Aliso: a Journal of Systematic and Evolutionary Botany Wood Anatomy of Lamiaceae. a Survey, with Comments on Vascular and Vasicentric Tracheids

Quantitative and qualitative data are presented for 44 collections representing 42 species in 27 genera. Lamiaceae basically have: vessels with simple perforation plates; vessel-to-vessel pitting alternate; imperforate tracheary elements alllibriform fibers, fibers commonly septate; axial parenchyma scanty vasicentric; rays Heterogeneous Type liB. These features ally Lamiaceae closely with Verbenaceae. In addition to the Mesomorphy ratio (based on vessel element dimensions), features that indicate wood xeromorphy in Lamiaceae, in probable increasing order of importance are: presence of indistinct to marked growth rings; presence of helical thickenings in vessels; presence of vasicentric or vascular tracheids; presence of vessel groups averaging three or more vessels per group. More mesomorphic woods in Lamiaceae occur in species that are arborescent to shrubby, and that occur in areas that have no prolonged drought. Helical sculpture in vessels is expressed not only as thickenings, but also as grooves interconnecting pit apertures (coalesced pit apertures), and inconspicuous thickenings in pairs along the grooves. Vascular tracheids (at ends of growth rings) occur in several genera. Where growth rings are very narrow (Salvia, Trichostema), vessels and tracheids are intermixed, thereby justifYing description as vasicentric tracheids of what may have originated phylogenetically as vascular tracheids. This may represent one pathway for origin of vasicentric tracheids. Wide bands of axial parenchyma in Cuminia and Leptosceptrum have originated as a result of fiber dimorphism. Abundance of upright cells in numerous Lamiaceae is indicative ofpaedomorphosis, and may point to secondary woodiness in some instances; the family as a whole appears ancestrally moderately woody. The ratio between libriform fiber length and vessel element length marks Lamiaceae as specialized. Crystal presence and cambial variants are newly reported for the family.


INTRODUCfiON
Although Lamiaceae are a large family (180 genera and 3500 species according to Willis 1985), their wood anatomy has been little studied to date.The main sources of information are the summary for the family by Metcalfe and Chalk (1950) and the more recent study ofwoods of the subtribe Hyptidinae by Rudall (1981).A few data on wood anatomy ofLamiaceae were offered by Greguss (1959) and by Hatzoupoulou-Belba and Psaras (1985).The lack of studies on wood of Lamiaceae is related to the stature of plants in the family: only a few are large shrubs or small trees (e.g., some species of Hyptis, Leucosceptrum, Prostanthera, and Westringia).The purpose of the present study is to survey woods in the family for characters of ecological, evolutionary, and systematic significance.The present study is a contribution to a survey of woods oftubiflorous families of dicotyledons, a summary of which is in progress.
Lamiaceae proved to have diverse expressions in wood anatomy, including some features that have not been reported for the family: banded axial parenchyma; axial parenchyma forming the background tissue for the wood; grooved vessel walls (coalesced pit apertures); various types ofhelical thickenings in vessels; occurrence of crystals in ray cells; and occurrence of variant cambial activity.
The number of species surveyed (42 species in 27 genera) is not exceptionally great considering the size of the family.An attempt has been made to study samples of optimal size, so that a mature pattern of wood anatomy is available.Twig material has not been used.Exclusion of smaller wood samples has lowered the number of species that could be included.Only a small proportion of Lamiaceae typically develop stems in excess of one em in diameter at the base.The selection of species here nevertheless represents a broad systematic coverage.The species studied would be distributed in the following infrafamilial categories in the system offered by Briquet (1895): Ajugoideae: The ecological interest of wood anatomy of Lamiaceae is considerable.Lamiaceae as a whole occupy moderately dry, or seasonally dry, sites.The majority of the taxa studied here are characteristic ofMediterranean-type climates.The prominence of specimens from California, the Canary Islands, South Africa, and Australia demonstrates this.Mechanisms for survival of the dry season in Mediterranean-type climates are the chief point ofinterest.In this connection, the question of vasicentric tracheids is pertinent.Vasicentric tracheids are considered a potential mechanism for drought survival in woody dicotyledons (Carlquist 1985a, b).Several Lamiaceae were noted earlier as having vasicentric tracheids (Carlquist 1985a:49).However, in that reference and in a later book (Carlquist 1988a:135), Lamiaceae were mentioned as exemplifying both vascular and vasicentric tracheids.In some species of the family, vascular tracheids (tracheids formed only in the last several layers of a growth ring) are formed in such abundance that vessels of the latewood are intermixed with them, and therefore those tracheids surrounding the vessels could justifiably be termed vasicentric.The nature of the transition between vascular and vasicentric tracheids in Lamiaceae will be examined and documented here.Features such as vessel diameter, vessel density, vessel element length, and helical thickenings (and other forms ofhelical sculpture) in vessels have been shown to be indicative of wood xeromorphy (Carlquist 1966).These features prove of great interest in analysis of woods of Lamiaceae.
