ESTABLISHMENT OF THE VASCULAR SYSTEM IN SEEDLINGS OF POPULUS DELTOIDES BARTR.

Seven seedlings ranging from 1 to 25 days old were embedded in Spurr's resin and serially sectioned at 1–2 μm. Sectioning extended from well above the apex downward to the hypocotyl base in the 1–day seedlings and to varying levels in the hypocotyl in the older seedlings. Procambial development was analyzed in its entirety for each seedling, and a composite two-dimensional diagram representing the procambial system of a 25-day-old seedling was prepared. Each cotyledon was served by a double-trace, one-half of which was derived from each of two embryonic bundles. The central traces serving the four primary leaves were in turn derived from the four cotyledonary bundles comprising the double traces. The procambial system serving the cotyledons and the four primary leaves approximated a decussate phyllotaxy. The central traces serving the secondary leaves were arranged in a helix that conformed at first to a 1/3 and then to a 2/5 phyllotaxy. Transitions to higher phyllotactic orders were systematic and reproducible, and they occurred in an orderly sequence in both the central and lateral leaf traces. The manner in which leaf traces diverged from parent traces to serve new leaf primordia provided for vascular redundancy. Thus, the entire vascular system was integrated into a highly functional whole.     

DEVELOPMENT AND ORGANIZATION OF THE SECONDARY VESSEL SYSTEM IN POPULUS GRANDIDENTATA

The apical 22 cm of a dormant, first-year sprout of Populus grandidentata was sectioned serially, and the primary and secondary xylem systems were studied microscopically and graphically reconstructed. A total of 15 nodes was present on the mature stem and 14 foliar primordia in the dormant bud. The vascular traces in the lower portion of the mature stem conformed to a 2/5 phyllotaxy while those of the upper portion and within the dormant bud conformed to a 3/8 phyllotaxy. The 2/5 to 3/8 phyllotactic transition occurred in an extremely precise and systematic two-step pattern: (1) The lateral traces shifted to a new point of origin on the parent central trace, and (2) three new central traces were initiated in sequence by divergences from left-traces. Metaxylem, when followed downward, conformed to the arrangement of the procambial trace system only within one orthostichy. Below this point, the metaxylem components of lateral traces physically separated from those of the protoxylem and continued downward on a new course. Metaxylem vessels produced by the trace cambium originated from a postulated vessel-generating center at the stem-petiole junction. Each metaxylem vessel developing basipetally through the primary body was continuous with a secondary vessel developing basipetally in the secondary body. Because secondary development closed the vascular cylinder, vessels originating from developing leaves or primordia situated at higher levels in the shoot were displaced radially outward when they entered the secondary xyelm. The distribution of vessels in the secondary xylem can therefore be accounted for by a knowledge of the production and distribution of metaxylem vessels in the primary body.     

VASCULAR ANATOMY OF THE NODAL REGION IN POPULUS DELTOIDES BARTR

The ontogeny of vascular bundles in the nodal region of Populus deltoides Bartr. was examined to understand more thoroughly the structure-function relation between leaf and stem. Three vascular traces from the stem independently enter each leaf in the nodal region. At the base of each developing leaf a region was observed in which both bundle size and vascular development was reduced; this region was referred to as the constricted zone. The constricted zone was described quantitatively at 13 locations within the nodal region of a leaf at LPI 5 by determining the number of metaxylem vessels and the total metaxylem vessel area in each of the three leaf traces. A plot of these data showed a distinct minimum value for total metaxylem vessel area within the constricted zone of each trace; the location of this minimum value was referred to as the constriction plane. Each vascular bundle within the nodal region is composed of independent subsidiary bundles that originate within the constricted zone. These bundles provide a direct connection between the leaf lamina and the stem. The node was defined anatomically on the basis of the ontogenetic development of the subsidiary bundles. The node began at the initial exit point of the central trace from the vascular cylinder and extended distally to the constriction plane. This definition allowed us to quantify the limits of each node. The origin of the initiating layer and metacambium was also examined within the nodal region. These precursors of the cambium develop continuously and acropetally from the stem into the leaf. The developmental implications of the constricted zone and the metacambium within the nodal region are discussed with respect to wood formation.     

