Wood Anatomy of Cassiope (Ericaceae)

Wood anatomical characteristics for 21 taxa of Cassiope are reported. Qualitative features differ somewhat from the general pattern of the Ericaceae. However, considerable deviation is found in the quantitative aspects of the wood features. The stems are very slender with narrow growth rings evident in all but one species. Vessel elements are very small and usually have scalariform perforation plates with several bars. Vessels are extremely abundant in transection. Vascular rays are mostly uniseriate with a few biseriates and all are composed exclusively of erect cells. The imperforate elements are tracheids which are the only conductive elements in the late wood. Calluna-type pith is found in most of the taxa while homogeneous pith is found in two species. The recognition of these in the separate genus Harrimanella is supported. Cassiope plants are often found in moist, protected microenvironments in arctic and alpine areas of extreme cold and desiccating winds. Short growing seasons and low mean temperatures probably contribute in large part to the dwarf stature of the plants, which in tum accommodates the reduced vascular system. The tracheids provide a safe conductive system in the late wood which is less susceptible to loss of function due to air embolisms. The vessel elements with scalariform perforation plates accommodate the presumably slow conductive rates in the earlier part of the growing season. The vessel elements of Cassiope are considerably smaller than most others known to have scalariform perforation plates. The values for vulnerability, an indicator of the degree of vessel redundancy and hence safety, are much lower than any yet calculated for any group. The same is true for figures for mesomorphy, which introduces vessel element length as an independent measure of degree of xeromorphy.


INTRODUCTION
While wood anatomy of trees and many larger shrubs has been studied, little is known about the wood anatomy of small shrubs.Even less is •known about woods of arctic-alpine shrubs which often would be classed as dwarf shrubs.The reduced stature of the plants contributes to the general lack of adequate material for study in wood collections.The species affected by the climatic extremes offer unusually interesting material for the study of ecological wood anatomy.The harsh conditions belie the fact that some of the taxa that occur in these areas are members of groups which would be considered to have un specialized woods.
Cassiope D. Don is a small genus of dwarf, evergreen shrubs in the Ericaceae.It is generally placed in the tribe Cassiopeae Stevens of the Vaccinioideae End!. (Stevens 1971).Cassiope is restricted to the Northern Hemisphere.The plants often occupy moist sites near those exposed to cold desiccating winds and winter snow abrasion.Most often Cassiope forms patches varying in size and density depending upon the species and environmental conditions of the site.Not uncommonly two species may be sympatric.Many of the Asiatic taxa are sparsely represented in herbaria and label data are often scanty.Ranges for most of these taxa are poorly understood.
Few anatomical treatments have included more than a few species in anyone genus.The following general description of the woods of Ericaceae, including the Vacciniaceae, is taken from Metcalfe and Chalk (1950).Vessels extremely small, less than 25 ~m in diameter, to moderately small, 50-100 ~m in diameter, exclusively solitary except for incidental overlapping of tapering end walls.Perforation plates scalariform, simple, or foraminate.Spiral thickenings sometimes present in the vessel elements.Vessel density rarely less than 40 per mm 2 and as high as 400 per mm 2 in Vaccinium.Lateral wall pitting scalariform to opposite.Axial parenchyma scanty to absent.Rays typically of two sizes, the heterogeneous often lack procumbent cells.The imperforate elements tracheids, vasicentric tracheids (Carlquist 1985b), libriform fibers, or septate fibers.

MATERIALS AND METHODS
Liquid-preserved samples were stored in 50% ethyl alcohol prior to further preparation.Dry specimens were boiled to remove air and restore the tissues, then stored in 50% ethyl alcohol.Macerations were prepared using Jeffrey's solution (Johansen 1940) and stained in safranin.Woody stems of small proportions do not lend themselves to standard preparation using a sliding microtome.Softening of the wood samples prior to paraffin embedding and sectioning made this study possible.The stems were softened in a 4% solution of ethylene diamine and subsequently embedded in paraffin according to procedures outlined by Carlquist (1982a).Sections were cut at 15 ~m and stained with safranin.Some were counterstained with fast green.Table 1 lists some pertinent information on the collections used in this study.Table 2 summarizes the quantitative and qualitative data for all samples studied.Means are based on 25 measurements unless otherwise indicated.Multiseriate rays were rare if present at all.Stem and pith diameters (Table 2) were measured with the ocular micrometer.