The samples of Lamiaceae studied here come from both cultivated specimens and plants growing in the wild.Cultivated specimens are often thought to have greater water availability than wild-occurring specimens.By comparing wood samples of Lamiaceae from cultivation to those collected in the wild, one can infer to what degree such features as vessel diameter and vessel density are altered by the conditions of cultivation.Bissing (1982) has shown differences between specimens collected in the wild and cultivated specimens with respect to wood features for a series of Californian native plants.However, his results indicated that degree of environmental modification differed among the species he studied.
Certain features of wood of Lamiaceae are of interest with respect to wood evolution.The ratio between libriform fiber length and vessel element length may be indicative of specialization (Carlquist 197 5).The origin of axial parenchyma bands, present in two genera ofLamiaceae (whereas vasicentric parenchyma characterizes the entire family) provides another question of phylogenetic interest.Many Lamiaceae are herbs, but a larger number are at least moderately woody at the base.The nature of the habit ancestral within the family is unclear, and wood anatomy may offer a few features useful in this regard.Predominantly upright ray cells, for example, have been cited as an indicator of juvenilism in wood (Carlquist 1962), and may point to secondary woodiness.

MATERIALS, METHODS, AND ACKNOWLEDGMENTS
Woods were available in dried condition.Samples were boiled in water and stored in aqueous 50% ethyl alcohol.Sections were prepared on a sliding microtome; softening techniques were not required.Some sections of selected species were dried between clean glass slides and examined with scanning electron microscopy.Sections of all species were stained with safranin and, in most species, counterstained with fast green to highlight pit details.

w w
Quantitative data in Table 1 are means based on 25 measurements, except for vessel wall thickness, libriform fiber diameter, libriform fiber wall thickness, and ray cell wall thickness.For these latter features, a few typical conditions were measured for each sample.Wood terminology follows that of the lAW A Committee on Nomenclature (1964).The growth-ring classification used is that of Carlquist (1988a).
The growth ring termed Type 10 was recognized only recently (Carlquist 1980), based upon an evergreen chaparral shrub, Eriodictyon of the Hydrophyllaceae.In this type, the growth ring begins with vessels of moderate diameter, then continues with wider vessels and concludes with narrow latewood vessels.This has been interpreted as a response to commencement of woody cylinder growth during a cool wet season when peak conduction has not yet been achieved, followed by production of wider vessels suited to peak conduction at the onset of warm weather (Carlquist 1980).Type 10 growth rings are clearly illustrated here by Lepechinia fragrans (Fig. 4).I can report Type 10 also for Dorystoechas hastata, Lavandula crenata, Salvia apiana, S. canariensis ( Carlquist 15 9 52 stem), and S. mellifera (Fig. 32).These are all shrubs of Mediterranean-type climates where the climatic regime mentioned above does occur.

Cambial Variants
In several of the species studied, secondary xylem is laid down in increments varying in thickness around the stem.One such example is illustrated for Monardella linoides (Fig. 2), in which several growth rings are markedly wider at the left side of the photograph than at the right.
A much more extreme condition is represented here for Hemiandra pungens (Fig. 5).In the portion illustrated, the most recent additions to the wood Oeft two thirds of the photograph) have been deposited in an orientation perpendicular to that of the earlier-formed wood (right third of photograph).Stems tend to split along rays in this species.Accordingly, cambia originate in these rays, and the wood produced from these ray cambia is perpendicular to that of the original woody cylinder.Although Lamiaceae have not hitherto been reported to have variant cambial structure ("anomalous secondary thickening"), the condition illustrated by H emiandra pungens qualifies as an exemplar of this category.

