Provascular Strands Research Articles

Rhizome architecture, development and vascularization in the water lily Nymphaea alba.

Water lilies are of particular interest with regard to the evolution of angiosperms. They live in an aquatic environment and have been regarded as links to the monocots by some authors. Vascular bundles are sometimes described as scattered or atactostelar as in monocots. However, this view needs to be clarified as the morphology and vascularization of Nymphaea rhizomes remain to be understood. The rhizome of Nymphaea alba was re-investigated morphologically and histologically. Developmental studies were conducted using scanning electron microscopy. Comprehensive histological analyses, including hand and microtome sections and a variety of specific staining procedures, were conducted to re-evaluate the composition of longitudinal and transverse tissue. The rhizome is covered by parenchymatous nodal cushions each bearing a leaf and several adventitious roots. Internodes are extremely short. The apex is flat and early overtopped by developing leaf primordia and cushions. The phyllotaxis is spiral and passes alternately through vegetative and reproductive phases. Flowers appear in the leaf spiral, and lack a subtending bract and a cushion below the peduncle. The reproductive phase includes two or three flowers which alternate with a single leaf. The rhizome is histologically subdivided into a central core, an aerenchymatic cortex, and a parenchymatic exocortex formed to a great extent by the nodal cushions. The core contains strands of vascular bundles united to a complex vascular plexus. Vascular elements continuously anastomose and change shape and direction. Provascular strands originating from leaf primordia merge with the outer core vascular tissue whereas the flower strands run into the centre of the core. Roots originating from the parenchymatous cushions show the characteristic actinostelic pattern, which changes into a collateral pattern inside the rhizome. Several root traces merge and form one strand leading to the central core. Early cell divisions below the apical meristem dislocate leaf, flower and root primordia and their provascular strands outwards. Consequently, fully developed vascular strands insert horizontally into the vascular plexus at advanced rhizome stages. The absence of bracts and cushions below the flowers, the alternate leaf-flower sequence and the course of the peduncle strand suggest that the rhizome is sympodially instead of monopodially organized. The spiral phyllotaxis extends in this case over several shoot orders, masking the branching pattern. The vascular strands in the central plexus differ considerably from vascular bundles in monocots, confirming the unique vascularization in Nymphaea. Sclerenchymatic bundle sheaths are lacking, and vascular bundles continuously split and anastomose throughout the rhizome. Though vascular bundles in petioles and peduncles of N. alba show similarities with some Alismatales, the vascular system of N. alba in general has little in common with that of monocots.

Morphological and histological analysis of anther-derived embryos of coconut (Cocos nucifera L.)

Doubled-haploids are a great source of material for heterozygous coconuts to shorten the time taken for crop improvement through hybridization programs. However, low frequency of embryo conversion and formation of weak plantlets are the major limitations in the protocol. In the present study anther-derived embryos were analysed at cellular level through the histological observations to understand the occurrence of shoot differentiation in those embryos. Attempts were also made to optimise the 6-Benzylaminopurine (BAP) concentration in the regeneration medium. Among the tested BAP levels (5, 15, 25, 35, 45, 55 μM), the regeneration medium containing 25 and 35 μM gave rise to 51.5% sprouted embryos against the control that gave only 22.7%. Among the analysed anther-derived embryos sequential events of differentiation including the formation of provascular strands followed by vascular bundle, differentiation in to the growing point, giving rise to the secondary embryos and polarization in to the shoot and root pole were identified. The blunt embryos devoid any morphological sign of sprouting showed different cellular arrangement viz. haustorial structure without any meristematic point (47%), bipolar with shoot and root meristems (8%) and unipolar with either of the pole (45%). Thick haustorial cover lead to the physical dormancy inhibiting further development of the polar structures. The germinating embryos containing a single shoot gave rise to the healthy plants where as double, multiple or fused shoots formed the week plantlets. The findings can be used for further optimisation of the protocol to achieve a greater plant regeneration frequency. 1. Culture media supplemented with increased levels of 6-Benzylaminopurine (BAP) enhanced the regeneration frequency of anther-derived embryos. 2. Thick haustorial tissue created an obstacle effect on further development of the differentiated shoot apical meristem. 3. Formation of clustered plants through secondary embryogenesis or shoot multiplication at the early stage of plant regeneration caused to give rise to the week plants.

