A SEASONAL STUDY OF THE VEGETATIVE SHOOT APEX AND THE PATTERN OF PITH DEVELOPMENT IN HIBISCUS SYRIACUS

Tolbert , Robert J. (West Virginia U., Morgantown.) A seasonal study of the vegetative shoot apex and the pattern of pith development in Hibiscus syriacus. Amer. Jour. Bot. 48(3): 249–255. Illus. 1961.—The shoot apex of Hibiscus syriacus L. is described as having a cytohistological zonation superimposed on a tunica-corpus configuration. The apex is flat-topped or may have a saddle-back or concave appearance as seen in median longitudinal section. The metrameristem, consisting of the tunica and corpus initials, is comprised of large, light-staining, vacuolate cells that have thick cell walls and exhibit much dark-staining intercellular substance. Surrounding the metrameristem is the flanking meristem, which is responsible for the outer layers of the shoot, and from which the leaf primordia arise. The pith rib meristem lies below the metrameristem and consists of files of cells that are responsible for the pith. There are no major seasonal changes in the structure of the apex during the yearly cycle. The pith displays a long-shoot type of development with the cells remaining in distinct files during the first flush of growth in the spring. As growth slows and internode elongation is gradually reduced, the pith displays the characteristic short-shoot type of development, consisting of a spongy tissue of rounded cells with many intercellular spaces and no distinct files of cells. A crown is differentiated across the top of the pith at the end of the growth period. This consists of a band of cells with thick, dark-staining cell walls, which separates by the apex from the last year's growth. In contrast to many gymnosperms, this crown is dispersed by renewed cell activity the following spring.     

THE SHOOT APEX OF BOUGAINVILLEA SPECTABILIS

The shoot apex of Bougainvillea spectabilis consists of five zones: A two- or occasionally three-layered tunica, a central mother cell zone, a cambium-like zone, a rib meristem (central meristem), and a peripheral meristem. The presence of a cambium-like zone is somewhat unusual in the apex of vascular plants, having only been reported for a few taxa. In B. spectabilis the cambium-like zone is consistently present throughout the plastochron and all yearly seasonal periods.     

ONTOGENY OF THE INFLORESCENCE OF SAURURUS CERNUUS (SAURURACEAE)

The spicate inflorescence of Saururus cernuus L. (Saururaceae) results from the activity of an inflorescence apical meristem which produces 200–300 primordia in acropetal succession. The inflorescence apex arises by conversion of the terminal vegetative apex. During transition the apical meristem increases greatly in height and width and changes its cellular configuration from one of tunica-corpus to one of mantle (with two tunica layers) and core. Primordia are initiated by periclinal divisions in the subsurface layer. These are “common” primordia, each of which subsequently divides to produce a floral apex above and a bract primordium below. The bract later elongates so that the flower appears borne on the bract. All common primordia are formed by the time the inflorescence is about 4.4 mm long; the apical meristem ceases activity at this stage. As cessation approaches, cell divisions become rare in the apical meristem, and height and width of the meristem above the primordia diminish, as primordia continue to be initiated on the flanks. Cell differentiation proceeds acropetally into the apical meristem and reaches the summital tunica layers last of all. Solitary bracts are initiated just before apical cessation, but no imperfect or ebracteate flowers are produced in Saururus. The final event of meristem activity is hair formation by individual cells of the tunica at the summit, a feature not previously reported for apical meristems.     

Arrangement of cortical microtubules in the shoot apex of Vinca major L.

The arrangements of cortical microtubules (MTs) and of cellulose microfibrils in the median longitudinal cryosections of the vegetative shoot apex of Vinca major L., were examined by immunofluorescence microscopy and polarizing microscopy, respectively. The arrangement of MTs was different in the various regions of the apex: the MTs tended to be arranged anticlinally in tunica cells, randomly in corpus cells, and transversely in cells of the rib meristem. However, in the inner layers of the tunica in the flank region of the apex, cells with periclinal, oblique or random arrangements of MTs were also observed. In leaf primordia, MTs were arranged anticlinally in cells of the superficial layers and almost randomly in the inner cells. Polarizing microscopy of cell walls showed that the arrangement of cellulose microfibrils was anticlinal in tunica cells, random in corpus cells, and transverse in cells of the rib meristem; thus, the patterns of arrangement of microfibrils were the same as those of MTs in the respective regions. These results indicate that the different patterns of arrangement of MTs and microfibrils result in specific patterns of expansion in the three regions. These differences may be necessary to maintain the organization of the tissues in the shoot apex.Abbreviations MT(s) microtubule(s) - lp length of the youngest leaf primordium     

