A survey of vessel dimensions in stems of tropical lianas and other growth forms

Summary Vessel dimensions (total diameter and length) were determined in tropical and subtropical plants of different growth forms with an emphasis upon lianas (woody vines). The paint infusion and compressed air methods were used on 38 species from 26 genera and 16 families in the most extensive survey of vessel length made to date. Within most stems there was a skewed frequency distribution of vessel lengths and diameter, with many short and narrow vessels and few long and wide ones. The longest vessel found (7.73 m) was in a stem of the liana (woody vine) Pithecoctenium crucigerum. Mean vessel length for 33 species of lianas was 0.38 m, average maximum length was 1.45 m. There was a statistically significant inter-species correlation between maximum vessel length and maximum vessel diameter. Among liana stems and among tree+shrub stems there were statistically significant correlations between stem xylem diameter and vessel dimensions. Lianas with different adaptations for climbing (tendril climbers, twiners, scramblers) were similar in their vessel dimensions except that scramblers tended to have shorter (but not narrower) vessels. Within one genus, Bauhinia, tendril climbing species had greater maximum vessel lengths and diameters than tree and shrub species. The few long and wide vessels of lianas are thought to hydraulically compensate for their narrow stem diameters. The many narrow and short vessels, which are present in the same liana stems, may provide a high resistance auxiliary transport system.     

Vessel dimensions in liana and tree species of Gnetum (Gnetales)

Maximum vessel diameters were examined in the secondary xylem of stems of Gnetum of various sizes. One tree (G. gnemon) and 13 liana species were compared. In three species, vessel length distributions were determined by the latex paint method, and showed many short and fewer long vessels. Latex and compressed air methods, used to find the maximum vessel lengths, showed that maximum vessel lengths were similar for three species of Gnetum. In old stems, mean and maximum vessel diameters tended to be greater in lianas than in the tree species. The skewed distribution of vessel lengths and the trend of wider vessels in lianas as compared to trees were similar to those distributions and trends described previously for angiosperms. In random samples of macerated wood of three species, simple perforation plates were most common in vessel members of all species. Foraminate and modified foraminate perforations were less frequent. Average diameter of vessel members with either foraminate or modified foraminate perforations was less than for those with simple perforations. The resemblance of Gnetum vessels to those of angiosperm trees and vines is most likely a case of convergent evolution (homoplasy) in xylem characteristics.     

TECHNIQUES FOR MEASURING VESSEL LENGTHS AND DIAMETERS IN STEMS OF WOODY PLANTS

Results were compared between the latex paint and compressed air methods for determining total vessel lengths, and between the sectioning and maceration methods for determining vessel diameters. The minimum, mean, median, and maximum vessel diameters were less with the sectioning method than with the maceration technique. Vessel diameter distributions were always nonnormal and had roughly similar patterns with the two techniques, but were statistically different from one another. In all six species where the paint and air methods for determining vessel length were compared, both methods showed a similar skewed vessel length distribution, with many short vessels and few long ones. Although there was no consistent pattern to the difference in results with these two methods, the vessel length frequency distributions were statistically different from one another. With the paint method, many vessels, especially many of the narrowest ones, were not paint-filled at the paint infusion port. The air method utilized the paint method, in part, and, in addition, is based upon the incorrect assumption that all vessels in the stem are the same diameter. Both techniques tended to exclude vessel lengths of the narrowest vessels. However, the narrow vessels, although numerous, contributed an insignificant amount to the total theoretical hydraulic conductance in stems.     