Spatially-resolved localization and chemical speciation of nickel and zinc in Noccaea tymphaea and Bornmuellera emarginata.

Hyperaccumulator plants present the ideal model system for studying the physiological regulation of the essential (and potentially toxic) transition elements nickel and zinc. This study used synchrotron X-ray Fluorescence Microscopy (XFM) elemental imaging and spatially resolved X-ray Absorption Spectroscopy (XAS) to elucidate elemental localization and chemical speciation of nickel and zinc in the hyperaccumulators Noccaea tymphaea and Bornmuellera emarginata (synonym Leptoplax emarginata). The results show that in the leaves of N. tymphaea nickel and zinc have contrasting localization, and whereas nickel is present in vacuoles of epidermal cells, zinc occurs mainly in the mesophyll cells. In the seeds Ni and Zn are similarly localized and strongly enriched in the cotyledons in N. tymphaea. Nickel is strongly enriched in the tip of the radicle of B. emarginata. Noccaea tymphaea has an Fe-rich provascular strand network in the cotyledons of the seed. The chemical speciation of Ni in the seeds of N. tymphaea is unequivocally associated with carboxylic acids, whereas Zn is present as the phytate complex. The spatially resolved spectroscopy did not reveal any spatial variation in chemical speciation of Ni and Zn within the N. tymphaea seed. The dissimilar ecophysiological behaviour of Ni and Zn in N. tymphaea and B. emarginata raises questions about the evolution of hyperaccumulation in these species.

Ontogeny of the sheathing leaf base in maize (Zea mays).

Leaves develop from the shoot apical meristem (SAM) via recruitment of leaf founder cells. Unlike eudicots, most monocot leaves display parallel venation and sheathing bases wherein the margins overlap the stem. Here we utilized computed tomography (CT) imaging, localization of PIN-FORMED1 (PIN1) auxin transport proteins, and in situ hybridization of leaf developmental transcripts to analyze the ontogeny of monocot leaf morphology in maize (Zea mays). CT imaging of whole-mounted shoot apices illustrates the plastochron-specific stages during initiation of the basal sheath margins from the tubular disc of insertion (DOI). PIN1 localizations identify basipetal auxin transport in the SAM L1 layer at the site of leaf initiation, a process that continues reiteratively during later recruitment of lateral leaf domains. Refinement of these auxin transport domains results in multiple, parallel provascular strands within the initiating primordium. By contrast, auxin is transported from the L2 toward the L1 at the developing margins of the leaf sheath. Transcripts involved in organ boundary formation and dorsiventral patterning accumulate within the DOI, preceding the outgrowth of the overlapping margins of the sheathing leaf base. We suggest a model wherein sheathing bases and parallel veins are both patterned via the extended recruitment of lateral maize leaf domains from the SAM.

Importance of structural integrity of somatic embryos for long-term cryopreservation of cocoa (Theobroma cacao L.) germplasm

Cryopreservation is a reliable means for the long-term conservation of plant genetic resources. It is of particular interest for cocoa (Theobroma cacao L.) whose seeds are recalcitrant to conventional storage methods and field collections susceptible to disease infestations. However, the encapsulation- dehydration procedure previously developed for cocoa somatic embryos has resulted in poor survival after retrieval from liquid nitrogen. To examine the causes of such failure, cocoa somatic embryos following each treatment step of the encapsulation-dehydration procedure were examined using a combination of confocal scanning laser microscopy and transmission electron microscopy. Results showed that the parenchyma’s cells of the hypocotyl and radicle were the major sites of injury possibly due to their large size and non-cytoplasmic nature, whereas the shoot meristem and provascular strand were well preserved throughout the treatments. In general, cell deformation and/or disruption was observed following the sucrose preculture, desiccation and freezing steps, with the extent of damage increasing at each treatment step. Post-thaw regrowth of injured embryos was possible through the proliferation of surviving cells, but these embryos often failed to convert into plantlets. The present study suggests that the maintenance of structural integrity of somatic embryos at each treatment step is essential for the successful cryopreservation of cocoa germplasm. Key words: Cocoa somatic embryos, confocal scanning laser microscopy, cryopreservation, structural integrity, transmission electron microscopy.