Quantitative Vessel Element Features
The mean number of vessels per group for the species studied is shown in Table 1, column 1.However, one must note that for some samples, no accurate figure for the number of vessels per group can be obtained.For example, in Rosmarinus officina/is (Fig. 25), cells easily identified in transection as vessels are intermixed with narrower cells that could be either narrow vessels or vasicentric tracheids (macerations reveal both of these cell types).Vessels per group can be calculated only on the basis of transections, yet macerations are required to distinguish between narrow vessels and vasicentric tracheids definitively.Similarly, the latewood of the temperate species of Lepechinia (Fig. 26, 27) contains narrow vessels that grade into vascular tracheids.In transection there is no way that one can establish what proportion of these are narrow vessels and what proportion are vascular tracheids.In Salvia apiana, S. dorrii, S. funerea, S. mellifera, and Trichostema lanatum, similar considerations apply.The figures given in Table 1, column 1, for the species discussed in this paragraph are doubtless minimal.
For species other than the above, the number of vessels per group is relatively low and ranges from 1.07 (Hoslundia opposita) to 4.10 (Tinnea rhodesiana).
Relatively low degrees of vessel grouping are seen in Figures 1 and 7; somewhat greater degrees of vessel grouping are represented in Figures 2, 3, 4, 5, and 6.Mean vessel diameter cannot be accurately determined from transections for species with vasicentric or vascular tracheids for reasons given above.Conceding that figures for those species may be slightly inaccurate, there is still a very wide range in mean vessel diameter in Lamiaceae (Table 1, column 2).Narrow vessels characterize the species with relatively large numbers of vasicentric or vascular tracheids, as well as Origanum majorana, Prostanthera aspalanthoides, Salazaria mexicana, and Teucrium heterophyllum.Metcalfe and Chalk (1950) cite two genera with vessels less than 25 ~-tm in mean diameter, Phlomis and Rosmarinus.Relatively wide vessels (more than 60 ~-tm in diameter) were observed in the present study in Hoslundia opposita, the species of Hyptis, Leucosceptrum canum, Phyllostegia lantanoides (Fig. 7), and Salvia canariensis (Carlquist 15952, root only).
Vessel wall thickness (Table 1, column 5) shows an appreciable range in Lamiaceae, although the photomicrographs in the present study do not demonstrate this clearly.The species in the present study with the greatest mean vessel diameter, Phyllostegia lantanoides, also has relatively thick (3.2 Jtm) vessel walls and illustrates a correlation often seen in dicotyledons.Leucosceptrum canum similarly has wide vessels with thick walls.However, the thickest vessel walls in woods of the present study (4.0 Jtm) were observed in Rosmarinus officina/is, which has relatively narrow vessels.Thus, more than one factor may be related to thickness of vessel walls.
Lateral walls of vessels in Lamiaceae bear alternate pits on vessel-to-vessel contacts.In size, these pits range from 2 Jtm in diameter in Teucrium heterophyllum and Tinnea rhodesiana to 8 Jtm in Plectranthus barbatus (Table 1, column 6).The mean for the family (approximately 5 Jtm) is close to the figure reported for pit diameter in most species.

Morphological Features of Vessel Elements
Vessel-to-vessel pits in Lamiaceae are alternate and circular in outline.Where crowded, pits may be somewhat angular in outline (Fig. 8).Apertures of pits are narrowly elliptical, as illustrated in Figure 8.
Two types of helical sculpture are present on vessel walls.One of these takes the form of grooves interconnecting two or more pit apertures (these have also been termed coalesced pit apertures), as illustrated in Figure 9. Short grooves interconnecting a few pit apertures, and without other forms of helical sculpture, were observed in vessels in Bystropogon canariensis, Hemizygia obermayerae, Hyptis arborea, H. mutabilis, Lavandula dentata, Leucosceptrum canum, Ocimum basilicum, Orthosiphon labiatus, Phyllostegia lantanoides, Salvia canariensis (Car/quist 2592), S. lanceolata, Teucriumflavum, and T. heterophyllum.Longer grooves were observed in vessels of Salvia apiana .