A comparison of two methods for measuring vessel length in woody plants

Vessel lengths are important to plant hydraulic studies, but are not often reported because of the time required to obtain measurements. This paper compares the fast dynamic method (air injection method) with the slower but traditional static method (rubber injection method). Our hypothesis was that the dynamic method should yield a larger mean vessel length than the static method. Vessel length was measured by both methods in current year stems of Acer, Populus, Vitis and Quercus representing short‐ to long‐vessel species. The hypothesis was verified. The reason for the consistently larger values of vessel length is because the dynamic method measures air flow rates in cut open vessels. The Hagen–Poiseuille law predicts that the air flow rate should depend on the product of number of cut open vessels times the fourth power of vessel diameter. An argument is advanced that the dynamic method is more appropriate because it measures the length of the vessels that contribute most to hydraulic flow. If all vessels had the same vessel length distribution regardless of diameter, then both methods should yield the same average length. This supports the hypothesis that large‐diameter vessels might be longer than short‐diameter vessels in most species.     

Xylem of rattans: vessel dimensions in climbing palms

We examined 11 species in four genera of rattans (Calamus, Daemonorops, Korthalsia, Plectocomia) growing in their native rainforest habitat in Singapore. Using aqueous safranin dye, we found that >95% of all vascular bundles at the base of a mature stem were functioning to transport water. We determined the frequency of vessel lengths in the long stems of these climbing palms by infiltration with dilute latex paint. Separate length distributions were made for metaxylem and protoxylem vessels; in both, there were many short and a few long vessels. The longest protoxylem vessels ranged from 7.5 to 62 cm in length, but one stem had an exceptional protoxylem vessel measuring 3.0 m. Maximum metaxylem vessel diameters were positively correlated to maximum vessel lengths in these species. The longest metaxylem vessel was found in K. rigida and was 3.96 m in length and was constructed from ～1200 vessel elements (cells). The widest vessel in that same stem was 532 μm in diameter. Long, wide vessels decrease resistance and increase water transport efficiency. In addition, we suggest that wide metaxylem vessels may have an important function in water storage.     

Wood anatomy of Gentianaceae, tribe Helieae, in relation to ecology, habit, systematics, and sample diameter

Twenty collections representing one species each ofSymbolanthus andTachia, and 17 species ofMacrocarpaea were studied by means of light microscopy and scanning electron microscopy (SEM). Wood details show that the three genera form a coherent group;Tachia differs from the others in only a few minor characters. Because the species studied form a natural group, wood variations within Helieae offer the basis for correlations and interpretations with respect to habit and ecology. Diameter of stems studied proves to be an important variable that must be taken into account. Correlations with stem diameter include wider vessels in outer wood of wider samples. This would correspond to deeper penetration of reliable water tables by roots of helioid trees or large shrubs. Ray height decreases with increase in stem diameter, an indication of paedomorphosis. Rays of all species are paedomorphic in histology by virtue of relative paucity or even absence of procumbent cells in multiseriate rays. Pseusoscalariform lateral wall pitting of vessels is also a feature characteristic of paedomorphosis. The assemblage of paedomorphic features correlates well with the conclusion, reached by authors who used cladistic methods, that Gentianaceae other than Gentianeae are derived from suffrutescent prennials. The Mesomorphy Ratio, which incorporates three vessel features, correlates with leaf length and with stem diameter. All Helieae are mesophytic, but to various degrees. Septate fiber-tracheids, where present, are typically near vessels and form a substitute for or an addendum to vasicentric axial parenchyma as a mechanism for photosynthate storage. Vestured pits occur on lateral wall pits of vessels of all Helieae, but not on the fibertracheids. Vestured pits show diversity withinMacrocarpaea, a feature of possible systematic significance.     

RELATIONSHIP AMONG VESSEL DIAMETER,VESSEL FREQUENCY,AND SPACING OF PARENCHYMA BANDS IN WOOD OF CARYA TOMENTOSA NUTT., MOCKERNUT HICKORY

