ONTOGENY OF THE PRIMARY THICKENING MERISTEM IN SEEDLINGS OF BOUGAINVILLEA SPECTABILIS

Anomalous secondary thickening occurs in the main axis of Bougainvillea spectabilis as a result of a primary thickening meristem which differentiates in pericycle. The primary thickening meristem first appears in the base of the primary root about 6 days after germination and differentiates acropetally as the root elongates. It begins differentiating from the base of the hypocotyl toward the shoot apex about 33 days after germination. The primary thickening meristem is first observable at the base of the first internode about 60 days after germination. It then becomes a cylinder in the main axis of the seedling. No stelar cambial cylinder forms in the primary root, hypocotyl, or stem because vascular cambium differentiation occurs neither in the pericycle opposite xylem points in the primary root nor in interfascicular parenchyma in the hypocotyl or stem. The primary vascular system of the stem appears anomalous because an inner and an outer ring of vascular bundles differentiate in the stele. Bundles of the inner ring anastomose in internodes, whereas those of the outer ring do not. Desmogen strands each of which is composed of phloem, xylem with both tracheids and vessels, and a desmogic cambium, differentiate from prodesmogen strands in conjunctive tissue. The parenchymatous cells surrounding desmogen strands then differentiate into elongated simple-pitted fibers and thick-walled fusiform cells that are about the same length as the primary thickening meristem initials.     

STRUCTURE OF FLORAL NECTARIES OF ALFALFA (MEDICAGO SATIVA L.) IN RELATION TO NECTAR PRODUCTION

Alfalfa (Medicago sativa L.) plants differ in nectar volume production per floret. These differences are heritable, but no information is available on what type of structural variation may be responsible for these differences. One high, one intermediate, and one low nectar-volume-producing clone was selected from each of two alfalfa cultivars. Results from light and scanning electron microscopy indicated that alfalfa has an annular nectary located on the staminal column and primarily on the receptacle. It is composed of several cell layers subtended by vascular bundles containing both xylem and phloem. Vascular tissue does not extend into nectariferous tissue. Permanently open stomata are present in the epidermal layer and are thought to function in nectar secretion. These stomata did not respond to stimuli known to affect leaf stomata. Number of stomata per nectary among the six clones ranged from 24.7 ± 1.9 to 6.8 ± 0.5. Nectar-reservoir diameters among clones ranged from 1.07 ± 0.09 mm to 0.70 ± 0.01 mm. Clones producing the most nectar were characterized by a large nectar reservoir and moderate to high numbers of stomata.     

VASCULAR DYSFUNCTION IN FUSARIUM WILT OF TOMATO

Leaves on tomato plants infected with Fusarium oxysporum f. lycopersici frequently wilt unilaterally when the vascular bundles supplying the affected leaflets are diseased. However, when the vascular bundles on one side of healthy petioles are severed by notching the petiole base, the entire leaf remains turgid. Leaflets on the notched side receive water by diffusion between bundles at the petiole tip. Lateral translocation of water out of individual vessels and between bundles in diseased xylem is impaired by the impregnation of vessel walls, intercellular spaces, and cells adjacent to vessels with the products of vascular discoloration. Waterproofing of vascular bundles can be induced in excised healthy leaves by culture filtrates of the pathogen and catechol. Waterproofing of vessels may play an important role in vascular dysfunction by confining water to individual vessels and thereby increasing the importance of vessel occlusions.     

HISTOLOGICAL STUDIES ON THE COLEOPTILE. I. TISSUE AND CELL TYPES IN THE COLEOPTILE TIP

The uppermost 1-4 mm of 25-mm coleoptiles of oats and wheat have been studied at the optical microscope level, using newer histological methods and sections 1-4 μ thick. The outer epidermal wall, which shows very fine wrinkling, is continuous with the thinner wall of the inner epidermis through the pore. The cells of both epidermal layers have acidophilic cytoplasm with long transvacuolar strands. Both inner and outer epidermis have stomata, those of the outer epidermis having kidney-shaped guard cells like those of dicotyledons. The guard-cell walls are lignified in their inner layers only and are thinly cuticularized. In the vascular bundles the sieve tubes terminate apically about 250 μ below the end of the xylem; the xylem in turn terminates about 400 μ below the extreme apex. A number of clearly undifferentiated cells, with highly basophilic cytoplasm and many mitochondria, separate the xylem elements from the inner epidermis. Towards the outer epidermis there are a few sieve elements, each of which is associated with a special cell having an elongated nucleus supported on fine cytoplasmic strands. The parenchyma of both the tip and the shaft of the coleoptile are generally interpenetrated by air-spaces, but where they are adjacent to the inner epidermis there is heavy interposition of readily stained intercellular material, especially in Triticum. Plastids are widely distributed throughout the tissue, but their greening in light takes place preferentially towards the phloem side of the vascular bundles. The observations are discussed in reference to earlier literature and with regard to the function of the coleoptile as a protecting and guiding organ for the shoot system of the seedling.     