TENDRILS OF PASSIFLORA FOETIDA: HISTOGENESIS AND MORPHOLOGY

Passiflora foetida bears an unbranched tendril, one or two laterally situated flowers, and one accessory vegetative bud in the axil of each leaf. The vegetative shoot apex has a single-layered tunica and an inner corpus. The degree of stratification in the peripheral meristem, the discreteness of the central meristem, and its centric and acentric position in the shoot apex are important plastochronic features. The procambium of the lateral leaf trace is close to the site of stipule initiation. The main axillary bud differentiates at the second node below the shoot apex. Adaxial to the bud 1–3 layers of cells form a shell-zone delimiting the bud meristem from the surrounding cells. A group of cells of the bud meristem adjacent to the axis later differentiates as an accessory bud. A second accessory bud also develops from the main bud opposite the previous one. A bud complex then consists of two laterally placed accessory bud primordia and a centrally-situated tendril bud primordium. The two accessory bud primordia differentiate into floral branches. During this development the initiation of a third vegetative accessory bud occurs on the axis just above the insertion of the tendril. This accessory bud develops into a vegetative branch and does not arise from the tissue of the tendril and adjacent two floral buds. The trace of the tendril bud consists of two procambial strands. There is a single strand for the floral branch trace. The tendril primordium grows by marked meristematic activity of its apical region and general intercalary growth.     

COMPARISON OF SHOOT APEX DYNAMICS AND EARLY LEAF ONTOGENY IN TWO SPECIES OF SARACA (LEGUMINOSAE)

Shoot apices of Saraca indica produce adult leaves that have 4 to 6 pairs of leaflets, whereas those of S. bijuga usually have only 2 pairs. In both species one leaflet pair is found during the juvenile phase. Juvenility lasts many plastochrons in S. bijuga but is restricted to a few in S. indica. The shoot apical meristems of these two taxa are similar in structure, cell number, and cell size; however, the shoot apex of Saraca bijuga is slightly more stratified, having 2–3 tunica layers as opposed to 1–2 in S. indica. For most of the plastochron the apical meristem in both species is situated laterally at the base of the most recently formed leaf. A newly forming primordium and its internode shift the apical meristem upward unilaterally; the meristem passes through a brief apical dome stage and becomes positioned 180° from its origin at the beginning of the plastochron. Hence, there is a true pendulum meristem in both species. In S. bijuga the maximum length of the youngest leaf primordium, just prior to the formation of its successor, is twice that of S. indica. The internodes immediately below the shoot apex and the axillary buds develop more rapidly in S. bijuga than in S. indica. It is suggested that the bijugate leaf of S. bijuga represents a case of neoteny in plants.     

THE SHOOT APEX AND EARLY LEAF DEVELOPMENT IN CLEMATIS

Tepfer , Sanford S. (U. Oregon, Eugene.) The shoot apex and early leaf development in Clematis . Amer. Jour. Bot. 47 (8): 655–664. Illus. 1960.—The high-domed shoot apex comprises a 2-layered tunica and shallow corpus. The rib meristem at times extends to within 5 cells of the summit. The cells of tunica and corpus are uniform cytologically, distinguishable only by the orientation of division planes. No zonation is visible within the corpus. No evidence was found of the existence of a méristème d'attente; mitotic figures appear frequently in the central region of the tunica and corpus. Decussately arranged leaf primordia arise high on the flanks of the apex. Periclinal divisions in the inner tunica and outermost corpus layers mark the site of initiation. Details of the growth and early differentiation of the leaf primordia follow the usual pattern of buttress formation, growth through apical and subapical initials. Apical growth continues beyond the early stages of leaf ontogeny; the blade-forming marginal meristems do not appear until after leaflet primordia are formed. There are 5 primary leaflets, pinnately arranged. Each leaflet is 3- to 5-lobed. In primordium P₃ expansion of the adaxial-lateral margins occurs at the base, but not above. This marks the upper limits of the basal pair of lateral leaflets. In P₄ the upper limits of the upper lateral leaflets become demarcated in similar fashion.     