Vessel-lumen diameter, vessel frequency, and spacing of concentric, tangential bands of parenchyma cells follow well defined trends across annual growth rings of Carya tomentosa Nutt., mockernut hickory. These cross-ring cellular changes were correlated with each other to a considerable degree but not tightly linked. Vessel-lumen diameter decreased in a strongly linear fashion, as a function of distance from the inner ring boundary. Distance between tangential, concentric bands of axial wood parenchyma cells also decreased across rings, but followed a less strongly linear trend. Vessel frequency decreased abruptly after the large-pore zone and then increased to the outer ring boundary. Radial growth rate, as indicated by ring width, strongly influenced these trends. For all three cellular characteristics, the cross-ring trend was abrupt in narrow rings and more gradual in wide rings. An anomalous ring of Carya tomentosa, and three similar rings of Carya glabra (Mill.) Sweet, showed a decrease in interband distance and then an abrupt increase followed by another decrease to the outer ring boundary, whereas vessel diameter remained small and uniform after an initial decrease. The three cellular characteristics were concluded to be under separate physiological controls, although growth rate seems to exert a considerable, parallel influence on them.     

WOOD ANATOMY OF ILLICIUM (ILLICIACEAE): PHYLOGENETIC,ECOLOGICAL, AND FUNCTIONAL INTERPRETATIONS

Quantitative and qualitative features are reported for 23 collections of ten species of Illicium, sole genus of Illiciaceae. Vessel elements are long, thin-walled, and angular; perforation plates are scalariform and range from long to moderately long; vessel-ray pitting is scalariform to opposite. Tracheids bear fully bordered pits. Axial parenchyma is sparse, abaxial to vessels with some diffuse cells also present. Rays are both multiseriate and uniseriate; the former are lacking near the pith in some species. No erect sheathing cells are present on multiseriate rays, and procumbent cells become more abundant as stems increase in size. Four species from montane subequatorial highlands lack growth rings and helical sculpture in vessels; the remainder of Illicium species have these features. Notably narrow vessels and large numbers of vessels per sq. mm characterize the temperate species from northern limits for the genus, I. anisatum, I. floridanum, and I. parviflorum. Greater vessel density offers redundancy and greater safety and is correlated with greater frost and possibly greater fluctuation in moisture availability. These two ecological features are probably also related to the narrow vessel diameter, which may retard entry of air embolisms (lowered air entry values) in accord with the physiological considerations of Slatyer. Reduction of number of bars per perforation plate within Illicium is also correlated with the more northerly climates. An additional hypothesis for evolution of shorter vessel elements in dicotyledons is introduced: if, as claimed by Slatyer, air embolisms in vessels tend to stop at ends of vessel elements and thus do not disable entire vessels, shorter vessel elements would maximally localize air embolisms. Presence of helical sculpture and other forms of relief within vessels has been difficult to explain in conifers and dicotyledons; such features might help resist cavitations by increasing adhesion of water molecules to cell walls (hydration).     

Polar auxin transport is implicated in vessel differentiation and spatial patterning during secondary growth in Populus

Premise of the Study

Dimensions and spatial distribution of vessels are critically important features of woody stems, allowing for adaptation to different environments through their effects on hydraulic efficiency and vulnerability to embolism. Although our understanding of vessel development is poor, basipetal transport of auxin through the cambial zone may play an important role.

Methods

Stems of Populus tremula ×alba were treated with the auxin transport inhibitor N‐1‐naphthylphthalamic acid (NPA) in a longitudinal strip along the length of the lower stem. Vessel lumen diameter, circularity, and length; xylem growth; tension wood area; and hydraulic conductivity before and after a high pressure flush were determined on both NPA‐treated and control plants.

Key Results

NPA‐treated stems formed aberrant vessels that were short, small in diameter, highly clustered, and angular in cross section, whereas xylem formed on the untreated side of the stem contained typical vessels that were similar to those of controls. NPA‐treated stems had reduced specific conductivity relative to controls, but this difference was eliminated by the high‐pressure flush. The control treatment (lanolin + dimethyl sulfoxide) reduced xylem growth and increased tension wood formation, but never produced the aberrant vessel patterning seen in NPA‐treated stems.

Conclusions

These results are consistent with a model of vessel development in which basipetal polar auxin transport through the xylem‐side cambial derivatives is required for proper expansion and patterning of vessels and demonstrate that reduced auxin transport can produce stems with altered stem hydraulic properties.     