Sink to Source Transition of Populus Leaves

Development of the Populus leaf is presented as a model system to illustrate the sequence of events that occur during the sink to source transition. A Populus leaf is served by three leaf traces, each of which consists of an original procambial trace bundle that differentiates acropetally and continuously from more mature procambium in the stem and a complement of subsidiary bundles that differentiates bidirectionally from a leaf basal meristem. During development these subsidiary bundles maintain continuity through the meristematic region of the node. The basipetally developing subsidiary bunles form phloem bridges that serve to integrate adjacent leaf traces of the stem vasculature. Distal to the node the acropetally developing bundles from all three leaf traces are reoriented in a precise and orderly sequence to form tiers of petiolar bundles. These tiers of bundles extend into the midrib where bundles diverge at intervals as the major lateral veins. The dorsal-most tier of bundles extends to the lamina tip and each successive tier of bundles contributes to lateral veins situated more proximally in the lamina. Although the midrib and the major vein system differentiate acropetally in the lamina, they mature basipetally. Maturation of the mesophyll and other lamina tissues also mature basipetally. As a consequence of the basi-petal maturation process, the lamina tip matures very early and begins exporting photosynthates while the lamina base is still importing from other leaves. The transition of a leaf from sink to source status must therefore be considered as a progression of structural and functional events that occur in synchrony.     

Xylem Development in the Gynoecium of the Apple (Malm pumila L.) Cv. Cox's Orange Pippin K. A. D. *J. COSTA TURA{dagger}

Lignification of the xylem within the carpellary bundles ofapple flowers spreads acropetally from a point 900–1400µm below the base of the locules. At the same time, anotherwave of lignification spreads basipetally from a point justbelow the stigma. The acropetal spread at first progresses morequickly, but at later stages the number of lignified xylem elementsjust below the stigma increases rapidly, reaching a peak justas the flower opens. This increase is very localized and thenumber declines greatly within only 25 % of the stylar distancebelow the stigma. Lignification of the xylem in the bundlesserving other flower parts precedes that serving the gynoecium,and spreads basipetally from a point above the base of the locules Malus pumila, L., anatomy, apple, carpel, Cox's Orange Pippin, development, flower, gynoecium, pedicel, pistil, stigma, style, vasculature, xylem regenreation     

The three-dimensional structure of vascular tissues in Agrobacterium tumefaciens-induced crown galls and in the host stems of Ricinus communis L.

The three-dimensional pattern of phloem and xylem in 10-d-to two-month-old tumors induced by Agrobacterium tumefaciens (C58) and in adjacent Ricinus communis L. stem tissues was studied in thick sections by clearing with lactic acid and by staining with lacmoid. The crown galls contained two types of vascular strands: treelike branched bundles, which developed towards the tumor surface in fast-growing regions, and globular bundles in the slowly developing parts. Both types of vascular bundles contained xylem and phloem and were continuous with the vascular system of the host plant. The tumor bundles were interconnected by a dense net of phloem anastomoses, consisting of sieve tubes but no vessels. These vascular patterns reflect the apparent synthesis sites, concentration gradients and flow pathways of the plant hormones additionally produced in the tumors upon expression of the T-DNA-encoded genes. The A. tumefaciens-induced crown gall affected vascular differentiation in the host stem. In the basipetal direction, the tumor induced more xylem differentiation directly below it, where the crown-gall bundles joined the vascular system of the host. In the centripetal direction, the crown gall caused the development of pathologic xylem characterized by narrow vessels, giant rays and absence of fibers. On the other hand, most probably as a consequence of its gibberellic acid content, the host plant stimulated a local differentiation of regenerative phloem and xylem fibers with unique ramifications, only at the base of the tumor. However, fibers were absent from the main body of the crown gall. The study shows that A. tumefaciens-induced crown galls are characterized by a sophisticated network of vascular tissues in the tumor and are accompanied by a perturbated vessel system in the host. The hormonal mechanisms controlling vascular differentiation in the tumor and neighboring host tissues are discussed. In addition, the gall constriction hypothesis is proposed for explaining the mechanism which gives priority in water supply to the growing gall over the host shoot.We thank Dr. Zs. Koncz (Max-Planck-Institut für Züchtungsforschung, Köln, Germany) for Agrobacterium strains and the Deutsche Forschungsgemeinschaft (SFB 199) for financial support to C.I.U.     