THE GROWTH OF THE STEM TIP OF KALANCHOËU CV. ‘BRILLIANT STAR’

Stein , Diana B. and o . L. Stein . (Montana State U., Missoula.) The growth of the stem tip of Kalanchoë cv. ‘Brilliant Star.‘ Amer. Jour. Bot. 47 (2) : 132—140. Illus. I960.–The purposes of this investigation were (1) to define as clearly as possible the events in the shoot apex and its immediate derivatives during the ontogeny of the shoot; and (2) to determine the changes which occur during the transition from a vegetative to a reproductive meristem. Rate of leaf production in Kalanchoë is basically constant. The rate of leaf growth subsequent to the early primordial state is, however, dependent on the age of the plant and on the environment in which the plant is grown. By keeping these factors constant a correlation can be demonstrated between the size of the youngest visible leaf and the microscopic primordia. Throughout its ontogeny the general architecture of the shoot apex remains essentially the same. Two tunica layers cover the corpus in the vegetative shoot apex, and even in the flowering meristem these 2 layers can be detected. The apex is essentially flat and blends into the adjacent leaf primordia early in the plastochron. About 10 days after flower induction has been started the apex changes its form to a dome, primarily by increased cell division. At the same time the rate of elongation of the youngest internodes increases thus placing the flowering stem tip atop an elongated stem. Axillary development is ultimately responsible for the development of a dichasium.     

EARLY HISTOGENESIS AND SEMIQUANTITATIVE HISTOCHEMISTRY OF LEAF INITIATION IN TRITICUM AESTIVUM

The shoot apex of Triticum aestivum cv. Ramona 50 was investigated histologically to describe cell lineages and events during leaf initiation. During histogenesis three periclinal divisions occurred in the first apical layer, with one or two divisions in the second apical layer. This sequence of cell divisions initially occurred in one region and spread laterally in both directions to encircle the meristem. Cells of the third apical layer were not involved in leaf histogenesis. Initially, young leaf primordia were produced from daughter cells of periclinal divisions in the two outer apical layers. Nuclear contents of protein, histone, and RNA in the shoot apex were evaluated as ratios to DNA by means of semiquantitative histochemistry. Daughter cells of periclinal divisions in the outer apical layer which produced the leaf primordia had higher histone/DNA ratios than cells of the remaining meristem. However, protein/DNA and RNA/DNA ratios were similar in both regions. Leaf initial cells had a higher ³H-thymidine labeling index, a higher RNA synthesis rate, and smaller nuclear volumes than cells of the residual apical meristem.     

CYTOKININ-ELICITED FORMATION OF THE PITH-RIB MERISTEM AND OTHER EFFECTS OF GROWTH REGULATORS ON THE MORPHOGENESIS OF ECHINOCEREUS (CACTACEAE) SEEDLING SHOOT APICAL MERISTEMS

Shoot apical meristems of Echinocereus engelmannii have only a tunica-corpus organization at germination, but the corpus rapidly develops central mother cells, a peripheral zone and a pith-rib meristem. The manner in which nutrition, darkness and various growth regulators at several concentrations and in several combinations affect the development of zonation was examined by growing derooted seedlings on agar which contained the nutrients or growth regulators. Benzylaminopurine was able to elicit the formation of the pith-rib meristem in an otherwise non-zonate corpus. Also, the rate of leaf initiation was greatly increased. Gibberellic acid severely inhibited the formation of corpus zones but had little effect on leaf initiation. Indoleacetic acid had no effects other than mild inhibition of zonation and a slight retardation of leaf initiation. Abscisic acid was strongly inhibitory. Sucrose only slightly increased the rate of leaf formation and did not affect apex size or zonation. To more closely examine the cytokinin-induced effects on the apical meristem, several growth regulators were applied in combination with the most effective concentration of cytokinin. Certain combinations were able to interfere with several of the cytokinin-induced responses, while other cytokinin-induced responses occurred even in the presence of high concentrations of these other growth regulators. Leaf initiation and meristem morphogenesis appeared to be remarkably stable and insensitive to the presence of most hormones except cytokinin and gibberellin.     