How weak twining lianas adapt to competition with host tree trunks: Case of Merremia boisiana

Fierce competition exists between most stem‐twining lianas and the trunks of host trees. However, Merremia boisiana, a vigorous invasive twining liana, never strangles the host tree. Here, we investigated how M. boisiana stems adjust their twining growth to avoid intense competition with host trees, and how hydraulic conductivity is maintained for rapid asexual reproduction. We evaluated the effects of competition on twining M. boisiana stems (E_m) and host tree trunks (E_h), compared differences in secondary growth between twining and creeping M. boisiana stems, calculated the total number of vessels (N_t), vessel density (Vmm⁻²), average vessel diameter (VD_ave), and percentage of vessels wider than 300 μm in diameter (P₃₀₀) in the secondary xylem, and traced how these parameters change with increasing cross‐sectional area of stem (SA). The results showed that twining M. boisiana stems were competitively weaker, and mean E_m (14.3%) was 21 times greater than that of E_h (0.7%). Secondary growth along the normal direction of the contact surface was significantly inhibited in stems twining on host trees. The lateral secondary growth of these stems was active, forming secondary vascular rings and/or arcs with abundant large vessels. Secondary growth in the central vascular cylinder was also significantly limited in extremely flat twining stems. N_t was positively and linearly correlated with SA. Vmm⁻² and VD_ave fluctuated greatly in younger stems and tended to be stable in older stems. N_t and Vmm⁻² did not significantly differ between twining and creeping stems, while VD_ave and P₃₀₀ were both higher in twining stems compared to creeping stems of the same size. In conclusion, well‐developed lateral anomalous secondary growth prevents twining M. boisiana stems from fiercely competing with their host trees, while stable vessel density and wider, newly formed, vessels ensured sufficient hydraulic conductivity for the rapid asexual reproduction of twining M. boisiana stems.     

Xylem structure and water transport in a twiner,a scrambler,and a shrub of Lonicera (Caprifoliaceae)

Summary Wood structure and function was investigated in different growth forms of temperate honeysuckles (Lonicera spp.). All three species had many narrow vessels and relatively few wide ones, with the measured K _h (flow rate/pressure gradient) approximately 24–55% of the theoretical K _h predicted by Poiseuille's law. Only the twiner, Lonicera japonica, had some vessels greater than 50 m in diameter. The twiner also had the narrowest stem xylem diameters, suggesting the greater maximum vessel diameter hydraulically compensated for narrow stems. Conversely, the free-standing shrub, L. maackii, had the greatest annual increments of xylem but the least percent conductive xylem implying that a great portion of the wood was involved with mechanical support. The scrambler, L, sempervirens had low maximum vessel diameter, high Huber values (= xylem area/leaf area), and low specific conductivities (= measured K _h/xylem area), much like the shrub. The greatest vessel frequency occurred in the scrambler (901 vessels · mm^-2), the highest thus far recorded in vines. The lowest Huber value and highest specific conductivity occurred in the twiner, suggesting little self-support but relatively efficient water conduction. LSC (= measured K _h/leaf area) and maximum vessel diameter of Lonicera vines were near the low end of the range for vines in general.     

WOOD FORMATION IN PROSOPIS: EFFECT OF 2,4-D, 2,4,5-T,AND TIBA

Treatment of erect stems of Prosopis with near phytotoxic levels of 2,4-D or 2,4,5-T causes the formation of an unusual wood with narrow, thick-walled vessels and axial parenchyma in which cell wall thickening is inhibited. Although reduced in diameter, the vessels formed during 2,4-D and 2,4,5-T treatment are so numerous that there is no significant difference between phenoxyacetic acid and control seedling groups with regard to total area of xylem occupied by vessels. The preferential maturation of xylem vessels over parenchyma and the transformation of fusiform initials into septate parenchyma strands in phenoxyacetic acid-treated Prosopis resemble the structural changes reported to occur after girdling in the cambial tissue of other arborescent angiosperms. Bending experiments indicate that tension-wood fibers of Prosopis differentiate in response to an auxin deficiency. However, xylogenesis in erect stems treated with TIBA is affected such that a significantly higher proportion of the cambial cell population becomes axial xylem parenchyma.     