VASCULAR DEVELOPMENT IN THE TRANSITION REGION OF POPULUS DELTOIDES BARTR. EX MARSH. SEEDLINGS

Seed and developing seedlings of Populus deltoides Bartr. ex Marsh., through 2 wk old, were embedded in Paraplast or Spurr's resin and serially sectioned at 7 or 2 μm, respectively, to examine vascular development in the root-hypocotyl transition region. The procambial template is well established in the dormant embryo. Protophloem forms a continuous system connecting the root with the cotyledons but protoxylem is restricted primarily to the cotyledons. With the onset of germination there is rapid elongation of both the root and hypocotyl. The basipetal progression of protoxylem from an initiating center at the cotyledonary node is correlated with the establishment of a second initiating center in the collet at the base of the hypocotyl. The relation between procambium, initiating layer, and metacambium, and their derivatives is similar to that described for the cottonwood shoot. The primary vascular system of cottonwood should be considered a single unit, both conceptually and functionally. The vascular pattern of the root may be, in part, determined by a basipetal stimulus from the cotyledonary node.     

LEAF DEVELOPMENT AND PRIMARY VASCULAR ORGANIZATION IN SHOOTS OF ANISOPHYLLEA DISTICH A

Anisophyllea disticha is characterized by strong shoot dimorphism. Orthotropic shoots with helically arranged scale leaves produce tiers of plagiotropic shoots, while plagiotropic shoots are anisophyllous and bear dorsal scale and ventral foliage leaves arranged in a unique tetrastichous system. In this study we compare the patterns of leaf development and primary vascular organization in the two types of shoots. Orthotropic shoots have an open vascular system with five sympodia. Expansion of orthotropic shoot scale leaves occurs from P1 to P10–12, and leaf tissues mature precociously. Plagiotropic shoots have a closed vascular system with six sympodia. Leaves in ventral and dorsal orthostichies do not differ significantly in size until ca. P15, but ventral leaves are distinct histologically from the second node in an orthostichy, P4–6. Ventral foliage leaves have a diffuse plate meristem, and leaf expansion continues until ca. P30. Differentiation of ventral and dorsal leaf trace procambium parallels the divergent patterns of leaf expansion. These observations demonstrate the strong correlation among shoot symmetry, leaf development, and vascular differentiation within dimorphic shoots of one species.     

STRUCTURAL CHANGES IN POPULUS DELTOIDES TERMINAL BUDS AND IN THE VASCULAR TRANSITION ZONE OF THE STEMS DURING DORMANCY INDUCTION

Morphological and anatomical changes in shoots of vigorously growing cottonwood plants (Populus deltoides Bartr.) were studied during dormancy induction in 8-hr short days (SD) and in control plants grown in 18-hr long days (LD). Pronounced structural changes occurred in terminal buds after 4 wk and full dormancy was achieved in 7 wk of SD. Leaf expansion ceased after 5 wk of SD as foliage leaves matured to the terminal bud base at leaf plastochron index 0 (LPI 0). Within the bud, total leaf length (lamina + petiole) decreased and stipule length increased progressively each week; thus, the ratio total leaf length/stipule length decreased rapidly, especially at the position of incipient bud-scale leaves LPI - 1 and LPI - 2. These bud-scale leaves were fully developed by wk 6 and were derived from enlarged stipules and aborted laminae. The full complement of primordia within the bud at the start of SD eventually matured as foliage leaves and the first bud-scale leaf (LPI - 1) was initiated immediately following transfer to SD. Acropetal advance of the primary-secondary vascular transition zone (TZ) was associated with leaf maturation. However, it did not advance throughout the entire vascular cylinder as in LD, but only in those leaf traces serving mature leaves beneath the terminal bud. In both LD and SD treatments the same linear relationship was maintained between LPI of the TZ and LPI of the most recently matured leaf; both parameters simultaneously increased in LD and decreased in SD. Thus, the relationship between leaf maturation and advance of the TZ was maintained irrespective of environment.     