Translocation and incorporation of 14C into the petiole from different regions within developing cottonwood leaves

Summary The ability of a developing cottonwood (Populus deltoides Bartr.) leaf to export ¹⁴C-labeled assimilates begins at the lamina tip and progresses basipetally with increasing LPI. This progression indicates that portions of leaves function quasi-independently in their ability to export ¹⁴C-photosynthate. Although most of the exported radioactivity was recovered in the petiole as water-80% alcohol-soluble compounds, there was also substantial incorporation into the chloroform and insoluble fractions. This observation indicates that assimilates translocated from the lamina are used in structural development of the petiole. Freeze substitution and epoxy embedding were used to prepare microautoradiographs for localization of water-soluble compounds. Radioactivity was found in all cell types within specific subsidiary bundles of the petiole. However, radioactive assimilates appeared to move from the translocation pathway in the phloem toward active sinks in the walls of the expanding metaxylem cells. Translocation in the mature xylem vessels was not observed.     

STRUCTURALLY PRESERVED SPHENOPHYTES FROM THE TRIASSIC OF ANTARCTICA: VEGETATIVE REMAINS OF SPACIINODUM,GEN. NOV.

Sphenophyte remains of Early-Middle Triassic age are described from silicified peat collected in the Transantarctic Mountains of Antarctica. The new sphenophyte, Spaciinodum collinsonii sp. nov., is represented by ribbed, jointed stems with characteristic pith and carinal canals. Stems are relatively small, ranging from 1.8–3.0 mm in diameter, lack secondary tissues, and are characterized by vallecular canals that are restricted to nodal regions. The internodal vascular system consists of 12–18 collateral bundles which alternate between successive internodes. A complete vascular ring is present in the nodal region and is surrounded by a continuous double endodermis. Xylem is endarch and composed of elements ranging from annular to reticulate. The Antarctic sphenophyte is compared with other Gondwana fossil articulates and extant Equisetum. Superficial stomata suggest affinities with modern Equisetum subgenus Equisetum; however, some anatomical differences preclude assignment with living species.     

EMBRYOGENESIS AND SEEDLING DEVELOPMENT IN CASSIPOUREA ELLIPTICA (SW.) POIR. (RHIZOPHORACEAE)

Embryogenesis in Cassipourea elliptica (Sw.) Poir, begins with a first division of the zygote which may be oriented transversely, obliquely, or rarely longitudinally. The orientation of the second division is also variable. Though the differentiation of suspensor and embryo proper occurs early, some derivatives of the terminal cell sometimes contribute to the suspensor. Provascular tissue “differentiates” after the initiation of the cotyledons. The radicle apical meristem originates subterminally, 5–10 cell layers from the juncture of the embryo proper and the suspensor. After germination, during early seedling establishment, radicle apical organization is of an unspecialized, columellate type. Vascular differentiation occurs before germination, and there are two loci of initial xylem differentiation: one in the hypocotyl and another in the median trace of the cotyledons. After germination, additional xylem differentiates de novo (without lateral or longitudinal continuity with already-mature vessels) inside the arcs of phloem in the hypocotyl, a pattern reported in few angiosperms. The cotyledonary node is one-trace, unilacunar.     

VASCULAR ORGANIZATION IN SHOOTS OF CACTACEAE. I. DEVELOPMENT AND MORPHOLOGY OF PRIMARY VASCULATURE IN PERESKIOIDEAE AND OPUNTIOIDEAE