PRESENCE OF VESSELS IN WOOD OF SARCANDRA (CHLORANTHACEAE); COMMENTS ON VESSEL ORIGINS IN ANGIOSPERMS

Sarcandra is the only genus of Chloranthaceae hitherto thought to be vesselless. Study of liquid-preserved material of S. glabra revealed that in root secondary xylem some tracheary elements are wider in diameter and have markedly scalariform end walls combined with circular pits on lateral walls. Examination of these wider tracheary elements with scanning electron microscope (SEM) demonstrated various degrees of pit membrane absence in the end walls. Commonly a few threadlike fibrils traverse the pits (perforations); these as well as intact nature of pit membranes in pits at ends of some perforation plates are evidence that lack of pit membranes does not result from damage during processing. Some perforations lack any remnants of pit membranes. Although perforation plates and therefore vessels are present in Sarcandra roots, no perforations were observed in tracheary elements of stems or lignotubers. Further, stem tracheids do not have the prominently scalariform end walls that the vessel elements in roots do. Presence of vessels in Sarcandra removes at least one (probably several) hypothetical events of vessel origin that must be postulated to account for known patterns of vessel distribution in angiosperms, assuming that they are primitively vesselless. Seven (perhaps fewer) vessel origin events in angiosperms could account for these patterns; two of those events (Nelumbo and monocotyledons) are different from the others in nature. Widely accepted data on trends of vessel specialization in woody dicotyledons yield an unappreciated implication: vessel specialization has happened in a highly polyphyletic manner in dicotyledons, and therefore multiple vessel origins represent a logical extension backward in time. If a group of vesselless dictyoledons ancestral to other angiosperms existed, they can be hypothesized to have had a relatively homogeneous floral plan now that Sarcandra-like plants no longer need be imagined within that group. Sarcandra and other Chloranthaceae show that the borderline between vessel absence and presence is less sharp than generally appreciated.     

Wood anatomy reflects the distribution of Krascheninnikovia ceratoides (Chenopodiaceae)

Wood anatomy characters of the Eurasian/North American half-shrub Krascheninnikovia ceratoides were investigated on plants from different parts of the wide distribution area. The secondary xylem is developed as result of anomalous secondary thickening. The vessels are small (commonly <50 μm) and the secondary xylem is rayless. Differences in vessel diameter exist between the main axis, basal branches and flowering shoots. The maximum as well as the minimum vessel diameter in the flowering shoots is significantly smaller compared to the basal branches, as well as in the main axis in all provenances. The length of thick-walled libriform fibre cells varies between the main axis and the basal branches of the same plant individual, but in the short main axis the length of libriform fibre cells is relatively constant (193-217 μm), independent of provenance and climate. Obviously, the length of libriform fibre cells is a conservative character and dependent on plant size. The vessel diameter is a more sensitive parameter for ecological studies. It varies more clearly, considering the vessel position in the plant axis system, in relation with climate differences of the provenances of the examined plants.Plants from a temperate, semi-humid climate have the largest vessel diameter in basal branches and flowering shoots, while the vessel diameter of plants from an arid temperate climate in Central Asia is smallest. The vessel diameters depend not only on climate, but also on the position in the axial system of the plants. There is a trade-off between average vessel number and their maximum diameter in flowering shoots of populations from Russia and Mongolia. Thus, vessel diameters depend on both, position in the axial system of the plant and climatic conditions.     