A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). I. SHOOT APICAL MERISTEMS AT GERMINATION

At germination the shoot apical meristems of Echinocereus engelmannii were discs with a volume of 666,000 μm³ and were composed of a unistratose tunica (volume: 260,000 μm³) and a corpus which was two cell-layers thick (volume: 406,000 μm³). Four days after germination the nucleus constituted 28.9% of the volume of the cell, and the vacuole constituted 24.5%. The mitochondria were 13.3% of the volume of the tunica cytoplasm, the chloroplasts 9.4%, and the dictyosomes only 1.2%. The endoplasmic reticulum was too sparse to be accurately measured. The organelles of the corpus were identical in size and shape to those of the tunica, but there were statistically significant differences in their cellular and cytoplasmic densities: the more distal corpus layer (C1) was less vacuolate (16.6% of the cell volume), and both corpus layers contained more chloroplasts, 12.0% of the cytoplasmic volume in C1 and 14.3% in the more proximal corpus layer (C2). During the first four days after germination there was a dramatic increase in the size of the central vacuole (e.g., from 15.4% to 24.5% in the tunica), and the mitochondria increased in density from 10.2% of the cytoplasmic volume to 13.3%. Chloroplast density also increased in all meristem layers, but the dictyosome density decreased, as much as a 30% loss in C2. There was also a highly significant reduction in the number of cisternae per dictyosome, from 5.47 to 4.77. Surprisingly, there was no change in heterochromatin: ca. 27% of the volume of the nuclei of all layers was heterochromatic at all stages studied. Thus, the organellar structure of corpus cells is distinctly different from that of tunica cells, and as the apical meristem becomes active after germination, the changes which occur are not uniform in the meristem but rather are zone-specific.     

FOLIAR ONTOGENY AND HISTOGENESIS IN MAGNOLIA GRANDIFLORA L. I. APICAL ORGANIZATION AND EARLY DEVELOPMENT

Foliar ontogeny of Magnolia grandiflora was studied to elucidate possible unique features of evergreen leaves and their development. The apex of Magnolia grandiflora is composed of a biseriate or triseriate tunica overlying a central initial zone, a peripheral zone and a pith rib meristem. Leaf primordia are initiated by periclinal divisions on the apical flank of the tunica in its second layer. This initiation and expansion is seasonal just as in related deciduous magnolias. Following leaf initiation, a foliar buttress is formed and the leaf base gradually extends around the apex. As growth continues, separation of the leaf blade primordium from the stipule proceeds by intensified anticlinal divisions in the surface and subsurface layers near the base. Marginal growth begins in the blade primordium when it reaches approximately 200 μm in height and results in the formation of two wing-like extensions, the lamina. This young blade remains in a conduplicately folded position next to the stipule until bud break.     

Ontogeny of Axillary and Accessory Buds in Clerodendrum phlomidis L.

Plastochronic changes in the vegetative shoot apex and originand development of axillary and accessory buds are studied. The flat shoot apex shows structural and dimensional changesin a plastochron. They are described in three phases, the pre-leafinitiation, the leaf initiation, and the post-leaf initiation.The youngest axillary bud meristem is identified near the axilat the second node when the subtending leaf primordium is 200–12µ long. The corpus of the bud meristem has a more activerole in bud development than has the tunica layers. The shellzone associated with a young bud meristem persists until thebud has attained the structural and functional attributes ofthe main shoot apex. It loses its histological identity by producingderivatives which merge with the ground tissue and procambialcells of bud traces. In a developing bud the provascular systemof the bud appears as an arc, a loop, or as a ring in transversesections at different levels. These configurations are composedof anastomosing procambial strands of bud trace and residualmeristem, both being differentiated from developing bud meristem.     