Primary shoot vasculature has been studied for 31 species of Pereskioideae and Opuntioideae from serial transections and stained, decorticated shoot tips. The eustele of all species is interpreted as consisting of sympodia, one for each orthostichy. A sympodium is composed of a vertically continuous axial bundle from which arise leaf- and areole-trace bundles and, in many species, accessory bundles and bridges between axial bundles. Provascular strands for leaf traces and axial bundles are initiated acropetally and continuously within the residual meristem, but differentiation of procambium for areole traces and bridges is delayed until primordia form on axillary buds. The differentiation patterns of primary phloem and xylem are those typically found in other dicotyledons. In all species vascular supply for a leaf is principally derived from only one procambial bundle that arises from axial bundles, whereas traces from two axial bundles supply the axillary bud. Two structural patterns of primary vasculature are found in the species examined. In four species of Pereskia that possess the least specialized wood in the stem, primary vascular systems are open, and leaf traces are mostly multipartite, arising from one axial bundle. In other Pereskioideae and Opuntioideae the vascular systems are closed through a bridge at each node that arises near the base of each leaf, and leaf traces are generally bipartite or single. Vascular systems in Pereskiopsis are relatively simple as compared to the complex vasculature of Opuntia, in which a vascular network is formed at each node by fusion of two sympodia and a leaf trace with areole traces and numerous accessory bundles. Variations in nodal structure correlate well with differences in external shoot morphology. Previous reports that cacti have typical 2-trace, unilacunar nodal structure are probably incorrect. Pereskioideae and Opuntioideae have no additional medullary or cortical systems.     

PROCAMBIUM VS. CAMBIUM AND PROTOXYLEM VS. METAXYLEM IN POPULUS DELTOIDES SEEDLINGS

The concept of a procambium-cambium continuum was examined in Populus deltoides by following its development in serially sectioned bud and stem tissues. As in other species, the term cambium is used to refer to that part of the continuum associated with the formation of secondary vascular tissues; i.e., with secondary growth. However, that part of the continuum associated with the formation of primary vascular tissues is subdivided to facilitate interpretation of the consecutive stages of primary xylem differentiation. Thus, the procambium as envisioned by other authors is subdivided into procambium, initiating layer, and metacambium, all of which develop acropetally and in complete continuity. The procambium is derived from the residual meristem in the form of acropetally developing strands and traces. The initiating layer is represented by the first, tangentially separated, periclinal divisions that delineate the position of the prospective cambium. The metacambium is a later stage during which additional periclinally dividing cells unite the initiating layer into a tangentially continuous meristem within a trace bundle. After establishment of the initiating layer, the procambial trace is completely phloem dominated. Protoxylem differentiation begins in an originating center at the base of the leaf primordium and it progresses basipetally to form the protoxylem pole. Cells of the initiating layer do not contribute to the formation of either protoxylem or protophloem. However, those cells of the initiating layer directly opposite the protoxylem pole divide precociously and later differentiate to metaxylem, thus forming a radial file of protoxylem-metaxylem elements. Protoxylem elements of lateral traces are longitudinally continuous with the protoxylem of their parent traces, whereas those of a central trace are longitudinally continuous with the metaxylem of its parent trace. Metaxylem is formed later than protoxylem and it is derived from the metacambium. Metaxylem does not form a continuous system with protoxylem of the same trace because of the different temporal and spatial origins of the two kinds of xylem. Rather, metaxylem is longitudinally continuous with secondary xylem of older traces below. An attempt was made to determine the functional significance of the pattern of protoxylem and metaxylem differentiation in relation to primary and secondary plant development.     

THE DEVELOPMENTAL ANATOMY OF OPUNTIA BASILARIS. I. EMBRYO,ROOT, AND TRANSITION ZONE

The large seeds of Opuntia basilaris Engelm. & Bigel. show an unusually high percentage of germination, followed by a rapid development of the seedling during the first 30 days of growth. The primary root has six xylem arms alternating with six phloem poles around a large central pith. Development of metaxylem opposite each of the primary phloem poles results in the formation of eight collateral bundles. Secondary and tertiary roots have four xylem and phloem poles with xylem developing to the center of the stele. The transition zone is characterized by a gradual disappearance of all but two of the primary xylem arms of the root. Metaxylem development in the central portion of the transition zone interconnects the protoxylem poles forming a primary xylem cylinder around the central pith. The collateral bundles pass through the transition zone essentially without change.     