Do lianas really have wide vessels? Vessel diameter–stem length scaling in non-self-supporting plants

Lianas and other climbing plants are known for their extraordinarily wide vessels. Wide vessels are thought to contribute to the extreme hydraulic efficiency of lianas and to play a part in their ability to dominate many tropical habitats, and even their globally increasing abundance with anthropic disturbance. However, recent hydraulic optimality models suggest that the average vessel diameter of plants generally is the result of tip-to-base vessel widening reflecting the effects of selection buffering conductive path length-imposed hydraulic resistance. These models state that mean vessel diameter should be predicted by stem length, by implication even in lianas. We explore vessel–stem relations with 1409 samples from 424 species in 159 families of both self- and non-self-supporting plants. We show that, far from being exceptional in their vessel diameter, lianas have average natural (not hydraulically weighted) vessel diameters that are indistinguishable for a given stem length from those in self-supporting plants. Lianas do, however, have wider variance in vessel diameter. They have a small number of vessels that are wider than those in self-supporting plants of similar stem lengths, and also narrower vessels. This slightly greater variance is sufficient to make hydraulically weighted vessel diameters in lianas higher than those of self-supporting counterparts of similar stem lengths. Moreover, lianas have significantly more vessels per unit of wood transection than self-supporting plants do. This subtle combination of slightly higher vessel diameter variance and higher vessel density for a given stem length is likely what makes lianas hydraulically distinctive, rather than their having vessels that are truly exceptionally wide.     

酸枣茎导管对自然梯度干旱生境响应的结构特征

近期因全球变暖高温干旱事件频发,严重影响植物的生长发育。逆境中植物的形态结构必定发生改变而得以生存。然而关于不同自然梯度干旱生境中植物导管形态特征变化及其规律方面的研究较少。以采自烟台、石家庄、银川和吐鲁番地区形成的自然梯度干旱条件下生长的酸枣植株为试验材料,离析观察酸枣茎次生木质部导管,研究比较不同干旱条件下酸枣茎中次生木质部导管的结构及适应特征。结果表明:酸枣茎中次生木质部导管有多种类型,不同生境中同种导管的比例不同。根据管尾情况可将导管分为三类:无尾型、一端有尾型和两端有尾型导管,且在结构上表现出特定的适应特征及规律。从烟台到新疆自然梯度干旱条件下,茎中环纹导管和螺纹导管的长度、宽度和管径总体呈减小的趋势,管壁逐渐增厚;网纹导管的宽度和直径逐渐增大;梯纹导管的长度总体呈增大的趋势,管径以宁夏的最大,新疆的最小;木纤维的宽度、管径和管壁的厚度逐渐增大。生长在不同自然梯度干旱生境中的酸枣,其导管结构的变化,提高了水分和无机盐的输导效率,可以快速补充酸枣在干旱环境中蒸腾散失的水分,抵御适应逆境,保证植株正常生长。     

华南地区常见树种导管特征与树干液流的关系

为探讨树木结构与功能的关系,对华南地区常见8种树木边材的导管特征进行观察,并利用Granier热扩散探针法测量干、湿季树干的液流密度,分析导管特征与树干液流的关系。结果表明,除红锥(Castanopsis hystrix)有两种导管外,大叶相思(Acacia auriculaeformis)、荷木(Schima superba)、火力楠(Michelia macclurei)、藜蒴(C.fissa)、马占相思(A.mangium)、柠檬桉(Eucalyptus citriodora)、尾巨桉(E.urophylla×E.grandis)的导管类型单一。导管特征在种间存在明显差异,且导管长度、密度和孔径之间存在明显相关性,它们与标准化的边材面积呈现显著相关。湿季液流最大值与导管特征无明显相关性,但整树最大蒸腾速率与导管特征呈显著相关;树木的日蒸腾量与导管特征也有明显相关性。因此,树木的液流速率并不受树干的导管影响;而树干的导管孔径与边材面积间的负相关权衡机制,可以降低树种间由于导管孔径差异引起的树干的水分输送速率的差异性。     

