A shoot meristem-like organ in animals; monopodial and sympodial growth in Hydrozoa

Thecate Hydrozoa produce stems from which polyps branch off. Similar to plants these stems form in two ways, either in a sympodial or in a monopodial type of growth. In the latter group a terminal organ develops which has similarities to a shoot apical meristem of higher plants: it elongates without a further differentiation. Similar to leaf formation in plants, thecate Hydrozoa produce polyps in a repetitive manner. This process continues during the whole life of the animal and has not yet been found to be limited by internal mechanisms. We studied the monopodially growing thecate Hydrozoon Dynamena pumila and suggest that the stem tip, the apical shoot meristem-like organ, is a polyp primordium hindered to develop into a polyp by the laterally developing polyps.     

Sympodial and monopodial branching inAcer (Aceraceae): Evolutionary trend and ecological implications

The evolutionary trend and its ecological implications in sympodial and monopodial branching patterns has been investigated in 20 JapaneseAcer spp. through comparison of shoot tip abortion and terminal bud formation. The genus is divided into two species groups according to its branching pattern, one (6 species) predominantly exhibiting sympodial branching with frequent monopodial branching in short shoots (sympodial species), and the other (14 species) exhibiting only monopodial branching (monopodial species). The early ontogeny of leaf and bud scales is described. Despite the difference in branching patterns, the bud scales of terminal buds are essentially the same in having a leaf base developed to function as a protecting organ. In all the sympodial species, during the abortion of a sympodium shoot tip, one or two pairs of primordia were found to occur on the apex, and later wither. These primordia resemble bud scales of terminal buds in their ontogeny and morphology, and appear to be rudimentary. It is suggested that a rudimentary terminal bud develops together with the establishment of sympodial branching, and that sympodial branching has originated from monopodial branching. Based on this proposed evolutionary trend, it is suggested thatAcer has moved from less shady habitats into shady habitats with monopodial branching (advantageous for vertical growth) changing into sympodial branching (advantageous for lateral spread).     

STRUCTURE AND DEVELOPMENT OF THE DIMORPHIC BRANCH SYSTEM OF THEOBROMA CACAO

The vegetative morphology of Theobroma cacao, the cacao tree, was studied in order to provide a foundation for further investigations on the morphogenesis of the cacao dimorphic shoot system. The seedling of cacao has a determinate orthotropic shoot with a (2+3) phyllotaxis. Branch dimorphism is initiated after 1 to 2 years of growth at which time the apical meristem of the orthotropic shoot aborts and a pseudowhorl of plagiotropic branches is initiated from axillary positions in the shoot tip. The plagiotropic branches are characterized by a distichous phyllotaxis and indeterminate growth. Subsequently an axillary bud below the pseudowhorl develops into a new orthotropic shoot. The apical meristem of this shoot eventually aborts and another pseudowhorl is formed. The apical anatomy of the two types of shoots is similar. The developmental potentiality of the orthotropic shoot axillary buds to form one or the other type of shoot was investigated. The phyllotaxis of the axillary buds of the orthotropic shoot is spiral and that of the axillary buds of the plagiotropic branch is distichous. Pruning and apical puncture experiments showed that the axillary buds of a plagiotropic branch, and of an orthotropic seedling shoot which has not yet formed a pseudowhorl, always give rise to the parent type of shoot. However, the axillary buds of an orthotropic shoot which already bears a pseudowhorl give rise to either type of shoot for several nodes below the point of origin of the pseudowhorl. The type of shoot has no influence on the form of branch which develops from an axillary bud grafted to it. This evidence supports the hypothesis that the axillary buds are initiated as one or the other type of shoot, i.e., once initiated they are predestined.     

Ammonia induces metamorphosis of the oral half of buds into polyp heads in the scyphozoan Cassiopea

Summary The scyphozoan polyp Cassiopea forms vegetative free swimming buds that metamorphose into sessile polyps. In sterile sea water metamorphosis does not take place. Buds keep swimming for weeks. Application of millimolar quantities of NH ₄ ⁺ causes the buds to metamorphose within one day. The resulting animals bear hypostome and tentacles, however, only occasionally peduncle and foot. Almost all transform either completely into solitary polyp head or only the oral half of the bud developes into a head while the aboral half remains bud tissue which becomes constricted off. Under suited conditions this small bud is able to transform into a normal shaped polyp.     