Cassiope abbreviata Hand.-Maz.and three recently described taxa C. pulvinalis T. Z. Hsu, C. nana T. Z. Hsu, C. argyrotricha T. Z. Hsu were not available at the time for inclusion in this study.Vouchers of my collections are deposited at LAM.
The kindness of the curators ofBM, CAL, CAS, DS, E, KUN, L, LIV, and UC for the loan and use of herbarium specimens is greatly appreciated.The author is deeply indebted to Dr. Sherwin Carlquist for the use of space and materials as well as photographic assistance.
ANATOMICAL DESCRIPTION Woods of Cassiope are generally ring porous (Fig. 1,6,10,15).The growth rings possess vessels in the earlywood, whereas vessels are almost entirely lacking in the latewood.Vessels are variously present after the earliest-formed wood (Fig. 2,7,10,22).Tracheids, the imperforate elements, comprise the bulk of the latewood in all of the taxa (Fig. 15,22).The vessel diameters decrease generally from earlywood to later wood.The average width ofthe growth rings ranges from 34 ~m in Cassiope hypnoides to 105.6 ~m in C. tetragona ssp.tetragona.One species, C. ericoides, lacks growth rings (Fig. 13).Stem and pith diameters are listed in Table 2.
The pith of Cassiope conforms to two different patterns.Most of the taxa have a central pith region composed of thin-walled, irregularly shaped parenchyma   cells surrounded by a cylinder of longitudinally elongate, thick-walled cells (Fig. 1,18,(22)(23).This type of pith has been called the Calluna-type (Watson 1964).The pith of C. hypnoides and C. stelleriana lacks the central region composed of thin-walled irregularly shaped parenchyma cells (Fig. 6,9,(20)(21).This corresponds to the homogeneous type (Watson 1964).All cells of the pith in these two taxa are square to vertically elongate and thick walled.
Vessel elements of Cassiope are thin walled compared to tracheids of the same wood (Fig. 2,7,13,19).Mean vessel diameters range from only JLm in C.     (Calder, Sa vile, Taylor 22825).Transection, large vessels restricted to earlywood; vessels somewhat grouped; some paratracheal parenchyma evident.(Fig. 10 ssp.tetragona.Often some of the bars are forked (Fig. 11) or irregular (Fig. 4).Simple perforation plates were found only in C. mertensiana and C. tetragona (Fig. 5).Figures in Table 2 indicate the percentage of simple perforation plates found among the two species and their sub specific taxa.
Lateral wall pitting of vessel elements is scalariform to opposite (Fig. 11 ,(16)(17).Some pitting approaching the alternate condition was observed in C. mertensiana ssp.gracilis.In some species such as C. myosuroides, vessel elements are so narrow that lateral wall pitting seems to merge with the perforation plate (Fig. 17).The pits are fully bordered (Fig. 16).
Tracheids constitute the imperforate elements in Cassiope.These elements have fully bordered pits and range in size from 179 .urn in C. tetragona ssp.tetragona to 367 .urn in C. macrantha.Tracheid wall thickness was similar for all the specimens studied (Table 2).The ratio of imperforate element length to vessel element length was low, ranging from 1.37 in C. mertensiana ssp.cali/ornica to 1.05 in C. mertensiana ssp.mertensiana.Axial parenchyma was extremely scanty, occurring only as a few isolated cells if present at all (Fig. 7, 14).
The vascular rays of Cassiope are composed exclusively of erect cells.The multi seriate rays are merely biseriate, by virtue of having a single erect cell alongside an otherwise uniseriate ray.Very few multiseriate rays were seen; the numbers upon which the figures are based are given in parentheses in  (Fig. 3,8,12,15,22).The height of the uniseriate rays ranges from 110 .urn in C. tetragona ssp.tetragona to 332 .urn in C. pectinata.

Systematic Conclusions
There have been few quantitative treatments ofthe wood anatomy of members of the Ericaceae.Some notable works include those of Cox (1948a, b).Metcalfe and Chalk (1950) provide a summary of the anatomical characteristics of the Ericaceae, although they separated the Vacciniaceae.The wood of the Ericaceae is considered to be relatively unspecialized, containing vessel elements, generally with scalariform perforation plates, and scalariform to opposite lateral wall pitting (Frost 1931).The woods of Cassiope differ only slightly from the qualitative features described by Metcalfe and Chalk (1950) for the Ericaceae and Vacciniaceae.Imperforate elements are tracheids and no spiral thickenings were observed in the vessels.The major features of deviation are the vascular rays.Multiseriate rays are rare and these are narrow biseriate rays.All of the ray cells are erect.Coville (1901) proposed separation of Cassiope hypnoides and C. stelleriana into a new genus, Harrimanella.He based this, in part, on the presence of alternate, decurrent leaves; terminal, ebracteate flowers; short sepals; pleated corolla with erect lobes; filaments oftwo unequal lengths and enlarged at base; and short stout styles.Not all of the characters proposed by Coville are consistent in H arrimanella, but the two species have been found to have homogeneous pith.Cassiope has opposite leaves; axillary bracteate flowers; long sepals; nonpleated corolla with reflexed lobes; filaments of a single length not enlarged at base; slender elongate cylindric styles; and heterogeneous pith.The heterogeneous pith has been termed the Calluna-type by Watson (1964).This view has been supported by Stevens (1970Stevens ( , 1971)).