Helical striations were observed on vessel walls of Hemizygia obermayerae(Fig.13).These striations are very likely produced by slight irregularities in cellulosic wall deposition, and are not to be confused with helical thickenings.These striations are not visible with light microscopy, but may occasionally be seen in a scattering of dicotyledon vessels when studied with SEM.
Only simple perforation plates were observed in the woods ofLamiaceae studied, except for a few bars on one plate of Salvia lanceolata.Solereder ( 1908) observed bars on a perforation plate of Cymaria elongata Benth.These bars, however, were at right angles to the usual (radial) orientation ofbars in scalariform perforation plates.

Libriform Fibers
Libriform fiber diameter at widest point is shown in Table 1, column 7.These mean diameters range from 16 to 34 Jtm.The wide diameters occur in Bystropogon p/umosus, Hyptis emoryi, and Lepechinia ganderi.
Libriform fiber wall thickness (Table 1, column 8) averages almost 3 Jtm for the family as a whole.Species in which wall thickness is 4.0 Jtm or more include Hyptis mutabilis, Lepechinia calycina, L. ganderi, Prostanthera aspalanthoides, P. rotundifolia, Rosmarinus officina/is, Salvia canariensis (Carlquist 2592), Teucriumjlavum, Tinnea rhodesiana, and Westringia rosmariniformis.All three species have smooth, hard woods in which the hardness may be attributed to the wall thickness of the libriform fibers.Notably thin walls characterize Hyssopus officina/is (0.4 Jtm) and Ocimum basilicum (0.6 Jtm).In Ocimum basilicum the wall thickness ranges from 0.3 to 2.0 Jtm.The thinner walls characterize the firstformed libriform fibers in this species; libriform fibers become thicker walled as growth proceeds in this annual.Libriform fiber length in Lamiaceae (Table 1, column 9) ranges from a low of 302 JLm (Teucrium heterophyllum) to a high of717 JLm in Hoslundia opposita and 728 JLm in Leucosceptrum canum.These latter two species are small trees.
The length of libriform fibers does seem to vary with plant size.However, a figure worth consideration is the ratio between libriform fiber length and vessel element length (sometimes termed "F/V ratio").For Lamiaceae as a whole, the figure for this ratio is 2.09.A low (1.29) is reached in Lepechinia cardiophylla.High ratios are represented by Leucosceptrum canum (2.73) and Rosmarinus officina/is (3.17).Vessel elements in Rosmarinus officina/is are relatively short.Within certain limits, the ratio of libriform fiber length to vessel element length is an indication of phyletic advancement.Differences within genera or among genera within a family in this ratio probably cannot be interpreted in this way for the most part, but differences among dicotyledon families in this ratio can be significant.
Walls of libriform fibers in Lamiaceae bear minute slitlike pits, rarely much more than 1 JLm long (Fig. 20).Pits are simple, although in Dorystoechas hastata, a very vestigial border was observed on some pits, although not on others, in libriform fibers.Libriform fibers are not lignified in Hemizygia obermayerae and Ocimum basilicum (earlier formed fibers).In Hyssopus officina/is and Phyllostegia lantanoides, the imperforate tracheary elements are supplanted by what I am terming a background of axial parenchyma.

Vascular and Vasicentric Tracheids
In earlier work (Carlquist 1985a(Carlquist , b, 1988a)), the terms vascular tracheid and vasicentric tracheid were defined in a way that I believe corresponds closely to the usage of other authors (e.g., Metcalfe and Chalk 1950) and the intent of the lAW A Committee on Nomenclature (1964).In my usage, vascular tracheids are formed at the end of a growth ring where vessels become progressively narrower, so that some of the last-formed vessels are very narrow, lack perforation plates, and therefore must be termed vascular tracheids.Vasicentric tracheids, on the other hand, characteristically occur around and adjacent to vessels, and they occur earlier than the last few layers of a growth ring (they may occur in diffuse porous woods).Although vasicentric tracheids are best known to wood anatomists in Quercus, where they form wide sheaths around vessels, they occur in many other families (Metcalfe and Chalk 1950;Carlquist 1985aCarlquist , b, 1988b) ) and woods in these families satisfY all of the criteria that have been applied to Quercus.In the instances cited for both vascular and vasicentric tracheids, the woods also contain either fiber-tracheids (with vestigially bordered pits) or libriform fibers (with simple pits).The lAW A Committee on Nomenclature (1964) defines these terms in this fashion.In woods in which tracheids are the only kind of imperforate tracheary element present, true tracheids are said to be present (Carlquist 1985a(Carlquist , 1988a)).