Unusual branch development in the palm, Chrysalidocarpus

Vegetative branch buds of C. lutescens are non-axillary and occur within an abaxial, tubular extension of the leaf sheath, either at the base of a shoot or aerially, a position unusual for palms. Buds are initiated on the abaxial surface of a leaf during its first plastochron (the youngest leaf primordium). The foliar origin of the vegetative bud appears to be unique for angiosperms. In contrast, inflorescence buds are axillary and are initiated as an adaxial ridge on the base of a leaf during its third plastochron (the third primordium from the apex). Aerial branches and basal suckers are developmentally identical and changes in their phyllotaxis are described. As far as can be established by comparative morphology, other species of Chrysalidocarpus have the same type of branch development as in C. lutescens. The development of branches is related to the morphogenetic characteristics of arborescent monocotyledons.     

Expression of a novel receptor tyrosine kinase gene and a paired-like homeobox gene provides evidence of differences in patterning at the oral and aboral ends of hydra

Axial patterning of the aboral end of the hydra body column was examined using expression data from two genes. One, shin guard, is a novel receptor protein-tyrosine kinase gene expressed in the ectoderm of the peduncle, the end of the body column adjacent to the basal disk. The other gene, manacle, is a paired-like homeobox gene expressed in differentiating basal disk ectoderm. During regeneration of the aboral end, expression of manacle precedes that of shin guard. This result is consistent with a requirement for induction of peduncle tissue by basal disk tissue. Our data contrast with data on regeneration of the oral end. During oral end regeneration, markers for tissue of the tentacles, which lie below the extreme oral end (the hypostome), are detected first. Later, markers for the hypostome itself appear at the regenerating tip, with tentacle markers displaced to the region below. Additional evidence that tissue can form basal disk without passing through a stage as peduncle tissue comes from LiCl-induced formation of patches of ectopic basal disk tissue. While manacle is ectopically expressed during formation of basal disk patches, shin guard is not. The genes examined also provide new information on development of the aboral end in buds. Although adult hydra are radially symmetrical, expression of both genes in the bud's aboral end is initially asymmetrical, appearing first on the side of the bud closest to the parent's basal disk. The asymmetry can be explained by differences in positional information in the body column tissue that evaginates to form a bud. As predicted by this hypothesis, grafts reversing the orientation of evaginating body column tissue also reverse the orientation of asymmetrical gene expression.     

SHOOT TIP ABORTION IN ULMUS AMERICANA

Millington , W. F. (Marquette U., Milwaukee, Wis.) Shoot tip abortion in Ulmus americana. Amor. Jour. Bot. 50(4): 371–378. Illus. 1963.—Phenological observations of American elm have shown that the phenomenon of shoot tip abortion in which the distal several plastochrons of each shoot turn yellow and abort, inducing sympodial growth, occurs over a period of several weeks starting at the time fruits are shed from older trees. Time of abortion varies among plants of different age, among individuals of the same age, and among shoots on the same individual. In the latter case, time of abortion is inversely correlated with the vigor of the shoot, the most vigorous shoots on a branch being the last to abort. Abortion is first evident in the yellowing of the entire shoot tip. Cytohistological studies show that necrosis commences at the sixth to eighth node back of the apex, where it is apparent in autolysis of internal cells of the stipules. Necrosis progresses acropetally in the stipules and young leaves and ultimately involves the leaf primordia at the shoot apex. Mitosis ceases in the shoot apex and its meristematic appearance is lost. These changes follow in basipetal sequence in the axillary buds down to the bud below the abscission site. This bud remains active and will resume growth the following season. The abscission site is evident externally as a green-yellow boundary in the basal part of the internode. No protective layer is present at the time of abscission, but it develops after the shoot tip abscises. There is no indication of blocking of vascular tissues before shoot tip abortion and limitation of water supply probably is not a causal factor. Photoperiod studies show that shoot tip abortion is accelerated in short days and delayed but not prevented in long days. Greenhouse experiments show that abortion is delayed also in seedlings, in plants supplied with organic fertilizer, or grown with the roots unconfined. Plants grown in a nutrient solution deficient in nitrogen aborted ahead of controls and plants deficient in calcium. Although shoot tip abortion occurs coincident with fruit drop, there is no indication of a causal relationship. The literature relating to shoot tip abortion is discussed in relation to the above observations.     

