BesitztAnemia Sori? Untersuchungen über die Entwicklung des Sporophylls vonAnemia phyllitidis (Schizaeaceae)

Observations of young fertile leaf primordia provide information about the development of the sporophyll ofAnemia phyllitidis Sw. The marginal meristem which surrounds the leaf primordium forms the pinna primordia, firstly the two “spore pinnae” by meristem fractionation. These are turned with their adaxial side towards the leaf apex and continue marginal meristem fractionations until products of the 5th order are formed.—In the sporophyll development two events are significant: (1) The fractionation products of the 2nd order reverse their direction of coiling. (2) From the marginal meristem of the fractionation products of the 5th order the sporangia arise in acropetal succession each originating from one initial cell.—Three observations—the fractionation products of the 2nd order being accessory outgrowths of the leaf margin, their reversed coiling direction, and the aggregation of the sporangia on the last segments—lead to the following concept of a sorus type: Each fractionation product of the 2nd order represents a marginal acropetal sorus with a branched receptaculum.     

LEAF GROWTH AND PHENOLOGY OF THE DIMORPHIC HERBACEOUS LAYER FERN DANAEA WENDLANDII (MARATTIACEAE) IN A COSTA RICAN RAIN FOREST

Leaf growth and spore release in Danaea wendlandii were monitored under temperate greenhouse conditions and in a rain forest at the La Selva Biological Station in Costa Rica. Sterile leaf (trophophyll) growth occurs in four phases, and fertile leaf (sporophyll) growth occurs in five phases. Trophophylls show an initial rapid increase in length for a 4-week period, attain full maturity at 8 weeks, and remain on the plant for about 3 years. There appears to be a seasonal pattern of trophophyll growth, with crozier emergence rates in May and June three times higher than the mean annual rate. Sporophyll growth shows a double sigmoid pattern. For about 3 weeks after crozier emergence, there is a rapid increase in length. This phase is followed by a 10-week period with no appreciable change in length. Rapid elongation over a 2-week period precedes spore release; this is followed within 2 weeks by disappearance of the sporophyll. Sporophylls in the rain forest are subject to 14% mortality prior to maturation, with most mortality occurring in later stages of growth. A projected phenology based on the sporophyll growth curve shows peaks in spore release late in the wet season with a concomitant peak in sporophyll emergence early in the wet season during the 15-month period from June 1986 through September 1987.     

INDETERMINATE GROWTH OF LEAVES IN GUAREA (MELIACEAE): A TWIG ANALOGUE

Leaves of seed plants are generally characterized as organs of determinate growth. In this regard, Guarea and related genera seem unusual in that the pinnately compound leaves of these plants contain a bud at their tip from which new pinnae expand from time to time. Previous studies (based upon superficial examinations of leaf-tip buds) have produced contradictory conclusions regarding how long the leaf apex remains meristematic and produces new pinna primordia. In order to determine whether leaf development in Guarea is truly indeterminate, we microscopically examined leaf-tip buds of G. guidonia and G. glabra. In both species, the leaf apex remains meristematic and continues to produce new pinna primordia as the leaf ages. Unexpanded leaves of G. guidonia contained an average of 23 pinna primordia, while the oldest leaves we examined had initiated an average of 44 total pinnae. In G. glabra, unexpanded leaves contained 8 pinnae, whereas an average of 28 pinnae had been initiated on the oldest leaves. These results indicate that leaf development in Guarea is truly indeterminate. Periodic examination of individual intact leaves indicated that the leaves commonly continue their growth for 2 or more years (observed maximum = 51 months). As new leaflets are initiated at the shoot apex (and subsequently expand in rhythmic flushes), older (basal) leaflets may abscise. In addition, the petiole and rachis of the leaf thicken and become woody as a result of the activity of a vascular cambium. Guarea leaves therefore seem to function as the analogue of a typical twig (stem) in general habit as well as in their indeterminate apical growth and secondary thickening.     