Vascular tracheids, according to the above definitions, are represented by Lepechiniafragrans (Fig. 22, 23, 27) and L. calycina (Fig. 26).Only in the last two or three layers of a growth ring do the vessels lack perforation plates and therefore qualifY as vascular tracheids.Perforation plates in some latewood vessels, but not all, can be seen in Figure 23.
On the other hand, Rosmarinus officina/is (Fig. 25) has vasicentric tracheids intermixed with groups of vessels throughout each year's accumulation of wood.This corresponds to the classic definition of vasicentric tracheids and shows the tendency toward diagonal vessel aggregations that characterize certain species with vasicentric tracheids, usually those in drier habitats (Carlquist 1987).By any definition, Rosmarinus has vasicentric tracheids but not vascular tracheids.
In other woods of Lamiaceae, however, there are transitional situations.For example, in Trichostema lanatum (Fig. 17), narrow growth rings in dry years form only tracheids and narrow vessels, and no libriform fibers.The narrow growth ring between the pairs of arrows in Figure 1 7 may be likened to latewood without any earlywood.The top and bottom third of Figure 1 7 shows wood that contains vessels plus libriform fibers.The difference between libriform fibers and tracheids cannot be readily seen in halftones, but there is a color difference in the preparations, and differences of pitting confirm the significance of the color differences.Thus, in the narrow growth rings of Trichostema, vessels are present and are embedded in tracheids.This can be demonstrated in a longisection (Fig. 18), in which one vessel (left, lighter in tone) is accompanied by tracheids.The term tracheid here can be used more specifically as "vasicentric tracheid" because the vessels are intermixed with the tracheids.
A situation similar to that of Trichostema lanatum is illustrated for Salvia dorrii (Fig. 24).Above the pair of arrows at the top of the photograph is a growth ring that begins with large vessels and that contains libriform fibers.Below the pair of arrows are several short growth rings that consist wholly of vessels plus tracheids that should be termed, because of their cooccurrence with vessels, vasicentric tracheids.Salvia dorrii is a shrub of the Mohave Desert, where there may be a succession of dry years that result in narrow growth rings such as the ones depicted.
All species of Lepechinia have vascular tracheids (Fig. 22,23,26,27) except for L. hastata, in which latewood is not strongly different from earlywood.Lamiaceae other than those noted in the above paragraphs that are like Trichostema lanatum or Salvia dorrii include Dorystoechas hastata, Prostanthera aspalanthoides (Fig. 6), P. rotundifolia, Salvia funerea, and S. mellifera.A few tracheids, which are probably best termed vascular tracheids because they were observed only in latter portions of growth rings, were observed in Hemiandra pungens, Lavandula dentata, Monardella linoides, Ocimum basilicum, Salazaria mexicana, Salvia apiana, and Teucrium jlavum.Should one or more narrow growth rings with few libriform fibers succeed each other in these woods, one could say that vascular tracheids transitional to vasicentric tracheids occur, as in Trichostema lanatum and Salvia dorrii.
Axial parenchyma consisting of nonsubdivided cells was recorded for Salvia dorrii.No axial parenchyma cells were observed in Prostanthera aspalanthoides, P. rotundifolia, and Teucrium heterophyllum.
Wide bands of axial parenchyma occur in a few Lamiaceae.These bands are figured here for Cuminiafernandeziana (Fig. 30,31).Banded parenchyma ofthis kind also occurs in C. eriantha and Leucosceptrum canum.These bands are composed of cells that are subdivided into strands of two or three cells or, less commonly, not subdivided.