Pith structure has been recognized as a useful generic character (Fritsch 1903;Stevens 1971;Watson 1964).The results here do, in fact, show that Cassiope sensu stricto has the Calluna-type pith composed of a central area of thin-walled irregularly shaped to boxy parenchyma cells surrounded by a multiple-celled layer of erect, cylindric, thick-walled parenchyma cells.The two species separated as Harrimanella have their entire pith composed ofthick-walled cells.These results support the recognition of Harrimanella hypnoides (L.) Coville and H. stelleriana (Pall.)Coville.These two species have distinctive homogeneous pith (Fig. 9, 21).
For comparison, however, in this paper both taxa will be listed as species of Cassiope.These two pith types do occur elsewhere in the Ericaceae (Stevens 1971).
There do not seem to be any other subdivisions of Cassiope warranted on the basis of wood anatomical features.

Evolution of Dwarf Habit and Significance to Wood Anatomy
There is general difficulty in separating cause and effect relationships with respect to habit in dwarf shrubs.Most, if not all, species of Cassiope could be considered dwarf shrubs.The tiny, often appressed and abaxially grooved, evergreen leaves would seem adapted to conserve water.The stems are slender, forming a decumbent system on a layer of older larger decumbent branches.Roots form irregularly along many of the stems at the soil interface.Cassiope hypnoides is among the smallest, with its evergreen leafy stems seldom longer than 10 cm.Forsaith (1920) reported several differences between woods from the harsh slopes ofMt.Washington, New Hampshire, and different species from the same genera from lower more protected areas nearby.The most significant of these were the decreased size of vessels and their elimination in the outer portions ofthe growth rings in Betula from the alpine area, and the simplification of the rays in all of the taxa studied.More recently, Baas, Chenglee, Xinying, Kerning, and Yuefen (1984) described some effects of dwarf growth on wood structure of taxa of normally taller stature.They studied conifers as well as hardwoods.Only their data on hardwood taxa will be considered here.Baas et al. (1984) reported a reduction in vessel element  lengths, and diameters, as well as an increase in the number of vessels per mm 2 in natural and artificially dwarfed plants as compared to normal sized plants of the same species from similar provenance.Only those species with complete data from Baas et al. (1984) are presented in Table 3 for comparison.From these data it may be seen that there is some correlation between decreased size of the plant and decreases in vessel element length, vessel diameter, and an increase in the number of vessels per unit area.Baas et al. (1984) attribute most ofthe differences to phenotypic plasticity rather than the selection of favorable genotypes.They consider these effects of dwarf growth the result of stress rather than functional adaptations.Experimental data would be required to substantiate this claim; however, in some cases this is undoubtedly the source of some of the observed anatomical differences between plants of one species of two habit types.
The basis of observed structure in dwarf shrubs and dwarfed specimens of normally larger shrubs or trees is not completely known although some evidence may be discussed here.Coe (1967) noted that elongation of internodes, normal for alpine plants transplanted to lower elevations in Kenya, could be averted by subjecting plants to nighttime low temperatures approximating typical diurnal fluctuations experienced in the native habitat.There is a tendency at low temperatures for photosynthates to be converted to storage products rather than cellulose for cell growth (Tranquillini 1979).Tranquillini cited this as a major factor in the decreased diameter of stems at high altitudes and latitudes.Increasing altitude delays the onset of the growing season which is then shortened, even though growing seasons may end at approximately the same time (Tranquillini 1979).This, coupled with the fact that growth rate decreases with increasing altitude, is responsible for a reduction in shoot increment at high altitudes (Tranquillini 1979).Tranquillini described some basic differences found between the deciduous larch and the evergreen spruce in their response to the delayed start of the growth period at higher altitudes.Larch extends new leaves which photosynthesize for up to two months prior to the period of vigorous shoot extension.The sudden occurrence of killing frosts in the latter part of the season would tend to eliminate the leaves after they had been functional for some period of time and eliminate that year's shoot formation at no loss to the plant.Spruce rapidly completes shoot growth in the early summer, which is followed by the completion of needle growth.Sudden killing frosts in the latter part of the season would kill off the newer unhardened leaves but this would not be detrimental to growth since photosynthetic activity is carried on throughout much of the year.Roots along the branches where they contact the substrate may provide a water source close to areas of active growth early in the growing season.They may also alleviate the need for large stems and favor numerous smaller stems instead.