FLORAL ONTOGENY OF ILLICIUM FLORIDANUM,WITH EMPHASIS ON STAMEN AND CARPEL DEVELOPMENT

The ontogeny of the flower and fruit of Illicium floridanum Ellis, the Star Anise, was investigated. Each of 5 or 6 bracts in each mixed terminal bud subtends either a vegetative or floral bud. The solitary flowers occur in terminal or axillary positions. Each flower has 3–6 subtending bracteoles arranged in a clockwise helix. The flowers in our material have 24–28 tepals, 30–39 stamens, and usually 13 (rarely 19) uniovulate carpels. Tepals and stamens are initiated in a low-pitched helix; carpels later appear whorled, but arise successively at different levels on the apical flanks. The floral apex is high-convex in outline with a tunica-corpus configuration; it increases in height and width throughout initiation of the floral appendages. Tepals, stamens, and carpels are initiated by one to several periclinal divisions in the subsurface layers low on the apical flanks, augmented by cell divisions in the outer layers of the corpus. The carpel develops as a conduplicate structure with appressed, connivent margins. Procambium development of floral appendages is acropetal and continuous. Bracteoles, tepals, stamens and carpels are each supplied by 1 trace; the carpellary trace splits into a dorsal and an ascending ventral sympodium. The latter bifurcates to form 2 ventral bundles. The ovular bundle diverges from the ventral sympodium. Ovule initiation occurs in a median axillary position to the carpel, an unusual type of ovule initiation. The fruit vasculature is greatly amplified as the receptacle and follicles enlarge. After carpel initiation an apical residuum persists which is not vascularized; a plate meristem develops over its surface to produce a papillate structure.     

COMPARATIVE FOLIAR HISTOGENESIS IN ACORUS CALAMUS AND ITS BEARING ON THE PHYLLODE THEORY OF MONOCOTYLEDONOUS LEAVES

A comparative histogenetic investigation of the unifacial foliage leaves of Acorus calamus L. (Araceae; Pothoideae) was initiated for the purposes of: (1) re-evaluating the previous sympodial interpretation of unifacial leaf development; (2) comparing the mode of histogenesis with that of the phyllode of Acacia in a re-examination of the phyllode theory of monocotyledonous leaves; and (3) specifying the histogenetic mechanisms responsible for morphological divergence of the leaf of Acorus from dorsiventral leaves of other Araceae. Leaves in Acorus are initiated in an orthodistichous phyllotaxis from alternate positions on the bilaterally symmetrical apical meristem. During each plastochron the shoot apex proceeds through a regular rhythm of expansion and reduction related to leaf and axillary meristem initiation and regeneration. The shoot apex has a three- to four-layered tunica and subjacent corpus with a distinctive cytohistological zonation evident to varying degrees during all phases of the plastochron. Leaf initiation is by periclinal division in the second through fourth layers of the meristem. Following inception early growth of the leaf primordium is erect, involving apical and intercalary growth in length as well as marginal growth in circumference in the sheathing leaf base. Early maturation of the leaf apex into an attenuated tip marks the end of apical growth, and subsequent growth in length is largely basal and intercalary. Marked radial growth is evident early in development and initially is mediated by a very active adaxial meristem; the median flattening of this leaf is related to accentuated activity of this meristematic zone. Differentiation of the secondary midrib begins along the center of the leaf axis and proceeds in an acropetal direction. Correlated with this centralized zone of tissue specialization is the first appearance of procambium in the center of the leaf axis. Subsequent radial expansion of the flattened upper leaf zone is bidirectional, proceeding by intercalary meristematic activity at both sides of the central midrib. Procambial differentiation is continuous and acropetal, and provascular strands are initiated in pairs in both sides of the primordium from derivatives of intercalary meristems in the abaxial and adaxial wings of the leaf. Comparative investigation of foliar histogenesis in different populations of Acorus from Wisconsin and Iowa reveals different degrees of apical and adaxial meristematic activity in primordia of these two collections: leaves with marked adaxial growth exhibit delayed and reduced expression of apical growth, whereas primordia with marked apical growth show, correspondingly, reduced adaxial meristematic activity at equivalent stages of development. Such variations in leaf histogenesis are correlated with marked differences in adult leaf anatomy in the respective populations and explain the reasons for the sympodial interpretation of leaf morphogenesis in Acorus and unifacial organs of other genera by previous investigators. It is concluded that leaf development in Acorus resembles that of the Acacia phyllode, thereby confirming from a developmental viewpoint the homology of these organs. Comparison of development with leaves of other Araceae indicates that the modified form of the leaf of Acorus originates through the accentuation of adaxial and abaxial meristematic activity which is expressed only slightly in the more conventional dorsiventral leaf types in the family.     