Initial parenchyma was recorded in wood of a few Lamiaceae.The earlywood vessels figured for Lepechinia fragrans (Fig. 22) are surrounded by initial parenchyma; libriform fibers are laid down only after the first vessels and their adjacent parenchyma are formed (top of photograph).Initial parenchyma is present, but in smaller quantities, in wood of Lepechinia calycina (Fig. 26).Subdivision of axial parenchyma into strands is illustrated for L. fragrans in Figure 27.Initial parenchyma of this sort also occurs in Lepechiniaganderi andMonardella linoides.A more limited quantity of initial parenchyma-often only a single cell layer-is illustrated for Sa/vis dorrii (Fig. 24).
The entire background of the wood is parenchyma in a few species ofLamiaceae.If one views a transection of Phyllostegia lantanoides wood (Fig. 28), one sees what might be interpreted as thin-walled libriform fibers.In longisection, however, most such cells proved to be strands of thin-walled axial parenchyma (Fig. 29).This is also true of the wood of Hyssopus officina/is.
In Table 1, column 10, a formula for ray histology is given.The species of Lamiaceae differ markedly from each other in ray histology.Lamiaceae can be said basically to have Heterogeneous Type liB rays, using the terminology of Kribs (1935).In this ray type the center of the multiseriate ray is composed of procumbent cells, as shown for Leucosceptrum canum in Figure 33.The margins of the rays contain upright cells, as shown for L. canum in Figure 35.The formula usP corresponds to this type, and is also reported here for Hoslundia opposita, Hyptis arborea, Lepechinia calycina, and Teucrium heterophyllum.One notes that this list includes the woodiest of the Lamiaceae studied.A moderate departure from this type, represented by the formula USP in Table 1, column 10, was recorded in Cuminia eriantha, Dorystoechas hastata, Hemizygia obermayerae, Hyptis mutabilis, Lepechinia cardiophylla, Prostanthera rotundifolia, Rosmarinus officina/is, Salvia apiana, Tinnea rhodesiana, and Trichostema lanatum.A further departure from the typical Heterogeneous Type JIB pattern, in which multiseriate rays have relatively few procumbent cells (USp), was observed in Cuminia fernandeziana, Hyssopus officina/is, Lavandula dentata, Leonotis leonurus, Lepechinia fragrans, L. ganderi, Orthosiphon labiatus, Phyllostegia lantanoides, Plectranthus barbatus, Prost ant hera aspalanthoides, Salvia canariensis, S. funerea, S. lanceolata, S. mellifera, and Westringia rosmariniformis.These are all clearly shrubby species.
Height of multiseriate rays is given in Table 1, column 11.Salazaria mexicana lacks multiseriate rays altogether; in Hyssopus officina/is, rays are mostly uniseriate with a few biseriate rays (Fig. 37); the latter were measured as multiseriate rays.Tall rays are characteristic of some species, such as H emizygia obermayerae (2143 p.m), Hyptis emoryi (2030 p.m), Phyllostegia lantanoides (2307 p.m), and Plectranthus barbatus (2069 p.m). Interesting with regard to these species is that there is no correlation between multiseriate ray height and vessel element length, although one frequently finds such a correlation in dicotyledons at large.Multiseriate ray height also does not correlate with multiseriate ray width.The multiseriate rays of Lepechinia fragrans (Fig. 34) are narrower but taller than those of Leucosceptrum canum (Fig. 35).Multiseriate rays of Lamiaceae as a whole (Table 1, column 12) average 3.8 cells at the widest point.Notably wide rays were recorded for Bystropogon plumosus, Cuminiafernandeziana, Lavandula dentata, Leucosceptrum canum (Fig. 35), Plectranthus barbatus, Salvia apiana, S. canariensis (Carlquist 15952, notably roots) Perforated ray cells were recorded in Phyllostegia lantanoides and Tinnea rhodesiana.Very likely perforated ray cells could be located in other Lamiaceae.Although perforated ray cells are a notable morphological phenomenon, their occurrence has not demonstrated phyletic or systematic significance.They may be more common in species with wide and tall rays than in species with short, narrow ones.

Storying
In several species, a few libriform fibers appeared storied.However, this condition was so vague and inconsistent that no storying can be claimed for the family.

Crystals and Other Cellular Contents
Metcalfe and Chalk ( 19 50) do not report crystals in secondary xylem ofLamiaceae.Prismatic crystals about three times as long as wide were observed most abundantly in Salazaria mexicana (Fig. 38).The only other species observed to have prismatic crystals of this description in ray cells was Plectranthus barbatus, in which crystals are sparser than they are in Salazaria mexicana.