The wood anatomical characteristics of dwarfed plants of normally full sized species are not, of course, comparable to those of normally dwarf species.This is evident by noting the figures given in Table 3.The woods of Cassiope, Misodendron, and those arctic plants studied by Miller (1975) would qualify as normally dwarf shrubs.These three groups have vessels that are narrower, shorter, and much more abundant than those in the group studied by Baas et al. (1984).Data on rock-crevice species are also given in Table 3.These compare in general to the figures given for desert shrubs.

Ecological Discussion and Conclusions
Some workers have attributed many trends in wood anatomical features to changes in latitude (Baas 1973;Graaff and Baas 1974).Generally these include shorter, narrower vessel elements, lower rays, stronger expression of growth rings, and higher vessel frequency at higher latitudes.Mesic and xeric habitats at the same latitude would not, however, be expected to support species with the same anatomical features.This was found to be the case by Forsaith (1920).He noted a decrease in vessel and ray sizes in plants from the alpine summit ofMt.Washington, New Hampshire, as compared to those from lower elevations on the same mountain.Vliet (1979) showed a decrease in vessel element length in Terminalia to be associated with decreased water availability.Dickison and Phend (1985) also note that mesic-xeric trends can override latitudinal trends in Styrax .Data presented in Table 2 show no consistent correlations with latitude.
The general ecological conditions under which Cassiope grows are xeric.Microenvironmental conditions are, however, often more moderate.Cassiope hypnoides, for example, at Mt. Washington, New Hampshire, occurs in the Snowbank Community protected from the cold, abrasive, high-velocity winds ofthe summit (Hadley and Bliss 1964).The mean summer temperatures are about 12 C, and the highest wind velocity recorded was 231 mph.Even though the woods of Cassiope are relatively unspecialized, they reflect some remarkable adaptations to their harsh arctic and alpine habitats.These are described in the following paragraphs.It is important to remember that an entire series of xerophytic adaptations occurs in these plants, especially in their leaves.All of the species are evergreen.The plants may experience severe low temperatures at any time of the year.During winter these plants are often covered to a considerable depth by snow.Under the snow, however, no water is lost due to transpiration in the saturated atmosphere (Tranquillini 1979).
Growth rings in high altitude or latitude species probably represent annual rings due to the brevity and abrupt termination of the growing season.The growth rings of Cassiope may be classified as Type V (Carlquist 1980).This type of growth ring combines greater flow rates in the early part ofthe growing season with greater safety in the latter part of the season.Under severe water stress from freezing, tracheids are less susceptible to incapacitation from air embolisms than are vessel elements.Air embolisms may be localized and reabsorbed more readily in tracheids than in vessels.Type V growth rings here may accommodate moderate flow rates associated with a primitive type of wood, one having tracheids as the imperforate elements and few vessels after earlywood vessels and microphylly as suggested by Carlquist (1980).This is possible, in part, because of the xeromorphic leaf modifications in most of the species of Cassiope.The absence of growth rings in C. ericoides may indicate a constant low availability of water coupled with the apparent xeromorphic leaf adaptations (Carlquist 1978).Persistence of relatively large vessels after the earliest wood in C. redowskii (Fig. 10) may reflect a habitat where the growing season is longer and water more abundant, coupled with a harsh cold season.