Thidiazuron-induced rapid shoot regeneration via embryo-like structure formation from shoot tip-derived callus culture of sugarcane

Rapid shoot regeneration system from callus cultures of a sugarcane (Saccharum officinarum L.) cultivar NiF8 was established. Apical meristematic tissues harvested from young sugarcane shoots were cultured on modified Murashige and Skoog medium containing 2 mg l−1 of 2,4-dichlorophenoxyacetic acid for callus induction. The sugarcane callus was then transferred onto media with different concentrations of thidiazuron (TDZ) (0.5, 1, 2 and 3 mg l−1) with or without 0.1 mg l−1 of 1-naphthaleneacetic acid (NAA) for shoot regeneration. The highest regeneration frequency (80.0%) was observed after three weeks of culture on medium containing 1 mg l−1 TDZ and 0.1 mg l−1 NAA. Histological observation showed that differentiation of proembryoid-like structure with pro-vascular strands were observed 3 days after transfer onto the optimum medium followed by formation of apical meristematic tissue and leaf-like structures after 5 to 7 days, suggesting that TDZ induced rapid shoot regeneration via a process similar to somatic embryogenesis. This study demonstrated a rapid, reproducible and efficient regeneration procedure of sugarcane, which is suitable for biotechnological application including genetic transformation.

MiR156-Regulated SPL Transcription Factors Define an Endogenous Flowering Pathway in Arabidopsis thaliana

The FT gene integrates several external and endogenous cues controlling flowering, including information on day length. A complex of the mobile FT protein and the bZIP transcription factor FD in turn has a central role in activating genes that execute the switch from vegetative to reproductive development. Here we reveal that microRNA156-targeted SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes not only act downstream of FT/FD, but also define a separate endogenous flowering pathway. High levels of miR156 in young plants prevent precocious flowering. A subsequent day length-independent decline in miR156 abundance provides a permissive environment for flowering and is paralleled by a rise in SPL levels. At the shoot apex, FT/FD and SPLs converge on an overlapping set of targets, with SPLs directly activating flower-promoting MADS box genes, providing a molecular substrate for both the redundant activities and the feed-forward action of the miR156/SPL and FT/FD modules in flowering control.

Localization of Iron in Arabidopsis Seed Requires the Vacuolar Membrane Transporter VIT1

Iron deficiency is a major human nutritional problem wherever plant-based diets are common. Using synchrotron x-ray fluorescence microtomography to directly visualize iron in Arabidopsis seeds, we show that iron is localized primarily to the provascular strands of the embryo. This localization is completely abolished when the vacuolar iron uptake transporter VIT1 is disrupted. Vacuolar iron storage is also critical for seedling development because vit1-1 seedlings grow poorly when iron is limiting. We have uncovered a fundamental aspect of seed biology that will ultimately aid the development of nutrient-rich seed, benefiting both human health and agricultural productivity.

Genetic regulation of vascular tissue patterning in Arabidopsis.

Plants transport water and nutrients through a complex vascular network comprised of interconnected, specialized cell types organized in discrete bundles. To identify genetic determinants of vascular tissue patterning, we conducted a screen for mutants with altered vascular bundle organization in Arabidopsis cotyledons. Mutations in two genes, CVP1 and CVP2 (for cotyledon vascular pattern), specifically disrupt the normal pattern of vascular bundles in cotyledons, mature leaves, and inflorescence stems. The spatial distribution of the procambium, the precursor to mature vascular tissue, is altered in cvp1 and cvp2 embryos, suggesting that CVP1 and CVP2 act at a very early step in vascular patterning. Similarly, in developing stems of cvp1 and leaves of cvp2, the pattern of vascular differentiation is defective, but the maturation of individual vascular cells appears to be normal. There are no discernible alterations in cell morphology in cvp2 mutants. In contrast, cvp1 mutants are defective in directional orientation of the provascular strand, resulting in a failure to establish uniformly aligned vascular cells, and they also show a reduction in vascular cell elongation. Neither cvp1 nor cvp2 mutants displayed altered auxin perception, biosynthesis, or transport, suggesting that auxin metabolism is not generally affected in these mutants.