Rhomboidal crystals are somewhat more common in woods of Lamiaceae.They are figured here for Westringia rosmariniformis (Fig. 39), in which they occur commonly in ray cells.Large and small crystals occur intermixed in ray cells of this species.Rhomboidal crystal occurrence of this nature was recorded for ray cells of Leonotis leonurus and Salvia canariensis (all collections).
Starch was observed abundantly in ray cells and septate fibers of Plectranthus barbatus (Fig. 21).Had wood samples for this study been preserved in liquid rather than dried, more instances of starch occurrence in ray cells and septate fibers would likely have been recorded.
Darkly staining deposits of resinlike compounds are not common in Lamiaceae.They were most conspicuous in Rosmarinus officina/is (Fig. 25), in which they fill some vessels as well as other cells.

ECOLOGICAL CONCLUSIONS
Three quantitative features are combined in the ratio termed Mesomorphy (Carlquist 1977): vessel diameter times vessel element length divided by number ofvessels per mm 2 • This ratio has been computed for Lamiaceae (Table 1, column 15).Notably low ratios ( < 15) occur in Lepechinia cardiophylla, L. ganderi, Man-ardella linoides, Poliomintha longiflora, Prostanthera aspalanthoides, P. rotundifolia, Rosmarinus officina/is, Salazaria mexicana, Salvia dorrii, S. funerea, S. mellifera, Teucrium heterophyllum, and Trichostema lanatum.Certainly all of these are species from regions with a prolonged dry season.The six species with the greatest Mesomorphy ratio figures (in descending numerical order) are Hoslundia opposita, Hyptis arborea, Leucosceptrum canum, Phyllostegia lantanoides, Bystropogon plumosus, and Cuminia eriantha.All of these six species are from relatively wet areas, in which either the dry season is not prolonged or is modified by high humidity or occasional condensation from clouds.
The other form of helical sculpture in vessels of Lamiaceae is represented by grooves interconnecting pit apertures ("coalesced pit apertures").These structures occur in Lamiaceae with woods less xeromorphic than those with helical thickenings.
Although one is accustomed to thinking of growth rings as an accommodation to peak flow by virtue of wider earlywood vessels, the adaptation to dry conditions by means of latewood conductive safety (narrow vessels, more numerous per mm 2 ) needs to be stressed, especially in such a family as Lamiaceae, in which many species occur in relatively dry localities.The majority of Lamiaceae have growth rings.Therefore, it is easier to cite the species that lack growth rings.These diffuse porous species are, together with their respective Mesomorphy figures in parentheses: Cuminia.fernandeziana(211), Hemiandrapungens (224), Hemizygia obermayerae (257), Hoslundia oposita (982), Hyptis arborea (800), Leonotis leonurus (208), Lepechinia hastata (53), Leucosceptrum canum (517), Origanum majorana (21), Orthosiphon labiatus (54), Phy/lostegia lantanoides (343), and Plectranthus barbatus (996).Some of these might have growth rings had they not been in cultivation.However, for these species, the mean Mesomorphy ratio figure is 389, well above the family mean (163).The mean Mesomorphy figure for the remaining Lamiaceae (with growth rings indistinct to prominent) is 78.
One can use the Mesomorphy ratio as a key to features other than the quantitative vessel element features directly involved in the ratio, as the paragraphs above indicate.By way of summarizing the findings of the above paragraphs, one can rank the average Mesomorphy figures for the species with those features: growth rings (78), helical thickenings in vessels (47), presence of vascular or vasicentric tracheids ( 4 7), presence of more than three vessels per group, average ( 17).One should note that any degree of growth-ring demarcation from indistinct to marked is recognized in the computation; had only strongly ring-porous conditions been recognized, a lower Mesomorphy figure for species with growth rings would have been obtained.