REGENERATION OF THE SUNFLOWER CAPITULUM AFTER CYLINDRICAL WOUNDING OF THE RECEPTACLE

Using the young capitulum of Helianthus annuus L., a cylindrical plug of undifferentiated receptacle tissue, 1 mm in diameter, was isolated from lateral communication with the rest of the receptacle surface by a vertical circular wound cut, while retaining continuity with the subapical meristem. Within 24 hr, active cell division was induced at the inner and outer surfaces of the wound and in the receptacle epidermis bordering the wound edges, creating a rounded rim at the top of the wound. Within 3–6 days, floral initials, spaced 133–166 μm apart appeared on the flanks of both rims and later on the top of the plug and surrounding receptacle surface. The first formed initials developed into involucral bracts or ray florets and the later ones into disc florets which were organized into contact parastichies, the number of which did not conform with the Fibonacci series. The base of the plug developed into a stem-like structure completing the regeneration of a fully formed functional capitulum. This operation was demonstrated for two sunflower cultivars and occurred in both long and short daylengths.     

COMPARATIVE ULTRASTRUCTURAL STUDIES OF SPERMATOGENESIS IN THE METZGERIALES (HEPATICAE). I. THE BLEPHAROPLAST OF PALLAVICINIA LYELLII

The blepharoplast in the young spermatid of Pallavicinia is similar to that of other hepatics in that it comprises a four-layered multilayered structure (MLS) and two staggered, dimorphic basal bodies. The spline, approximately 40 μm in length and extending through nearly two full gyres, comprises 20 parallel microtubules at its anterior end and narrows to 17 at the posterior limit of the subjacent lamellar strip (LS). Behind this, the spline shank, approximately 32 μm in length, is reduced to six tubules. The LS curves around the spermatid, following the anterior one-third of the first gyre of the spine, and is approximately 7.5 μm in length, the longest yet recorded for the bryophytes. It is spatulate in outline, equaling the width of the spline anteriorly but tapering steeply from the right-hand side behind the anterior basal body (ABB). It then extends posteriorly as a narrow strip beneath the left-hand margin of the spline. The basal bodies of the greatly staggered flagella are nonoverlapping and separated by a distance of about 4.4 μm. The subapical ABB and PBB measure (including the ventral triplet extensions and transition zones) 1.2 μm and 2.4 μm in length, respectively. A short, narrow aperture equaling one tubule-diameter in width is located in the spline directly beneath the ABB. The anterior mitochondrion is about 7 μm long and follows the outline of the overlying LS, while a cupshaped posterior mitochondrion is appressed to the plastid. Comparisons with other taxa indicate that major distinguishing features of metzgerialian blepharoplasts are highly staggered, nonoverlapping basal bodies, greatly elongate anterior mitochondria, and six-tubule shanks. Great differences between the spermatids suggest wide phylogenetic discontinuities between the genera of the Metzgeriales.     

CHARACTERIZATION OF STARCH GRAINS IN THE NONARTICULATED LATICIFER OF EUPHORBIA PULCHERRIMA (POINSETTIA)

Starch grain morphology in laticifer amyloplasts of Euphorbia pulcherrima Willd. (poinsettia) was examined for evidence of starch metabolism in vegetative and flowering plants. Laticifer starch grains in vegetative plants were rod shaped with lengths ranging from 3 to 60 μm. Average grain size was significantly larger in stems than leaves, and in older than younger tissues. Starch grain length frequency was unimodal and approximated a normal probability distribution in stems, but was skewed positively toward smaller grains in leaves. Frequency distributions were shifted toward larger grains in older tissues. Under short-day photoperiod (flowering) conditions, round starch grains formed in latex of stems, and the average length of rod-shaped grains decreased in latex of stems and leaves. Round grains did not occur in laticifers of leaves or bracts. Round starch grains often occurred in aggregates of two or more subunits. Changes in size and shape of latex starch grains indicate that amyloplasts in fully differentiated laticifers metabolize starch. Identification of metabolically active amyloplasts in differentiated laticifers suggests that the function of these organelles may involve starch mobilization under certain physiological conditions.