ORGANIZATION AND ONTOGENY OF THE VASCULAR SYSTEM IN THE PETIOLE OF EASTERN COTTONWOOD

The topologic arrangement of petiolar bundles varies within the length of the cottonwood petiole. Each petiolar bundle is formed by the subdivision and aggregation of acropetally differentiating subsidiary bundles in a predictable pattern. The subsidiary bundles provide vascular continuity between the stem and specific portions of the leaf lamina. Spot-labeling of individual veins with ¹⁴CO₂, freeze substitution, and microautoradiography were used to establish the relation between the secondary veins of the lamina and the vasculature of the petiole. Within the petiole vasculature each subsidiary bundle was continuous with a specific portion of the lamina and seemed to have a separate function. Subsidiary bundles continuous with the central leaf trace were closely related functionally to the tip region of the lamina, while the subsidiary bundles continuous with the lateral leaf traces were functionally related to the middle and basal portions of the lamina.     

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.     

ONTOGENY OF MAJOR VEINS IN THE LAMINA OF POPULUS DELTOIDES BARTR

The ontogeny of the major venation in the lamina of Populus deltoides Bartr. leaves was investigated in relation to the development of original procambial bundles, subsidiary bundles, and their derivatives. Serial sections and clearings were used to show that the midrib region is a composite structure consisting of several independent vascular bundles, each of which eventually diverges into the lamina to become a secondary vein. The sequence of events in the ontogeny of major secondary veins is: (1) an original procambial strand develops acropetally and becomes the precursor of the first vascular bundle of the midrib region of the lamina, (2) ground tissue at the forefront of acropetally developing subsidiary procambial bundles differentiates in a wavelike continuum; meristematic regions precede the acropetally developing procambial bundles, (3) discrete subsidiary bundles differentiate in the meristematic regions as they advance acropetally, (4) subsidiary bundles diverge obliquely in the lamina margin giving rise to the secondary veins in a basipetal fashion, and (5) subsequent differentiation and maturation of the secondary veins occurs within the lamina. The original procambial bundles and first-formed subsidiary bundles become the secondary veins of the uppermost portions of the lamina, the next-formed subsidiary bundles become the secondary veins of the middle portions of the lamina, and the last-formed subsidiary bundles become the secondary veins of the lowermost portion of the lamina.     

THE ORGANIZATION OF THE VASCULAR SYSTEM IN THE STEMS OF THE NYMPHAEACEAE. III. VICTORIA AND EURYALE

Organization of the stem vascular system was analyzed in Victoria species and Euryale ferox. The stem vascular system consists of a number of concentrically-organized continuing axial stem bundles. At the node each leaf is supplied with a root trace, two lateral leaf traces, and a median leaf trace. A peduncle fusion bundle is also present at each node. The peduncle fusion bundle supplies vascular tissue to the median leaf trace and to the peduncle trace. Flowers are nonmedian axillary but have specific vascular, spatial, and developmental relationships to leaves in a manner that resembles the genus Nymphaea. On the basis of the analysis of the stem vascular system, Victoria and Euryale are more similar to each other than to Nymphaea. However, the vascular system in Victoria and Euryale is similar enough to Nymphaea to suggest that Nymphaea, Victoria, and Euryale form a natural taxon of unique angiosperms. The organization of the stem vascular system in Victoria and Euryale is dicotyledonous.     

THE RELATIONSHIP OF GELATINOUS FIBERS TO WOOD STRUCTURE IN EASTERN COTTONWOOD (POPULUS DELTOIDES)

Wood samples from Populus deltoides Marsh. were used to investigate the relationship of gelatinous fibers to the size and number of other wood elements. An increase in the amount of gelatinous fibers was related to a decrease in diameter and an increase in wall thickness of non-gelatinous fibers. The relative sizes of rays, vessels, and fibers were inter-related, but all of these wood elements decreased in size with an increase in the amount of gelatinous fibers. Apparently environmental conditions controlling the differentiation of gelatinous fibers also influence the development of associated wood elements. Thus, the adverse physical properties of reaction wood must be attributed to all of these structural differences in the wood and not merely to the presence of large numbers of gelatinous fibers.     