The above calculations are valid if wood from cultivated specimens is not much more mesomorphic than wood of specimens collected in the wild.To be sure, the cultivated Salvia canariensis stem had a higher Mesomorphy figure (103) than the stem from the natural habitat (56).However, most of the collections with very low Mesomorphy figures are cultivated specimens: Hyssopus officina/is (15), Monardella linoides (16), Ocimum basilicum (26), Origanum majorana (21), Poliomintha longiflora (14), Prostanthera aspalanthoides (9), P. rotundifolia (11), Rosmarinus officina/is (7), Salazaria mexicana (5), Teucrium heterophyllum (9), and Trichostema lanatum (6).Gardens may not be as mesic compared to the wild as one might think, but a more likely explanation is that cultivation for these species does not alter quantitative wood features to a major extent.
In an earlier study (Carlquist 1985a), a correlation was noted between vascular tracheids and drought-deciduous foliage on the one hand, and vasicentric tracheids and evergreen foliage on the other.The hypothesis was offered that if conductive cells other than vascular tracheids cavitated, the cambium would be protected but pathways to leaves would not be protected.In species with vasicentric tracheids, the pathways of vessels to leaves are accompanied by vasicentric tracheids, so that a subsidiary conductive system (vasicentric tracheids) could supply leaves even if the vessels were embolized.In Lamiaceae, this correlation is evident.Rosmarinus, with clear vasicentric tracheids, has evergreen leaves.Lepechinia, with vascular tracheids, tends to have drought-deciduous leaves.Salvia and Trichostema, in which a condition transitional between vascular and vasicentric tra- cheids occurs, have leaves that crisp somewhat during the hot months, but tend to persist.

MORPHOLOGICAL CONCLUSIONS
Vessel elements ofLamiaceae show both grooves interconnecting pit apertures (coalesced pit apertures) and helical thickenings.A condition perhaps intermediate, represented in several Lamiaceae, is the formation of thickenings on either side of a groove (Fig. 11).This was termed "paired bands" in my earlier papers on wood of Asteraceae.Neither of these phenomena is as well recognized as helical thickenings in vessel elements.Attention is called to the three phenomena, each of which is in itself diverse.
Both the diameter and density of vessels of H emiandra pungens are above the means for Lamiaceae as a whole, despite the fact that wider vessel diameter tends to be associated with lowered vessel density.The nature of these features in Hemiandra pungens suggests the wood of a vine (Carlquist 1975), and indeed, the stems of Hemiandra pungens could be likened to those of a prostrate vine.
In this connection, one should note that Hemiandra pungens has unusual cambial activity: cambia formed from rays.Cambial variants of this sort are more common in vines and lianas (Carlquist 1985b).The relatively long stems of Phyllostegia lantanoides also remind one of vine wood structure in terms of their wide vessels (Fig. 7), although the mean vessel diameter in this species is not as great as one finds in most vines.Phyllostegia can be called a sprawling shrub, the stems of which extend through shrubbery and extend outward from the plants on which they climb.The relatively wide vessels of Hoslundia opposita, Hyptis arborea, and Leucosceptrum canum are related to the arborescent habit and relatively moist habitats of these species.
Septate fibers occur in the majority of Lamiaceae.These may be a site for storage of starch, as is observable in Plectranthus barbatus.Septate fibers can form a parenchymalike tissue.Axial parenchyma is not abundant in most dicotyledon woods with septate fibers, and Lamiaceae are no exception.Axial parenchyma was not observed in Prostanthera aspalanthoides, P. rotundifolia, and Teucrium heterophyllum.Because of the paucity of axial parenchyma in dicotyledons with septate or living fibers, the development of imperforate tracheary elements into a parenchymalike tissue may phyletically be accompanied by lessening or even disappearance of axial parenchyma.
The axial parenchyma of Cuminia and Leucosceptrum canum is a good example of fiber dimorphism, a phenomenon discovered in Asteraceae (Carlquist 1958), but now reported in other families (Carlquist 1988a).The parenchyma that results from fiber dimorphism does not represent a modification of preexisting parenchyma, but an innovation of parenchyma from a new source.A phenomenon allied to evolution of parenchyma bands in woods of Cuminia and Leucosceptrum is the conversion of the ground tissue of wood of Hyssopus officina/is and Phyllostegia lantanoides into parenchyma.The earlier-formed imperforate tracheary elements of Ocimum basilicum are nonlignified and like parenchyma, although the later-formed imperforate tracheary elements are typicallibriform fibers.I am unaware of another example like this, where formation of cells producing mechanical strength is deferred.Initial parenchyma occurs in a few Lamiaceae, most