The anatomy of the shoot apex ofHyoscyamus niger L. reversed from the generative to the vegetative state

A study was made of the anatomical structure of the shoot apices ofHyoscyamus niger L. in plants which were transferred from a long-day to a short-day regime after the initiation of the inflorescence. After a certain time these plants are reverted to the vegetative stage with the inhibition of the development of further flower buds and the renewed production of rosette leaves. The inflorescence apex consisted of a few superficial layers of cells and a corpus composed of slightly elongated cells. The anatomical structure of the apices which were reverted into the vegetative state resembled that of shoot apices in the intermediate stage. The apex had several layers of small cells, under which there was a group of small but irregularly arranged cells which passed into the rib meristem. The shoot apices of plants transferred from a long to a short-day regime at different time intervals after fulfilling the requirements of minimal photoperiodic induction thus, on the short day, display morphological and anatomical characteristics of various degrees of transition from generative to vegetative stage.     

PROBABILITY OF DIVISION OF CELLS IN THE EPIDERMIS OF THE PHLEUM ROOT

Erickson , Ralph O. (U. Pennsylvania, Philadelphia.) Probability of division of cells in the epidermis of the Phleum root. Amer. Jour. Bot. 48(3): 268–274. Illus. 1961.—Photomicrographic records of the growth of a Phleum root, made at R. H. Goodwin's laboratory, in which individual epidermal cells can be identified, have been analyzed to provide estimates of the probability that cells at various distances from the apex will divide. In the apical part of the meristem, from 0μ to about 100μ from the apex, all cells divide (prob. = 1.0). From about 100μ to 275μ, the probability of division falls progressively to 0.0. The relationship of these estimates to rates of cell division and elongation in the same root is discussed.     

THE INDUCTION OF FLOWERING IN NICOTIANA. I. MORPHOLOGICAL DEVELOPMENT OF THE APEX

The morphological changes which the stem apex undergoes in the course of photoperiodic induction are described and presented photographically in order to provide a basis for subsequent physiological investigations. In both a short-day Nicotiana (N. tabacum cultivar ‘Maryland Mammoth') and a long-day one (N. sylvestris), the vegetative meristem is narrow and flat. It becomes wider and dome-shaped when flower induction has become irreversible. The speed at which the various morphological steps succeed each other varies with the intensity of light during the day, especially in the case of N. sylvestris.     

STRUCTURE AND DEVELOPMENT OF THE PERIANTH IN DOWNINGIA BACIGALUPII

The structure and ontogeny of the calyx and corolla of Downingia bacigalupii Weiler (Campanulaceae; Lobelioideae) were investigated for the purpose of comparing perianth development with previous observations on the floral bract, as well as elucidating the mechanism of development of the zygomorphic, sympetalous corolla. Sepals are uni-traced with a palmate, reticulate venation. They have basal and apical hydathodes, as well as storage tracheids. Sepals show a reduction in size, venation and hydathode number when compared to the bract. The pentamerous, zygomorphic corolla is bilabiate, consisting of a three-lobed adaxial lip and a two-lobed abaxial lip connected by a short tubular region. The constituent petal lobes are also uni-traced and have a reticulate venation, resembling that of the sepal and bract, but lack storage tracheids and hydathodes. Sepals arise in an adaxial to abaxial succession and are initiated in the outer corpus layer of the floral apex. Expansion of the floral apex follows and is accompanied by the establishment of a second tunica layer. Sepals undergo apical, marginal, and intercalary growth accompanied by acropetal differentiation of procambium. The petals arise simultaneously and are initiated in the second tunica layer and the outer corpus cells. After initiation, the petals exhibit a period of apical and marginal growth followed by intercalary growth. Apical growth in petals is less protracted than in sepals, but plate meristem activity is more extensive. The free petal lobes become temporarily fused by an interlocking of marginal epidermal layers, but they separate at anthesis. Zonal growth beneath the originally free lobes forms the tube and lip regions of the sympetalous corolla. Zygomorphy is evident from the time of initiation of petals and is accentuated by later differential growth. Comparative observations of corolla ontogeny in autogamous species of Doumingia indicate that the reduced corollas in these taxa are derived by a simple process of neoteny.