Supercooling characteristics of some deciduous fruit trees as related to water movement within the bud

SummaryTo characterize and identify the freezing resistance mechanism of some deciduous fruit trees, woody stem and dormant buds of apple (Malus domestica ‘Tsugaru’), grape (Vitis labruscana ‘Muscat Bailey A’), peach (Prunus persica ‘Shimizu Hakuto’), Japanese pear (Pyrus pyrifolia ‘Nijisseiki’), and persimmon (Diospyros kaki ‘Hiratanenashi’) were used in the following experiments. On thermal analysis (TA), the woody stem tissues or the buds with woody stem tissue of all fruit trees used showed two exotherms, the high temperature exotherm (HTE) and the low temperature exotherm (LTE). The flower buds without woody stem tissue of peach and pear also had two exotherms, the HTE and LTE, and the LTE temperatures coincided with the LT50S of flower buds although the detection frequency of LTE in pear was considerably lower (<10%). Furthermore, in peach and pear, the LTE of buds without woody stem tissue and the LT50 of flower buds on woody stem were cooling-rate dependent, and those temperatures fell with a decrease in cooling rate. Water migration from the flower primordia of both buds to the surrounding tissues was also observed during freezing. In apple, although the LTE of bud without woody stem tissue was not detected in our experiment, the LT50 of the bud was cooling-rate dependent, and water migration from the primordia to the other tissues in bud was observed during freezing. These facts indicate that the mechanisms of freezing tolerance in apple flower buds are the same as in peach and pear flower buds. Contrarily, in persimmon and grape, flower buds without woody stem tissue showed only one exotherm whose temperature was closely associated with the LT50 of the bud. Also, both the exotherm temperature of bud without woody stem tissue and the LT50 of buds on woody stem were independent of cooling rate. Persimmon buds showed no changes in the water content of either the primordium or the surrounding tissue during freezing, indicating no water migration from the primordia to the surrounding tissues. In addition, provascular strands were found between the bud axis and flower primordia in apple, peach, and pear buds, but not in persimmon and grape buds. From these results, the buds of apple, peach and pear may acquire freezing resistance by extra-organ freezing, but grape and persimmon buds are likely to supercool by themselves.

Histological Observation of Callus Formation in Mungbean(Vigna radiata(L.) Wilczek) Cotyledon Culture.

Histological observation was conducted to evaluate the origin, initiation site and development of callus in mungbean cotyledon culture. Results showed that callus formation in mungbean occurred in three sites: 1) in the peripheral area of the provascular strands where the initiation of endogenous callus was noted; 2) at the cut portion of the cotyledon where external primary callus developed; and 3) in the ruptured epidermal layer where the primary callus emerged. Callus formation differed in each site. Endogenous callus formed due to the rapid cell division of the cells in the peripheral zone of the provascular tissues. The primary callus from the cut portion of the cotyledon and reptured portion of the epidermal layer were mostly formed out of cell enlargement of the parenchymatous cells. Although the primary callus cells soon became highly vacuolated, this does not seem to contribute to the increase in callus cell mass. Owing to low dividing activity of the primary callus cells, the cytoplasm-rich and meristematic callus cells are apparently derived from the small callus cells which were located on the periphery of the original provascular tissues. And so, some cells in the provascular cells could be seen as the prominent origin of callus in the excised cotyledon system of mungbean.