AXILLARY BUD DEVELOPMENT IN POPULUS DELTOIDES. I. ORIGIN AND EARLY ONTOGENY

Origin and early development of axillary buds on the apical shoot of a young Populus deltoides plant were investigated. The ontogenetic sequence of axillary buds extended from LPI –1 (Leaf Plastochron Index) near the apical bud base to LPI –11, the fifth primordium below the bud apex. Two original bud traces diverged from the central (C) trace of the axillant leaf and developed acropetally. During their acropetal traverse the original bud traces gave rise to three pairs of scale traces. All subsequent scale traces, and later the foliar traces, were derived by divergencies from the first two pairs of scale traces. Just before the bud vascular system separated from that of the main axis, a third pair of traces diverged from the original bud traces to vascularize the adaxial scale. Concomitantly, the original bud traces were inflected toward the main vascular cylinder where they developed acropetally and eventually merged with the left lateral trace of the leaf primordium situated three nodes above the axillant leaf; they did not participate in further vascularization of the bud. During early ontogeny a shell zone formed concurrent with initiation of the original bud traces and lay interjacent to them. The shell zone defined the position of the cleavage plane that formed between the axillary bud and the main axis. The axillary bud apex first appeared in the region bounded laterally by the original bud traces and adaxially by the shell zone. Following divergence of the main prophyll traces from the original bud traces, the apex assumed a new position intermediate to the prophyll traces. Ontogenetic development suggested that the axillary bud apex may have been initiated by the acropetally developing original bud traces under the influence of stimuli arising in more mature vegetative organs below.     

AXILLARY BUD DEVELOPMENT IN POPULUS DELTOIDES. II. LATE ONTOGENY AND VASCULARIZATION

A nearly mature axillary bud of Populus deltoides was embedded in epoxy and serially sectioned at 6 μm. Sectioning extended from the cataphyll tips to a level in the subtending internode about 6 mm below the bud base. Vascular development was followed through the serial microsections and the vascular system was mapped in its entirety from initiation of the original bud traces to termination of the last recognizable leaf trace beneath the bud apex. Each vascular trace was identified as to its origin, its termination within a foliar organ, and its relation to other traces comprising the bud vascular cylinder. Analysis of these data confirmed the procambial patterns found in Part I of this study. Two original bud traces that diverged from the central trace of the axillant leaf gave rise to two pairs of scale traces in quick succession, and these scale traces become the progenitors of all subsequent vascular traces that were perpetuated within the bud. Just before the bud vascular system separated from that of the stem, a third pair of scale traces diverged from the original bud traces; the latter then receded toward the stem to eventually merge with its vasculature. The third pair of scale traces produced a horizontal vascular connection between stem and bud before terminating in the adaxial cataphyll. The vascular system at first conformed with a ½ vascular phyllotaxy when the original bud traces were initiated, progressed through a ⅓ vascular phyllotaxy in the scale trace system, and terminated at the time of sampling with a ⅖ vascular phyllotaxy in the foliage leaf primordia.     

LEAF DEVELOPMENT,PHOTOSYNTHESIS, AND C14 DISTRIBUTION IN POPULUS DELTOIDES SEEDLINGS

Rates of net photosynthesis and dark respiration and distribution of C¹⁴ from selected leaves were determined for young cottonwood (Populus deltoides) trees at different stages of development. Four series of five trees—one series for each of four treated leaf positions—were included in the study. Maximum C¹⁴ export occurred when a leaf had just attained maximum size. Lower stem leaves reached maturity quickly and began exporting photosynthate when demands of the young seedling were high. Leaves at higher stem positions matured more slowly, but senescence was also delayed so their effective export life was prolonged. Translocation from a newly exporting leaf was primarily upward to developing leaves and the apex. As a leaf at any one position aged, the translocation pattern gradually shifted from upward to bi-directional and finally to a predominantly downward direction. Photosynthate translocated downward was incorporated into stem wood and roots. Maximum photosynthetic efficiency coincided with the downward shift of C¹⁴ export. Thereafter, net photosynthesis began to decline, at first slowly and then more rapidly. The patterns of photosynthesis, respiration, and C¹⁴ export associated with leaf age all varied according to leaf position on the stem.     