The vessels of Cassiope are all very narrow as may be seen from Table 2. Vessels with narrow diameters are less suceptible to incapacitation from air embolisms than are wide vessels (Zimmermann and Brown 1971).Carlquist (1975) cites short, narrow vessel elements as indicative ofxeromorphy in woods.The narrow vessels of Cassiope would be better able to remain functional under severe water stress than wide vessels.The number of vessels per group is low in Cassiope and for most taxa the vessels are effectively solitary.This is usually the case for woods in which tracheids represent a subsidiary conducting system (Carlquist 1984).Figures for the mean numbers of vessels per group are given in Table 2. Numbers just above 1.0 indicate that solitary vessels predominate but that some grouped vessels are also present.The explanation for figures higher than 1.0 may lie in two factors.The stems of Cassiope are very narrow and the growth rings are very narrow so that vessels would be more likely to be grouped incidentally than in plants with wider stems and growth rings.There does not seem to be such a correlation from the data in Table 2.A second possibility seems more likely.Vessels occur mostly in the earlywood while the tracheids occur to the exclusion of the vessels in the latewood.The occasional grouping of vessels in the earlywood may possibly increase conductivity.Solitary vessels are usually interspersed among tracheids in the earlywood but are effectively absent from the latewood.
The abundance of vessels in the woods of Cassiope reflects the safety emphasized by Carlquist (1977a) in his index of vulnerability.Baas et al. (1984) attributed this feature as well as others found in dwarf trees to stress.Aloni and Zimmermann (1983) ascribe increases in vessel diameter and decreases in vessel density to decreasing auxin concentration.This they could control by the exogenous application of lanolin containing naphthaleneacetic acid.Small vessel diameters and high vessel densities in Cassiope could be the result of a decrease in auxin concentration, due to the late development of new leaves each season (Tranquillini 1979).Graaffand Baas (1974), Oever, Baas, and Zandee (1981), and Dickison and Phend (1985) have attributed increases in vessel frequency to increasing latitude.This is obviously not the case in Cassiope.
Perforation plates in Cassiope are predominately scalariform, although simple perforation plates are found mixed among vessels in two species.Scalariform perforation plates are the primitive condition (Frost 1930;Carlquist 1975).Carlquist (1975) stated that groups of dicotyledons with scalariform perforation plates seem to be those with an unbroken history of occupation of mesic habitats.Zimmermann (1978) claimed that phylogenetic retention of scalariform perforation plates facilitated the breakup of air bubbles formed as a result of thawing of woods in the spring.The vessels of Cassiope have presumably low conductivity rates since all of the plants have tiny xeromorphic leaves, are covered by winter snows, and have tracheids to perform conductive functions in the latter part of the growing season.Scalariform perforation plates are not a selective disadvantage under these conditions as noted by Carlquist (1980).The evergreen Cassiope may likewise experience a decreased demand for water in the spring as leaves develop later in the season as described for spruce by Tranquillini (1979).The decreased demand for rapid conduction of water would acommodate the retention of scalariform perforation plates in Cassiope.Salix herbacea and S. glauca both have simple perforation plates typical of this deciduous genus (Miller 1975).This observation coupled with the low conductivity rates noted above do not support Zimmermann's theory as to the reasons for phylogenetic retention of scalariform perforation plates.There is a great and immediate demand for water in the early spring to facilitate expansion of new leaves.This would be accentuated by the delayed onset of the growth period due to the increased latitude as noted for altitude by Tranquillini (1979).Fortunately, spring in these regions is also the season of greatest abundance of water.Simple perforation plates accommodate this situation.However, this is not the situation found in the evergreen genus Cassiope.In fact the only occurrences of simple perforation plates are in taxa from more mesic environments and may have evolved in response to an earlier seasonal need and availability of water rather than a shorter growth season.Perforation plate morphology is simplified with increasing xeromorphy according to Carlquist (1975).Bissing (1982) noted a lower stability of the ratio of simple perforation plates in plate-dimorphic woods correlated with decreasing moisture availability.Bissing (1976) noted in the plate-dimorphic woods he studied, Myrica cali/ornica Cham.& Schlecht., Rhododendron occidentale (T.& G.) Gray, and Platanus racemosa Nutt., that the scalariform perforation plates were found principally in the latewood.The same positioning was found in the plate-dimorphic taxa of Cassiope.These findings would accommodate consideration of the presence of scalariform perforation plates in narrower elements in the latewood as a xerom orphic adaptation.Carlquist (1975) delineated the probable minimal vessel element length and diameter for vessels with scalariform perforation plates.The data for Cassiope in Table 2 greatly decrease the vessel element length and diameter possible for elements with scalariform perforation plates.Scalariform perforation plates were associated with vessel elements at least 30 ~m in diameter and at least 600 ~m long by Carlquist (1975).Frost (1930) gave the average vessel element length 0.41 mm as indicative of vessels with simple transverse perforation plates.Vessel elements with entirely scalariform plates were found on elements whose average length was 1.09 mm.The mean vessel element lengths for all taxa of Cassiope with scalariform perforation plates are 0.328 mm or less.The bars on the perforation plates of Cassiope are thin, although on some of the perforation plates with few bars, a few bars are thicker.The success of plants with scalariform perforation plates in the harsh arctic-alpine habitats may be due in part to the fact that water is relatively plentiful early in the growing season when the vessels are the functional conductive system.Later in the season tracheids are the only functional conductive elements in Cassiope.This possibility is generally supported by Miller (1975).His data on nine dicotyledons show the presence of scalariform perforation plates to be correlated with the occurrence of what he called fibertracheids in six of seven taxa having scalariform perforation plates.The libriform fibers he noted for Betula are probably in error (Metcalfe and Chalk 1950).The presence oftracheids as a backup conductive system may relieve selective pressure for simplification of the scalariform perforation plate.The only two plants in Miller's (1975) study with simple perforation plates are Salix glauca L. and S. herbacea L. Both are deciduous, and both have libriform fibers as the imperforate elements.There is no backup conductive system in the form of tracheids which requires some sacrifice, in this case the leaves.The lack of leaves in the early spring, in turn, requires large vessels in the earlywood to facilitate rapid growth.In fact, Salix glauca was noted as having vessels of greater diameter than all other arctic species he studied except Vaccinium uliginosum L. ssp.microphyllum Lange.