Evidence that cytokinin controls bud size and branch form in Norway spruce

Shoot elongation in many coniferous species is predetermined during bud formation the year before the shoot extends. This implies that formation of the primordial shoot within the bud is the primary event in annual shoot growth. Hormonal factors regulating bud formation are consequently of utmost importance. We followed the levels of the endogenous cytokinins zeatin riboside (ZR) and isopentenyladenosine (iPA) in terminal buds, whorl buds and lower lateral buds of the uppermost current-year whorl shoots of 15- to 20-year-old trees of Norway spruce [ Picea abies (L.) Karst.] from June to September. Cytokinins were isolated with affinity chromatography columns, purified by high performance liquid chromatography, and quantified by ELISA. The level of ZR was low in June but increased gradually in all buds until September. Throughout the measurement period, the ZR level was highest in terminal buds and lowest in the scattered lateral, buds, with the whorl buds intermediate. The level of iPA peaked in July and decreased later without any consistent differences among the three classes of buds. The development of different kinds of buds was followed by scanning electron microscopy. We found that bud growth was greatest during August and September. The final size of primordial shoots within the buds varied considerably and the weight of the terminal bud was three times that of the whorl buds and more than five times that of the other lateral buds.
We conclude that the increase in ZR level during the period of active bud development is indicative of the importance of cytokinin for this process. Furthermore, the positive correlation between the level of ZR and bud growth during the period of predetermination of next year's branch growth suggests that this hormone indirectly controls the form of single branches in the spruce tree.     

Hormonal control of shoot branching

Shoot branching is the process by which axillary buds, located on the axil of a leaf, develop and form new flowers or branches. The process by which a dormant bud activates and becomes an actively growing branch is complex and very finely tuned. Bud outgrowth is regulated by the interaction of environmental signals and endogenous ones, such as plant hormones. Thus these interacting factors have a major effect on shoot system architecture. Hormones known to have a major influence are auxin, cytokinin, and a novel, as yet chemically undefined, hormone. Auxin is actively transported basipetally in the shoot and inhibits bud outgrowth. By contrast, cytokinins travel acropetally and promote bud outgrowth. The novel hormone also moves acropetally but it inhibits bud outgrowth. The aim of this review is to integrate what is known about the hormonal control of shoot branching in Arabidopsis, focusing on these three hormones and their interactions.     

Evidence that cytokinin controls bud size and branch form in Norway spruce

Shoot elongation in many coniferous species is predetermined during bud formation the year before the shoot extends. This implies that formation of the primordial shoot within the bud is the primary event in annual shoot growth. Hormonal factors regulating bud formation are consequently of utmost importance. We followed the levels of the endogenous cytokinins zeatin riboside (ZR) and isopentenyladenosine (iPA) in terminal buds, whorl buds and lower lateral buds of the uppermost current-year whorl shoots of 15- to 20-year-old trees of Norway spruce [ Picea abies (L.) Karst.] from June to September. Cytokinins were isolated with affinity chromatography columns, purified by high performance liquid chromatography, and quantified by ELISA. The level of ZR was low in June but increased gradually in all buds until September. Throughout the measurement period, the ZR level was highest in terminal buds and lowest in the scattered lateral, buds, with the whorl buds intermediate. The level of iPA peaked in July and decreased later without any consistent differences among the three classes of buds. The development of different kinds of buds was followed by scanning electron microscopy. We found that bud growth was greatest during August and September. The final size of primordial shoots within the buds varied considerably and the weight of the terminal bud was three times that of the whorl buds and more than five times that of the other lateral buds.
We conclude that the increase in ZR level during the period of active bud development is indicative of the importance of cytokinin for this process. Furthermore, the positive correlation between the level of ZR and bud growth during the period of predetermination of next year's branch growth suggests that this hormone indirectly controls the form of single branches in the spruce tree.     