Anatomical Features Facilitating Supercooling of the Flower within the Dormant Peach Flower Bud

Excision of the flower from the peach [Prunus persica (L.) Batsch.] flower bud raised the 50% injury temperature of flowers (cooled at 1C/hour) from -18 and -20C to -10 and -13C on two test dates, 26 Feb. 1988 and 5 Dec. 1990, respectively. Ice inoculation of the excised flowers at -3C further raised the 50% injury temperature to -7 and -8C for the two dates, respectively. These observations suggest that supercooling is a true mechanism for avoiding freezing injury. Low temperature scanning electron microscopy of freeze fractured cells verified that the flower froze intracellularly, whereas the subtending tissue froze extracellularly. Ice inoculation of the flower and the flower bud axis from which the scales were removed demonstrated that a barrier to ice propagation (effective to -11C) from the flower bud axis to the flower was present. This barrier may involve the provascular strands and a cell zone at the flower base (BZ) that were devoid of intercellular spaces. These tissues also had smaller cells, smaller vacuoles, greater ratio of cell wall thickness to cell size than tissue just below the BZ, which may result in greater cell rigidity and restriction of extracellular freezing. The cells outside the provascular strands at the base of the flower were also lacking in intercellular space, were smaller in size, and had a higher ratio of cell wall thickness to cell size compared to cells near the base of the scales. In the intact flower buds in which the flowers supercool below -11C, the presence of the first and second scales was important to full expression of supercooling because their removal raised the supercooling point, whereas the removal of lower scales did not. Sequestration of ice by the first two subtending scales during the early stages of freezing may be important to the creation of a dry region at the flower base that prevents ice propagation into the flower at temperatures below -11C.

Transcription of the Zea mays homeobox (ZmHox) genes is activated early in embryogenesis and restricted to meristems of the maize plant

AbstractFour Zea mays homeobox (ZmHox) genes have been analyzed in their spatial expression pattern during development of the maize plant. The ZmHox1a or b and ZmHox2a or b gene pairs encode putative plant transcriptin factors; the mRNA transcripts belong to the rare class of mRNA molecules. Expression of the ZmHox1 and 2 genes is activated very early in embryonic development and restricted to the embryo proper in the proembryo stage. After establishment of the root/shoot axis, transcripts are prevalent in the embryonic root and shoot apical meristems but later are also found in provascular tissues and young leaf primordia. In the vegetative plant body the transcription of the ZmHox genes marks several types of meristems of the shoot and root system, the descending proliferating regions and provascular strands. Expression persists in the developing reproductive organs, but results obtained in the male flower indicate that transcription of ZmHox genes here may become confined to specific cell types. The data obtained during this study show that these four ZmHox genes are expressed very specifically in cells where developmental decisions contribute to the ontogeny of the maize plant. The overall expression pattern suggests that the ZmHox class of maize homeobox genes will be involved in transcriptional control during maize development from the embryonic to the reproductive phase. © 1995 Wiley‐Liss, Inc.

Nuclear and cytoplasmic localization of maize embryo and aleurone dehydrin

The subcellular location of dehydrins was determined by immunomicroscopy in maize kernels imbibed in the presence of ABA. Antibodies were specific to the consensus sequence that exists near the carboxy terminus of virtually all dehydrins. Aleurone, scutellar epithelium, scutellar provascular strands, and the outermost embryonic leaves contained the highest concentration of dehydrins. In these tissues, and in scutellar parenchyma cells and inner embryonic leaves, dehydrins were present in the cytoplasm and associated with nuclei. In shoot and root apex cells, dehydrins appeared to be present principally in the cytoplasm. This localization pattern of dehydrins, together with the known amino acid sequences and phosphorylation patterns, leads to analogies with other proteins that are present in both the nucleus and cytoplasm of eukaryotic cells.

The pattern of histone H4 expression in the tomato shoot apex changes during development.

Two histone H4 cDNA clones were isolated from a tomato (Lycopersicon esculentum Mill.) shoot-tip cDNA library using a heterologous probe from barley (Hordeum vulgare L.). Both cDNAs, which are 81% identical in the coding region, are polyadenylated and belong to a small gene family in the tomato genome. Histone H4 message is abundant in young tissues and rare in older tissues. In the shoot apical meristem, the distribution of H4-expressing cells changes during development. In a juvenile vegetative apex, H4 message is detectable in the central region and the peripheral parts of the meristem. In a mature vegetative apical meristem, H4-expressing cells are localized in the peripheral zone extending into the provascular strands and the rib meristem whereas the central zone is almost devoid of H4 mRNA. After floral transition, H4 mRNA is found throughout the floral meristem, indicating a second change in the pattern of H4 expression. The observed changes in H4 expression are indicative of changes in the distribution of mitotic activity in the shoot apical meristem during plant development. In addition, H4-expressing cells were found to occur frequently in clusters, which may indicate a partial synchronization of cell divisions in the shoot apex.