Scalariform to opposite lateral wall pitting on the vessel elements predominates in Cassiope.Frost (1931) associated this type oflateral wall pitting with elements 1.13 mm long, on the average.Vessel elements of Cassiope are 0.328 mm or less in length.One would expect all lateral wall pitting to be alternate according to the categories delineated by Frost (1931).Opposite lateral wall pitting predominates in Cassiope.A condition approaching alternate occurs rarely and characterizes taxa from areas which are presumably more mesic, and/ or have a longer growing season.
The presence of helical thickenings has been considered to be possibly related to increasing xeromorphy (Baas 1973;Carlquist 1975Carlquist , 1978)).Carlquist (1975) noted previous citations of correlations with latitude.Cassiope lacks helical thickenings and consequently is not accommodated by either of the above explanations.The elements in Cassiope may be too narrow to have helical thickenings or they may not have evolved in this phyllad.There is no correlation to the occurrence of frosts as noted for Staphyleaceae (Carlquist and Hoekman 1985a).
Tracheids provide greater safety under conditions of high water stress due to lack of water or due to freezing.The numerous narrow vessel elements in the earlywood are sufficient for conducting water available in greater quantity in the early part of the season.The conductive function is taken over by the tracheids in the latewood.Water is less abundant in the latter part ofthe season due in part to rapid runoff and evaporation in the dry atmosphere.The retention oftracheids in Cassiope is undoubtedly an ancestral condition.
The ratio of tracheid length to vessel element length is quite low for Cassiope.Comparison of figures in Table 3 shows Bruniaceae to be similar in this regard as are the rock-crevice species and those taxa studied by Miller (1975).These data generally agree with comments by Carlquist (1986) for woods in which the imperforate elements are tracheids.Carlquist and Hoekman (1985b) show that vessel elements longer than might be expected in their "alpine" category are associated with the presence of tracheids.Misodendron differs considerably because the figure for imperforate element length is based on the lengths of the libriform fibers .Misodendron lacks a supplementary conductive system in the form of tracheids.One can readily appreciate the release from selective pressure to form shorter though fewer vessel elements with simple perforation plates afforded by the presence of tracheids by comparing figures for Cassiope and Misodendron in Table 3. Misodendron lacks tracheids so the vessel elements, which must sustain the plant throughout the season, are much shorter, although not much wider than those of Cassiope.Presumably, the shorter vessel elements are better able to withstand the water stress.
The rare occurrence or absence of axial parenchyma is not surprising in Cassiope.It is generally considered to be a storage site for photosynthates.There is little storage required in these evergreen dwarf plants.Axial parenchyma is scanty or absent in the Ericaceae as a whole (Metcalfe and Chalk 1950).