VARIATION IN THE ORGANIZATION OF THE STOMATAL COMPLEX IN THE LEAF EPIDERMIS OF MONOCOTYLEDONS AND ITS BEARING ON THEIR PHYLOGENY

Stebbins , G. L., and G. S. Khush . (U. California, Davis.) Variation in the organization of the stomatal complex in the leaf epidermis of monocotyledons and its bearing on their phylogeny. Amer. Jour. Bot. 48(1): 51–59. Illus. 1961.—Using macerated pieces of epidermal tissue from living plants and herbarium specimens, stomatal complexes of 192 species belonging to 49 families of monocotyledons were studied. Four categories are recognized, 2 with 4 or more subsidiary cells surrounding the guard cells, 1 with 2 subsidiaries, and 1 with none. Development of the 2-subsidiary type, studied in acetocarmine preparations of Juncus effusus and Sagittaria montevidensis, resembles that in Gramineae previously described. No correlation was found between type of stomatal complex and either leaf shape or type of xylem vessel, but some correlation exists between this character and type of seed germination, vascular anatomy of seedling, growth habit of mature plant, and geographic distribution. Types with 4 or more subsidiaries are most commonly phanerophytes with tropical distribution, many vascular bundles in the cotyledon, and hypogeal germination. Complexes with 2 subsidiaries occur in many families of a diverse nature, but the most primitive plants with this type are hydrophytes or helophytes with tropical or temperate distribution, 1 vascular bundle in the cotyledon, and epigeal germination. Stomatal complexes without subsidiaries are almost confined to the Liliales and their more specialized derivatives. These plants are predominantly geophytes with temperate or tropical distribution, 2 bundles in the cotyledon, and epigeal germination. Reasons are advanced for suggesting that the type with many subsidiaries is the most primitive and the other 2 types have been derived from it independently by reduction of the number of subsidiary cells.     

Light enhances differentiation of the vascular system in the fruit of Actinidia deliciosa

Light is recognized as crucial in determining high quality of fleshy fruits, for example, kiwifruit [Actinidia deliciosa var. deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson]. Among the possible mechanisms through which light improves the quality of kiwifruit berry, there may be a direct morphogenic role on the differentiation of the fruit's vascular system, though this has not yet been investigated. The present study's aim was to determine (1) whether light positively affects the differentiation of the vascular system of the fruit and/or the pedicel, and, if so, (2) which component (xylem, phloem, or both) is more affected, and (3) in which period of the berry's development the improvement of the vascular differentiation (if any) occurs. To this end, fruit morphogenesis of kiwifruit was studied in two developmental environments (i.e., in full sunlight and in paper bags that reduced the full sunlight to 10%), and in two phases of fruit development (i.e., 1 and 5 months [harvest] after anthesis). During the growth period, the type of environment did not affect the differentiation pattern of the vascular system in the three types of bundles present in the fruit. However, in comparison with shade, light improved the vasculature in the fruit pericarp and pedicel, inducing a consistently higher extent of the xylary component in the main bundles of the fruit and pedicel, principally due to an increase in the number of xylem elements. The phloic component was also increased by light, but to a much lesser extent than that of the xylary. During the entire period of development, light-grown fruits contained higher concentrations of calcium and magnesium, as compared with shade-grown fruits. In conclusion, in the berry of Actinidia deliciosa, light enhances the differentiation of the vascular system, in particular the xylary component. The hypothesis that fruit quality is improved through a more efficient translocation of specific mineral nutrients (e.g., calcium) via the xylem is presented.     

SEASONAL DEVELOPMENT OF THE SECONDARY PHLOEM IN PINUS

Functional sieve cells are present at all times in the secondary phloem of Pinus banksiana Lamb., P. resinosa Ait., and P. strobus L. With regard to a given year's growth increment, all but the last-formed sieve cells (2-4 layers) cease functioning the same season they are derived from the cambium. The former overwinter and remain functional until new sieve cells differentiate in spring. Toward the end of March undifferentiated cells in the outer margin of the cambial zone begin to differentiate into sieve cells. About a week later, cambial activity (cell division) commences. All early phloem is produced by early May before new xylem differentiation begins. Most sieve cells are differentiated by late August, but a few not until late September. Cessation of function begins in late May or June with formation of definitive callose on sieve areas of the sieve cells which overwintered and continues slowly to sieve cells of the current season's early phloem. By mid-December all but the last-formed sieve cells (i.e., those which will overwinter in a functional state) are devoid of contents. Phloem differentiation precedes xylem differentiation by approximately 1 1/2 months. Xylem and phloem production cease more or less simultaneously in August, xylem and phloem differentiation in September.     