Vascular Connection Between Lateral Roots and Stem in the Ophioglossaceae

The vascular connection between lateral roots and stem in the Ophioglossaceae and in two leptosporangiate fern species was examined. Two types of connections were found: “gradual” connections, which resemble leaf traces in ontogeny and morphology, and “abrupt” connections, which resemble the connections between lateral roots and their parent roots. Gradual root-stem connections occur in the genera Ophioglossum and Helminthostachys and in Woodwardia virginica. They are initiated in shoot apices distal to the level where cauline xylem elements mature. They resemble leaf traces in being provascular (procambial) strands that connect the cauline stele with the future vasculature of lateral appendages. As with leaf traces, gradual connections are part of the provascular and, later, protoxylem continuity between stems and lateral appendages. Gradual connections have many features in common with leaf traces, and the term root trace is applicable to them. The order of radial maturation of the primary xylem in gradual connections varies in different parts of the connections. It is endarch near the intersection with the cauline stele and exarch where the connections intersect root steles. Gradual connections resemble the transition regions of certain seed plants where protoxylem is also continuous from stem to root and the order of maturation is found to change continuously from stem to root. Abrupt connections occur in Botrychium and Osmunda cinnamomea. They develop in shoot apices at levels where cauline xylem is mature or maturing. The mature xylem does not dedifferentiate, so provascular and protoxylem continuity of the kind found in root traces does not occur. Also, reorientation of the order of maturation does not occur in abrupt connections. Xylem connectors are found in the region where radially oriented elements of the connections abut the longitudinally oriented cauline elements. Abrupt connections resemble the connection of secondary roots with their parent root systems since xylem connectors and the lack of continuity are also features found in these vascular systems. The resemblance of the vascular pattern of the fern root trace to the transition region of seed plants suggests that the radicle is more closely comparable to the cladogenous roots of pteridophytes than hitherto supposed.

VASCULAR CONNECTION BETWEEN LATERAL ROOTS AND STEM IN THE OPHIOGLOSSACEAE

The vascular connection between lateral roots and stem in the Ophioglossaceae and in two leptosporangiate fern species was examined. Two types of connections were found: “gradual” connections, which resemble leaf traces in ontogeny and morphology, and “abrupt” connections, which resemble the connections between lateral roots and their parent roots.Gradual root‐stem connections occur in the genera Ophioglossum and Helminthostachys and in Woodwardia virginica. They are initiated in shoot apices distal to the level where cauline xylem elements mature. They resemble leaf traces in being provascular (procambial) strands that connect the cauline stele with the future vasculature of lateral appendages. As with leaf traces, gradual connections are part of the provascular and, later, protoxylem continuity between stems and lateral appendages. Gradual connections have many features in common with leaf traces, and the term root trace is applicable to them.The order of radial maturation of the primary xylem in gradual connections varies in different parts of the connections. It is endarch near the intersection with the cauline stele and exarch where the connections intersect root steles. Gradual connections resemble the transition regions of certain seed plants where protoxylem is also continuous from stem to root and the order of maturation is found to change continuously from stem to root.Abrupt connections occur in Botrychium and Osmunda cinnamomea. They develop in shoot apices at levels where cauline xylem is mature or maturing. The mature xylem does not dedifferentiate, so provascular and protoxylem continuity of the kind found in root traces does not occur. Also, reorientation of the order of maturation does not occur in abrupt connections. Xylem connectors are found in the region where radially oriented elements of the connections abut the longitudinally oriented cauline elements. Abrupt connections resemble the connection of secondary roots with their parent root systems since xylem connectors and the lack of continuity are also features found in these vascular systems.The resemblance of the vascular pattern of the fern root trace to the transition region of seed plants suggests that the radicle is more closely comparable to the cladogenous roots of pteridophytes than hitherto supposed.