The predominance of erect-celled uniseriate rays is one of the more striking features of the woods of Cassiope.Biseriate rays are rare and are evidenced only by the presence of single cells along an otherwise uniseriate ray.Kribs (1935) described the structure of his primitive ray type (Heterogeneous I).In this type the uniseriate rays are high, numerous, and composed of vertically elongate cells.The multi seriate rays in this type are composed of interior portions with radially or vertically elongate cells with elongate uniseriate wings.Barghoorn (1941 a) noted a phylogenetic trend in elimination of the multiseriate rays.He further agreed with Kribs that the exclusive presence of uniseriate rays was highly specialized.Barghoorn (1941a) stated that many species of highly modified shrubs and semishrubs possess exclusively uniseriate, vertically elongate rays.Uniseriate rays have been noted in the early secondary xylem of Illicium (Bailey and Nast 1948;Carlquist 1982c), Sarcococca (Carlquist 1982b) and Bursera (Barghoorn 1941a), although those in Bursera are heterogeneous.There is no apparent correlation between stem diameter and multi seriate rays (Table 2).Forsaith (1920) found a preponderance ofuniseriate rays among alpine plants from Mt. Washington, New Hampshire, and related this to the lack of nutritive substances for storage.Carlquist, Eckhart, and Michener (1984) note exclusively erect cells in the rays of smaller stems while procumbent cells predominate in the larger stems in Acanthogilia gloriosa (Bde.)Day & Moran, a subshrub of the Polemoniaceae.These authors noted the prevalence of exclusively erect ray cells in the Polemoniaceae.The authors consider this to indicate a potential derivation of Acanthogilia from an herbaceous group in the Polemoniaceae.Carlquist et al. (1984) point out that only minor modification is required to convert ray areas to zones of fibers in woods with exclusively erect ray cells and short cambial initials.This is not the case in Cassiope since the Ericaceae are a primarily woody group.Barghoorn ( 1941 b) noted that a reduction in rays by suppression of ray initials was the condition found chiefly in dwarf plants or plants growing in unfavorable environments.He illustrated Corema conradii Torr.(Empetraceae), Staavia glutinosa (L.) Thbg. (Bruniaceae), and Tetratheca ciliata Lindl.(Tremandraceae) as all having exclusively uniseriate rays composed of erect cells.Carlquist (1976) suggested that erect-celled uniseriate rays represented a mechanism for vertical transport in small stems, not over 5 cm in diameter, in Roridula.
The very slender stems of Cassiope have little requirement for storage or lateral transport of materials, so reduction in the ray tissue is not at all unexpected.Vertical transport is a more serious constant need.Uniseriate, erect-celled rays probably function quite well in this regard with the similar safety built in that would be found in the tracheids.Carlquist (1977a) noted the value of redundancy of vessels in xeromorphic woods ofTremandraceae.Where vessels are numerous and narrow, air embolisms formed under conditions of water stress or freezing would have a lesser overall effect on the conductive capacity of the wood.Woods with fewer, wider vessels would be at greater risk of severe incapacitation under similar water stress.In the former instance there are more vessels, so the loss of conductive capacity in some has a lesser effect.Carlquist (1977 a) formulated a ratio called vulnerability, dividing mean vessel diameter in J.l.m by number of vessel elements per mm 2 • He considered species with ratios below 0.30 to have exceptionally numerous narrow vessels.This indicates low vulnerability to incapacitation from air embolisms and consequently high xeromorphy.Carlquist (1977a) also formulated another index, using the independently controlled vessel element length, by multiplying the vulnerability index by the mean vessel element length.This index he termed mesomorphy (Carlquist 1977 a).In his interpretation of the values for these two indices Carlquist (1977 c) considered vulnerability values below 1.0 to be indicative of a high degree of xeromorphy and a figure around 3.0 to mark the lower limit for true mesomorphy.Mesomorphy values below 30.0 probably indicate true xeromorphy.Low mesomorphy values have been attributed to the occurrence of frosts in Illicium (Carlquist 1982c).The length of vessel elements is dictated by the length of the fusiform cambial initials and consequently not readily modified compared to the diameter of the vessel elements, which often differ across a single annual growth ring.This indicates that the mesomorphy index is a more conservative indicator of basic mesomorphy in a group and that the vulnerability index is a sensitive indicator of xeromorphic structure.Carlquist (1977 b) provided some very interesting data regarding the floristic distributions of wood anatomical features for which he calculated values for vulnerability and mesomorphy.Values for vulnerability and mesomorphy for all specimens of Cassiope examined for this study reflect the considerable degree of xeromorphy of the woods (Table 2).The occurrence of xeromorphic adaptations in the woods of Cassiope is not surprising considering the harsh environmental conditions under which the plants exist.The plants occupy micro environments in many cases that are close to the surface and have temperatures higher than ambient air temperatures (Bliss 1962).Nearby sites are often exposed to highvelocity, cold, dry winds.The habitats are subject to abrupt cessation of growing season and to drought caused by freezing even on a diurnal basis.Water is available slowly in spring at shallow depths.The microphyllous, overlapping, appressed nature of the leaves, the occurrence of stomata often in abaxial grooves, and low appressed habit of most of the species reflect the xeric conditions under which these plants exist.The abundant, narrow vessel elements, reduced rays, and particularly the exclusive occurrence oftracheids in the latewood of Cassiope all point to extreme xeromorphy in a primitive wood type.