MORPHOLOGICAL STUDIES OF THE NYMPHAEACEAE SENSU LATO. XVII. FLORAL ANATOMY OF ONDINEA PURPUREA SUBSPECIES PURPUREA (NYMPHAEACEAE)

Classification and phylogeny of the Nymphaeaceae are unresolved. This study provides floral anatomical data that will assist in elucidating generic interrelationships and systematic relationships to other taxa of angiosperms. The floral anatomy of Ondinea purpurea den Hartog subsp. purpurea has been examined utilizing light microscopy. The peduncle possesses stelar vascular bundle complexes and cortical vascular bundles. Cortical bundles terminate within the peduncle. Each bundle complex consists of 2 collateral bundles on the same radius, the inner bundle inverted; 2 protoxylary lacunae occur yet differ in structure and function. Progressing acropetally, the inner xylary lacunae become discrete mesarch strands surrounded centrifugally by a vascular cylinder formed by divisions and anastomosing of the bundle complexes. Together these become the massive receptacular vascular plexus. The plexus provides collateral traces to the floral organs. Each sepal receives 3 traces that separate from the plexus as 1–3 lateral traces. Petals are absent and no vestigial petal traces have been observed. Distally, the plexus forms several large strands of connate gynoecial and androecial traces termed the principal vascular bundles (PVBs). Ventral veins separate from the PVBs and the latter extend acropetally through the outer ovary wall. Branches of the ventrals and PVBs contribute to septal vascular reticula from which each ovule is supplied by one vascular bundle. Each stamen receives 1 trace from branches of the PVBs. The ventrals and PVBs terminate within the carpellary lobes. A comparative anatomical study is offered that supports the inclusion of Ondinea in the Nymphaeaceae sensu stricto.     

The Initiation and Development of Secondary Xylem in Axillary Branches of Populus deltoides

The initiation of secondary xylem in elongating axillary branchesof Populus deltoides Bartr. ex Marsh. is independent of thatin the main stem. Although secondary xylem differentiates acropetallyin the main stem, it does not differentiate from the stem intothe axillary branch. Secondary xylem is usually initiated ininternode 4 (occasionally 3) of the axillary branch, and fromthis site it develops both acropetally in the elongating branchand basipetally toward the main stem. Secondary vessel differentiationalways precedes fibre differentiation. Although secondary xylemdifferentiates in internodes that have ceased elongation, itdifferentiates first in traces of the vascular cylinder servingrapidly expanding and maturing foliage leaves. As younger leaveson the branch expand and mature, secondary xylem differentiatesin their traces eventually producing a complete secondary vascularcylinder. Scale leaves do not initiate secondary xylem independentlyin their traces; they are activated by adjacent traces in thevascular cylinder serving foliage leaves. Once established,the primary-secondary vascular transition zone advances acropetallyin a branch just as it does in the main stem. Populus deltoides Bartr. ex Marsh., cottonwood, axillary branches, secondary xylem, plastochron index, post-dormancy development, xylem.     

Primary Phloem Development in the Shoot Apex of Rhizophora mangle L. (Rhizophoraceae)

The primary phloem in the shoot apex of the mangrove Rhizophora mangle L. is largely confined to the comparatively condensed area between the first three leaf pairs. The main extension zone, surrounded by the stipular sheath of the third leaf pair, contains vascular bundles arranged in a procambial ring and characterized by a well-developed primary phloem and a less advanced xylem. The phloem consists of a great number of sieve elements, an equal number of associated companion cells, and a few phloem-parenchyma cells. The differentiation of the sieve-element protoplast (with e.g., chromatolytic nuclear degeneration, loss of the vacuole and most organelles) proceeds largely according to a well-known pattern. Their P-type plastids, however, form their protein crystals rather late and therefore cannot be used as an early cell marker. Lateral sieve-element walls are distinct from other wall parts and walls of other cells by their heavy nacreous thickenings, the formation of which is shown to be strictly correlated with the occurrence and orderly arrangement of cortical microtubules.     

Wound-induced and naturally occurring regenerative differentiation of xylem in Zea mays L.

The regenerative differentiation of xylem, both around a wound in the stem and at the root junction was studied in seedlings of maize. The regeneration of vessels around a wound was very small, being limited to the very young internodes and sharply declining basipetally. There were more regenerative vessel elements and they differentiated faster above the wound than below it. The regenerative vessel elements around the wound were characterized by helical or annular pattern of secondary wall thickenings. Wounding also resulted in the development of additional vascular anastomoses in the leaf immediately above the wound, and in differentiation of discontinuous vessels in adjacent bundles. Regenerative vessel elements were very common where the adventitious roots connected with the stem internodes, and exhibited pitted or reticulated secondary wall thickenings.