Croissance des coenocytes provasculaires dans la racine de Marsilea diffusa (Marsiléales)

Five types of provascular strands of coenocytes with 2, 4, 8, 16 or 32 nuclei have been recognized in the central region of the root apex of Marsilea diffusa Lepr. This observation is relevant to studies of cellular differentiation and to the pattern of root growth and can be summarized as follows. (i) Nuclear division in the coenocytes of each strand is well synchronized. (ii) The other phases of the nuclear cycle show less synchronization. (iii) The levels of deoxyribonucleic acid (DNA) vary between 2C and 4C and endomitosis does not occur. (iv) Nuclear division is followed by a final S phase. (v) Correlations exist between (a) the content of DNA and the volume of the nucleus and (b) the volume of the coenocytes and the volume of their nuclei. (vi) The nuclear cycles of the coenocytes in the same provascular strand are of similar lengths. (vii) The growth of the coenocytes follows a standard allometric curve: their volume is doubled with each nuclear cycle. (viii) During each nuclear cycle, the central region of the root apex grows at the rate of 62%.

DEVELOPMENTAL MORPHOLOGY OF THE EMBRYO AND SEEDLING OF RHIZOPHORA MANGLE L. (RHIZOPHORACEAE)

The embryo of Rhizophora mangle L. is initially attached to the integument by a long multiseriate suspensor. Its basal cells lyse, and intrusive growth of the endosperm envelops the embryo, forces the micropyle open, and often carries the embryo out of the integument. Thus, “germination” is effected by growth of the endosperm rather than of the embryo. The surface of the endosperm differentiates into a layer of peculiar transfer cells. The cotyledonary body initiates as a toroidal primordium, which later becomes lobed; most of the free portions ultimately fuse. After “germination,” the axis of the viviparous seedling grows by a diffuse intercalary meristem below the cotyledonary node. Before seedling abscission, the shoot apex produces three pairs of leaves, the first of which aborts, leaving the rest of the plumule protected by their stipules. The (immersed) radicle apex is nearly inactive, but lateral roots arise early in seedling development; these are usually the first or only roots to grow during establishment. Ten provascular strands “differentiate” in the cotyledons; a hollow provascular cylinder develops in the hypocotyl. Initial vascular differentiation in the latter is of many alternate poles of xylem and phloem; later, de novo differentiation of metaxylem opposite the protophloem poles, and vice versa, produces collateral bundles. Xylem maturation is endarch over most of the length of the hypocotyl, but tangential and random series of metaxylem vessels occur in the radicle end.

Developmental Morphology of the Embryo and Seedling of Rhizophora mangle L. (Rhizophoraceae)

The embryo of Rhizophora mangle L. is initially attached to the integument by a long multiseriate suspensor. Its basal cells lyse, and intrusive growth of the endosperm envelops the embryo, forces the micropyle open, and often carries the embryo out of the integument. Thus, “germination” is effected by growth of the endosperm rather than of the embryo. The surface of the endosperm differentiates into a layer of peculiar transfer cells. The cotyledonary body initiates as a toroidal primordium, which later becomes lobed; most of the free portions ultimately fuse. After “germination,” the axis of the viviparous seedling grows by a diffuse intercalary meristem below the cotyledonary node. Before seedling abscission, the shoot apex produces three pairs of leaves, the first of which aborts, leaving the rest of the plumule protected by their stipules. The (immersed) radicle apex is nearly inactive, but lateral roots arise early in seedling development; these are usually the first or only roots to grow during establishment. Ten provascular strands “differentiate” in the cotyledons; a hollow provascular cylinder develops in the hypocotyl. Initial vascular differentiation in the latter is of many alternate poles of xylem and phloem; later, de novo differentiation of metaxylem opposite the protophloem poles, and vice versa, produces collateral bundles. Xylem maturation is endarch over most of the length of the hypocotyl, but tangential and random series of metaxylem vessels occur in the radicle end.