Development of woody branch attachments in Schefflera (Araliaceae or Apiaceae)

Attachment of branches in Schefflera is unusual in that it involves fingerlike woody extensions that originate in the cortex and pass gradually into the woody cylinder of the parent shoot. We tested the hypothesis that these structures could be roots since Schefflera is a hemi-epiphyte with aerial roots. These branch traces originate by secondary development in the many leaf traces (LTs) of the multilacunar node together with associated accessory traces. In the primary condition, the LTs may be described as cortical bundles. Leaves are long persistent and can maintain a primary stem connection across a broad cylinder of secondary xylem. Under the stimulus of branch development, the LTs form a template for secondary vascular development. Because the LT system is broad, with many traces, the branch attachment is also broad. The fingerlike extensions are attached to the surface of the woody cylinder of the parent stem but are progressively obscured as a continuous cambium is formed. Bark tissues are included within the branch axil because of the extended cortical origin of the initial attachment. The results are discussed in the context of branch-trunk unions in tropical plants, an important component of canopy development.     

Signalling by the FGFR-like tyrosine kinase, Kringelchen, is essential for bud detachment in Hydra vulgaris

Signalling through fibroblast growth factors (FGFR) is essential for proper morphogenesis in higher evolved triploblastic organisms. By screening for genes induced during morphogenesis in the diploblastic Hydra, we identified a receptor tyrosine kinase (kringelchen) with high similarity to FGFR tyrosine kinases. The gene is dynamically upregulated during budding, the asexual propagation of Hydra. Activation occurs in body regions, in which the intrinsic positional value changes. During tissue displacement in the early bud, kringelchen RNA is transiently present ubiquitously. A few hours later - coincident with the acquisition of organiser properties by the bud tip - a few cells in the apical tip express the gene strongly. About 20 hours after the onset of evagination, expression is switched on in a ring of cells surrounding the bud base, and shortly thereafter vanishes from the apical expression zone. The basal ring persists in the parent during tissue contraction and foot formation in the young polyp, until several hours after bud detachment. Inhibition of bud detachment by head regeneration results in severe distortion, disruption or even complete loss of the well-defined ring-like expression zone. Inhibition of FGFR signalling by SU5402 or, alternatively, inhibition of translation by phosphorothioate antisense oligonucleotides inhibited detachment of buds, indicating that, despite the dynamic expression pattern, the crucial phase for FGFR signalling in Hydra morphogenesis lies in bud detachment. Although Kringelchen groups with the FGFR family, it is not known whether this protein is able to bind FGFs, which have not been isolated from Hydra so far.     

标准切花菊杂交F1代群体侧枝侧蕾性状的杂种优势和遗传分析

以标准切花菊〔Dendranthema morifolium(Ramat.)Tzvel.〕品种'优香'('Yuuka')为母本、品种'神马'('Jinba')为父本进行杂交,对杂交F1代群体的单株侧枝平均长度、单株侧枝数、单株侧枝数与单株叶节数的比值(R1)、主蕾直径与侧蕾直径的比值(R2)、单株侧蕾数以及主蕾与侧蕾间距离6个性状进行杂种优势和相关性分析,并利用主基因+多基因混合遗传模型检测这些性状的主基因效应.结果显示:杂交F1代群体6个侧枝侧蕾性状的变异系数为2378%~5065%,且侧枝性状的变异系数总体上高于侧蕾性状;各性状的频次均呈现连续性的正态分布趋势,说明这些性状可能属于多基因控制的数量性状.杂交F1代群体的6个侧枝侧蕾性状均在001水平上表现出显著的中亲优势,表明各性状均存在显著的杂种优势.6个性状中,单株侧枝平均长度的中亲值最大(6230 mm),R1的中亲值最小(026);单株侧枝平均长度、R2和主蕾与侧蕾间距离的中亲优势均为正值,单株侧枝数、单株侧蕾数和R1的中亲优势均为负值.6个性状的中亲优势率为-5374%~3128%,其中,单株侧枝数的中亲优势率最小,而主蕾与侧蕾间距离的中亲优势率最大.相关性分析结果显示:单株侧枝平均长度和单株侧枝数均与R1呈极显著正相关,并与R2和单株侧蕾数呈极显著负相关;R2与侧蕾数也呈极显著正相关,且二者均与主蕾与侧蕾间距离呈极显著正相关.混合遗传分析结果显示:单株侧枝平均长度、R1、R2和单株侧蕾数均受2对主基因控制,符合B-1模型,主基因表现为"加性-显性-上位性",这4个性状的遗传率分别为7707%、9672%、6438%和5307%;单株侧枝数也受2对主基因控制,符合B-2模型,主基因表现为"加性-显性",该性状的遗传率为7438%,表明这5个性状的遗传存在主基因控制效应.而主蕾与侧蕾间距离符合A-0遗传模型,说明该性状无主基因控制,易受环境影响.     

