axial parenchyma in scandent woody plants cited above, it may also substitute very effectively for rays in these species, in which stem diameter may not exceed about 1 em.Asystasia zambiana is a lianoid species that does correspond to the Aristolochia pattern, for it has notably wide primary rays altered little during secondary growth.

The possible function of rayless ness as a way of conferring mechanical strength to stems of short duration has been mentioned above.Thick-walled libriform fibers appear related to habit in canthaceae, because they are most prominent in genera that are large shrubs or even small trees (Sanchezia, Trichanthera).The ratio between imperforate tracheary element length and vessel element length is of potential interest with relation to habit.The ratio has a comparatively low value in Gesneriaceae, an intermediate value in Acanthaceae, and a high value in Myoporaceae.This may accord with the fact that woody Gesneriaceae are relatively small shrubs, woody Acanthaceae are medium-sized to large shrubs, and Myoporaceae are medium-sized shrubs to trees.

Fig. 1
1
Fig. 1-4.Wood sections of Acanthaceae.-1-2.Anisacanthus thurberi, SJRw-2670J .-1.Transection, showing growth rings.-2.Tangential section; over half of cells shown are vasicentric tracheids.-3-4.Whitfieldia colorata, SJRw-J5093 .-3.Transection; note narrowness of v ssels.-4.Tangential section; multiseriate rays have predominantly erect ray cells.(Fig. 1-4, magnification scale above Fig. 1 [finest divisions = 10 I'm).)


Fig. 9 -
9
Fig. 9-12.Wood sections of Graptophyllum insularum, SJRw-2454/ .-9. Transection; growth rings demarcated by thick-walled libriform fibers.-IO.Tangential section; multiseriate and un iseriate rays are about equally abundant.-II.Portion of radial section; perforation plates are appreciably narrower than vessel elements.-12 .'SEM photograph of radial section, showing small rhomboidal crystals in ray cells.(Fig. 9,10, magn ification scale above Fig.I; Fig. II , scale above Fig.7; Fig. 12, scale on photograph = 10 um.)




Fig. 17-20.Wood sections of Acanthaceae.-17.Sanchezia williamsii, MADw-2677.Transection, showing simple pits in ray cells.-18.Sanchezia decora, MADw-38350.Tangential section ; cystoliths in ray cells.-19.Bravaisia integerrima , MADw-7158.Tangential section; cystoliths in ray cells.-20.Bravaisia floribunda, M Dw-7154.Radial section; cystoliths in ray cells.(Fig. 17-20, magnification scale above Fig.7.)




Fig. 17-20.Wood sections of Acanthaceae.-17.Sanchezia williamsii, MADw-2677.Transection, showing simple pits in ray cells.-18.Sanchezia decora, MADw-38350.Tangential section ; cystoliths in ray cells.-19.Bravaisia integerrima , MADw-7158.Tangential section; cystoliths in ray cells.-20.Bravaisia floribunda, MADw-7154.Radial section; cystoliths in ray cells.(Fig. 17-20, magnification scale above Fig.7.)


Fig.

Fig. Wood sections of Thunbergia.-25.T. laurifolia, cult.LASCA.Transection; note gelatinous fibers in phloem, above.-26-28.T. alata, cult.LASCA.-26.Transection; as in section of T. laurifolia, gray areas = interxylary phloem.-27.Transection portion showing large diameter f sieve tubes, slightly darker, surrounded by parenchyma (gray).-28.Tangential section portion to show vessel (above), with narrow pit apertures.(Fig. 25, 26, magnification scale above Fig.I; Fig. 27, scale above Fig.22; Fig. 28, scale above Fig.7.)


Table 1 .
1
Wood characteristics of Acanthaceae.Salpinxantha coccinea Hook.
IV 0 ,:.
& Hook.f.N o ANATOMICAL FEATURES

t"" en
ACKNOWLEDGMENTSWood samples from the SJRw and MADw collections of the Forest Products Laboratory, Madison, Wisconsin, have been made available through the courtesy
The present study has been aided by a grant from the National Science Foundation, BSR 84119469. Appreciation is expressed to William L. Stem and Dieter C. Wasshausen, who read the manuscript and offered helpful sugge