PHYLLOTAXIS AND VASCULAR ORGANIZATION OF THE CARPELS IN MICHELIA FUSCATA

Tucker , Shirley C. (U. Minnesota, Minneapolis.) Phyllotaxis and vascular organization of the carpels in Michelia fuscata. Amer. Jour. Bot. 48(1): 60–71. Illus. 1961.—Phyllotaxis pattern and vascular organization are closely related in the floral receptacle of Michelia fuscata (Magnoliaceae). The carpels arise in a spiral or helix. They are initiated alternately along each of 7, 8 or 10 helical parastichies according to a complex repetitive sequence. The pattern of the dorsal carpellary trace fusions is orderly for each of the 10 flowers investigated. The dorsal carpellary traces in each parastichy diverge from the same vascular sympodium. Among flowers one finds differing numbers of parastichies, differing angles of divergence, and varying sequences of parastichies which reflect the order of carpel initiation. The angle of divergence, although consistent for any 1 parastichy in a flower, can vary greatly between parastichies. The nature and importance of the organizers which determine appendage position at the apical meristem are considered. Changes in apical size, configuration, and activity are shown to be related to phyllotaxis.     

THE ORGANIZATION OF THE VASCULAR SYSTEM IN THE STEMS OF THE NYMPHAEACEAE. I. NYMPHAEA SUBGENERA CASTALIA AND HYDROCALLIS

The vascular system in the stems of Nymphaea odorata and N. mexicana subgenus Castalia, and N. blanda subgenus Hydrocallis consists of continuing axial stem bundles with eight being the usual number. The stem bundles are concentric and xylem maturation is mesarch. Xylem elements consist of tracheids with spirally or weakly reticulated secondary wall thickenings. The phloem is made up of companion cells and short sieve tube members with simple sieve plates that are nearly transverse. At the node each leaf is supplied with two lateral leaf traces and a median leaf trace. A root trace is also present and supplies a series of adventitious roots borne on the leaf base. Flowers and vegetative buds develop directly from the apical meristem and occupy leaf sites in a single genetic spiral. Each flower or vegetative bud is related to a leaf through specific spatial and vascular association. The related leaf is separated from the related flower by three members of the genetic spiral and occupies an adjacent orthostichy. Vascular tissue for the related flower arises from the inner surfaces of the four stem bundles supplying leaf traces to the related leaf and extends through the pith to the flower or vegetative bud via a peduncle fusion bundle. The vascular system organization in the investigated species of Castalia and Hydrocallis is not typically monocotyledonous or dicotyledonous, nor can it be considered transitional between them. The ontogeny of the vascular system is similar to typical dicotyledons and the investigated species of Nymphaea can, therefore, be considered to represent highly specialized and modified dicotyledons.     

DEVELOPMENT AND STRUCTURE OF THE VASCULAR CONNECTION BETWEEN THE PRIMARY AND LATERAL ROOT OF LYCOPERSICON ESCULENTUM

The primary xylem connection between the diarch parent root and the diarch lateral root was derived from the pericycle and stelar parenchyma. Early in lateral root development stelar parenchyma that was positioned between the parent xylem and the primordium divided transversely. These transverse divisions produced a plate of cells, most of which subsequently differentiated into vessel element connectors. After emergence of the lateral root, xylem maturation began in the stelar vessel element connectors and maturation proceeded acropetally into the lateral root. Protoxylem of the lateral root was connected to the metaxylem of the parent root via stelar vessel element connectors. The circular phloem connection was pericyclic in origin. Axial phloem connections which vascularized the lateral root were established with sieve tube elements of both parent phloem poles. Maturation of the phloem connection occurred prior to lateral root emergence. Transaxial phloem, positioned in arches above and below the lateral root vascular cylinder, was derived from the pericycle; and each arch consisted of three to four sieve tube elements. No transfer cells were found in the transaxial phloem.