Relationships among shoot sinks for resources exported from nodal roots regulate branch development of distal non-rooted portions of Trifolium repens L

Two manipulative experiments tested hypotheses pertaining to the correlative control exerted by nodal roots on branch development of the distal non-rooted portion of Trifolium repens growing clonally under near-optimal conditions. The two experiments, differing in their pattern of excision to manipulate the number of branches formed at the first 9-10 phytomers distal to the youngest nodal root, each found that after 20 phytomers of growth the total number of lateral branches formed on the primary stolon remained between five and seven regardless of where the branches formed along the stolon. Additional treatments established that nodal roots influenced branch development via relationships among shoot sinks for the root-supplied resources rather than through variation in the supply of such resources induced by fluctuations in photosynthate supply to roots from branches. Regression analysis of data pooled from treatments of both experiments confirmed that shoot-sink relationships for root- supplied resources controlled the branching processes on the non-rooted portion of plants. A disbudding treatment, which removed all the apical and axillary buds present on basal branches, but left other branch tissues intact, increased branch development of the apical region in the same way as did complete excision of the basal lateral branches. The apical buds and the elongation processes occurring immediately proximal to the buds were thus identified as strong sinks for the root-supplied resources. Such results suggest that branch development on the non-rooted shoot portion distal to the youngest nodal root is regulated by competition among sinks for root-derived resources, of limited availability, necessary for the processes of elongation of axillary buds and the primary stolon apical bud.     

Expression and developmental regulation of the Hydra-RFamide and Hydra-LWamide preprohormone genes in Hydra: evidence for transient phases of head formation

Hydra magnipapillata has three distinct genes coding for preprohormones A, B, and C, each yielding a characteristic set of Hydra-RFamide (Arg-Phe-NH2) neuropeptides, and a fourth gene coding for a preprohormone that yields various Hydra-LWamide (Leu-Trp-NH2) neuropeptides. Using a whole-mount double-labeling in situ hybridization technique, we found that each of the four genes is specifically expressed in a different subset of neurons in the ectoderm of adult Hydra. The preprohormone A gene is expressed in neurons of the tentacles, hypostome (a region between tentacles and mouth opening), upper gastric region, and peduncle (an area just above the foot). The preprohormone B gene is exclusively expressed in neurons of the hypostome, whereas the preprohormone C gene is exclusively expressed in neurons of the tentacles. The Hydra-LWamide preprohormone gene is expressed in neurons located in all parts of Hydra with maxima in tentacles, hypostome, and basal disk (foot). Studies on animals regenerating a head showed that the prepro-Hydra-LWamide gene is expressed first, followed by the preprohormone A and subsequently the preprohormone C and the preprohormone B genes. This sequence of events could be explained by a model based on positional values in a morphogen gradient. Our head-regeneration experiments also give support for transient phases of head formation: first tentacle-specific preprohormone C neurons (frequently associated with a small tentacle bud) appear at the center of the regenerating tip, which they are then replaced by hypostome-specific preprohormone B neurons. Thus, the regenerating tip first attains a tentacle-like appearance and only later this tip develops into a hypostome. In a developing bud of Hydra, tentacle-specific preprohormone C neurons and hypostome-specific preprohormone B neurons appear about simultaneously in their correct positions, but during a later phase of head development, additional tentacle-specific preprohormone C neurons appear as a ring at the center of the hypostome and then disappear again. Nerve-free Hydra consisting of only epithelial cells do not express the preprohormone A, B, or C or the LWamide preprohormone genes. These animals, however, have a normal phenotype, showing that the preprohormone A, B, and C and the LWamide genes are not essential for the basic pattern formation of Hydra.     