Comparison of the values for vulnerability and mesomorphy for all of the material reflects extreme xeromorphy in Cassiope.The figures for vulnerability are among the lowest known to date as are those for mesomorphy.If one compares these figures with those of some of the groups listed in Table 3, Cassiope is the most xeromorphic group of woods or wood florula.One of the most similar groups, as far as the extreme xeromorphy of the woods, is Misodendron (Carlquist 1985a).Misodendron, a group of stem parasites with simple perforation plates, may be so adapted to maintain sufficiently high transpiration rates to obtain nitrogen from the xylem of the host as was suggested for the mistletoes (Bloom, Chapin, and Mooney 1985).The data from taxa studied by Miller (1975) are summarized in Table 3.His data would have lower vulnerability and mesomorphy values if the data for the deciduous Salix glauca were deleted.Figures extracted from Miller (1975) for the Ericaceae result in a lower vulnerability and a mesomorphy index that is cut nearly in half.The effect of dwarf habit on woody plants is apparent from data in Table 3 extracted from Baas et al. (1984).There is a decrease in vessel diameter and length of vessel elements as well as an increase in vessels per mm 2 • However, the figures for vulnerability and mesomorphy are indicative of more mesomorphic woods.This is not unexpected since the plants studied by Baas et al. (1984) were not taxa evolutionarily adapted to a dwarf habit but rather dwarf specimens of normally larger mesomorphic taxa from Japan.The remaining figures in Table 3 are derived from normally small plants, from their native habitats.Figures for coastal shrubs of Western Australia are included for comparison as are those for desert shrubs (Carlquist 1977 b).
Data for four rock-crevice species from relatively primitive Bruniaceae and Penaeaceae are included in Table 3 for comparison.Additionally, members of both of these families have tracheids as the imperforate element type which makes figures can be predictive of vegetative adaptations if, for instance, the mesomorphy index is higher than the vulnerability index otherwise indicates (Carlquist and Hoekman 1985a).Oever et al. (1981) presented data on relative conductivity (C = radius of vessel to the fourth power times vessel frequency) for Symplocos.
They expected to find relationships between vessel diameter and frequency that would result in equally efficient water transport systems for Symplocos throughout its range.This is not possible since the taxa have different foliage systems, different habits, and undoubtedly different microenvironmental requirements.Figures for conductance are included for information in Table 2.They are based on the dimensions as applied to uniform unobstructed tubes which is not the case in vessel elements.There is also no consideration given to liabilities of woods adapted for rapid conductive capacity during periods of water stress.

Table 2 .
Continued.to 25 ~m in C. mertensiana ssp.mertensiana.The length of vessel elements ranges from 148 ~m in C. tetragona ssp.tetragona to 328 ~m in C. macrantha.Transections of woods show that most species have less than two vessels per group.Cassiope macrantha, C. lycopodioides ssp.lycopodioides, and C. lycopodioides ssp.cristapilosa are the only taxa with more than two vessels per group(Fig.14,18,23).No spiral thickenings were observed in the vessel elements.Scalariform perforation plates predominate in the genus (Table2).The mean number of bars per perforation plate is 17 in C. mairei and only 3.1 in C. dendrotricha Kaplin 3094).Transection, no growth rings evident; vessels thin walled compared to tracheids; pith toward bottom.-14.C. lycopodioides ssp.cristapilosa

Table 2 .
The multiseriate rays range in height from 121 .urn in C. tetragona ssp.tetragona to 510 .urn in C. jastigiata.The width of the multi seriate rays is 11 .urn in C. jastigiata and ranges up to 25 .urn in C. lycopodioides ssp.cristapilosa.Uniseriate rays predominate in Cassiope

Table 3 .
Vaccinium uliginosum L. ssp.microphyllum Lange, Dryas integrifolia M. Vahl, Salix glauca L., and S. herbacea L. Those for which Miller (1975) gave complete data are presented in All of the taxa examined by Miller (1975) had predominately uniseriate rays except Dryas integrifolia.Here again is an indication of the decrease in ray complexity in dwarf shrubs.Only Empetrum and Cassiopewere reported by Miller to have rays composed exclusively of erect cells.