Bud and crown architecture of white spruce and black spruce

Needle primordia in buds and branch lengths were assessed in the crown of a plantation-grown white spruce tree. There was a gradation in needle primordia in buds in branches within the crown. The largest number of primordia was in the terminal bud of the leading main stem shoot, with the number in first-order whorl lateral shoot terminal buds decreasing from whorl 1 to whorl 4, below which buds contained a similar small number of primordia (about one-third as many as in the terminal shoot). Previous year's shoot elongation followed a similar pattern (i.e., elongation of whorl branches was greater closer to the top of the tree and elongation in the fourth through ninth whorls was about one-third that of the main stem leader). Higher order branches within whorls had within-branch gradation in shoot elongation and number of needle primordia, with older branches having as few as 16–30 primordia in buds and 3–4 cm elongation for high-order branches on older main stem whorls. There were strong correlations between the number of primordia in branch terminal buds and branch length/diameter and bud length/diameter/volume. In both black spruce and white spruce, there were strong correlations of number of needle primordia in main stem leader terminal buds with number of needle primordia in terminal buds of first and second whorl leaders.     

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.     

A model for budding in hydra: pattern formation in concentric rings

Current models of pattern formation in Hydra propose head-and foot-specific morphogens to control the development of the body ends and along the body length axis. In addition, these morphogens are proposed to control a cellular parameter (positional value, source density) which changes gradually along the axis. This gradient determines the tissue polarity and the regional capacity to form a head and a foot, respectively, in transplantation experiments. The current models are very successful in explaining regeneration and transplantation experiments. However, some results obtained render problems, in particular budding, the asexual way of reproduction is not understood. Here an alternative model is presented to overcome these problems. A primary system of interactions controls the positional values. At certain positional values secondary systems become active which initiate the local formation of e.g. mouth, tentacles, and basal disc. (i) A system of autocatalysis and lateral inhibition is suggested to exist as proposed by Gierer and Meinhardt (Kybernetik 12 (1972) 30). (ii) The activator is neither a head nor a foot activator but rather causes an increase of the positional value. (iii) On the other hand, a generation of the activator leads to its loss from cells and therewith to a (local) decrease of the positional value. (iv) An inhibitor is proposed to exist which antagonizes an increase of the positional value. External conditions like the gradient of positional values in the surroundings and interactions with other sites of morphogen production decide whether at a certain site of activator generation the positional value will increase (head formation), decrease (foot formation) or increase in the centre and decrease in the periphery thereby forming concentric rings (bud formation). Computer-simulation experiments show basic features of budding, regeneration and transplantation.     

ORGANOGRAPHY,BRANCHING, AND THE PROBLEM OF LEAF VERSUS BUD DIFFERENTIATION IN THE VINING EPIPHYTIC FERN GENUS MICROGRAMMA

Investigation of the development and organography of the shoot systems of Microgramma vacciniifolia and M. squamulosa was undertaken for the purpose of determining: (1) the features of shoot growth that are responsible for the distinctive vining character of these epiphytic ferns; and (2) the mode of origin of branches and their contrast with leaf initiation. Shoots of both species are dorsiventral and plagiotropic (i.e., parallel to the substrate) in habit. Since the shoot apical meristem is radial in transectional symmetry, shoot dorsiventrality in Microgramma is a postgenital or secondary developmental event, and its inception is related to the initiation of lateral appendages. Leaves and buds arise in a distichous phyllotaxis and occupy opposite and alternating positions on the dorsal surfaces and flanks of the rhizome. Endogenous roots are initiated in two rows from the ventral surface of the stem, in the vicinity of the rhizome meristem; however, they do not emerge from the rhizome until some distance behind the tip and do not elongate until the region of substrate contact. We conclude that the vining nature of this fern rhizome is a result of precocious internodal elongation and the concomitant delay of leaf and bud expansion in the region of stem elongation. In addition, observation of branch origin confirms previous suggestions that branching in Microgramma is strictly lateral and extra-axillary and not a dichotomous derivative as proposed by some workers. Leaf and bud primordia differ not only in the nature of their respective vascular supplies but also in their actual course of initiation. In the case of the leaf, the primordium is precociously emergent and exhibits a lenticular apical cell at its summit when it is only one plastochron removed from the flanks of the apical meristem. By contrast, initials of the bud primordium divide less actively and remain in a sunken position for at least 5–6 plastochrons; only when the bud apex becomes expanded and emergent does a tetrahedral apical cell become recognizable at the tip of the bud promeristem. Because of the distinctive pattern of branch and leaf origin, as well as the lack of adventitious and phyllogenous origin of branch primordia, we suggest that the shoot of Microgramma is a useful test organism for the re-examination of the problem of leaf and bud determination in the ferns.