The determination of pea leaves,leaflets, and tendrils

Pea leaf determination was examined by culturing excised leaf, leaflet, and tendril primordia of different ages on a nutrient medium. Pinna primordia were designated as 1) determined, if they grew normally in culture; 2) undetermined, if they grew into differentiated structures that were morphologically and anatomically different from either leaflet or tendril; or 3) partially determined, if the two pinnae of an opposite pair developed unequally in isolation, or for leaflet pinnae only, if laminae were initiated but did not develop completely. The compound pea leaf as a whole is determined over four plastochrons of development. Proximal pinnae are determined during the second leaf plastochron, approximately 0.8 plastochron after their initiation. The second most proximal pair of pinnae is determined during the third plastochron, and the terminal portion of the rachis is determined last, during the fourth plastochron. Determination of leaflet dorsiventrality is gradual, requiring a critical minimum period with the leaf in physiological contact with the shoot system. The rachis primordium, when isolated from the shoot, does not affect determination of its pinnae as leaflets or tendrils. Afila and tendril-less homeotic mutations do not alter the timing of pinna determination.     

Developmental studies on the leaf and the extra-axillary bud ofHistiopteris incisa

Anatomical and developmental studies have been made ofHistiopteris incisa in order to obtain a reasonable interpretation of the so-called extra-axillary bud. Single, or rarely two extra-axillary buds arise on the lateral side of the petiolar base. The branch trace appears to depart from the basiscopic margin of the leaf trace. At the earliest stage of the leaf initiation, the leaf apical cell is cut off in one of the prismatic cells of the shoot apical meristem. The leaf apical cell, then, cuts off segments successively to form a well-defined group of derivatives. On the other hand, a well-recognized cell group called “outer neighboring cell group”,onc, is found adjacent to the abaxial boundary of the derivatives of the leaf apical cell. This group of cells does not originate directly in the mother cell of the leaf apical cell. The primordium of the extra-axillary bud is always initiated in the superficial pillar-shaped cell layer ofonc. The leaf primordium may consist of two parts, the distal part derived from the leaf apical cell and the basal part from the adjacent cells includingonc. These facts suggest that the extra-axillary bud is of foliar nature. This study was partly supported by a Grant-in-Aid for Encouragement of Young Scientists by the Ministry of Education of Japan; no. 374222 in 1978.     

STUDIES OF PALEOZOIC FERNS: THE FROND OF ANKYROPTERIS GLABRA

Eggert , Donald A. (Southern Illinois U., Carbondale.) Studies of Palerzoic ferns: The frond of Ankyropteris glabra. Amer. Jour. Bot. 50(4): 379–387. Illus. 1963—The major features of the frond of A. glabra are described on the basis of preserved parts found in Middle Pennsylvanian coal ball material from Illinois. The frond is planated and has well-developed foliar laminae. Primary pinnae arise from the petiole in 2 alternating series, and secondary pinnae arise in a similar fashion from the primary pinnae. Foliar laminae occur on the secondary pinnae and have dichotomous venation. The xylem of the petiole has a diupsilon configuration in the lower part of the axis, while higher in the petiole the xylem forms a strand resembling that of the European species A. westfaliensis. The xylem strands of the primary pinnae arise from the adaxial antennae of the petiolar vascular strand as somewhat C-shapcd bodies and develop antennae and become H-shaped at higher levels. A gap occurs in the antenna of the petiole vascular system above the level of departure of the primary pinna trace. Terete vascular strands occur in the secondary pinna axes which arise from the adaxial antennae of the xylem of the primary pinnae. The foliar laminae are relatively thin, have an irregular outline, and their histology is like that found in many living ferns. The frond of A. glabra illustrates that leaf evolution had progressed in at least one species of the coenopterid family Zygopteridaceae to the extent that an essentially 2-dimensional frond of modern aspect, and with well-developed foliar laminae, was present by Middle Pennsylvanian time.     

Developmental study onHypolepis punctata (Thunb.) Mett.

The origins of the first and second petiolar buds ofHypolepis punctata were clarified in relation to the early development of the leaf primordium, which arises from a group of superficial cells of the shoot apical meristem. One of these superficial cells produces a two-sided leaf apical cell which subsequently cuts off segments to make a well-defined cell group, called here the leaf apical cell complex, on the distal part of the leaf primordium. Meanwhile, cells surrounding the leaf apical cell complex also divide frequently to form the basal part of the leaf primordium. Two groups of basal cells of the leaf primordium located on the abaxial and the adaxial sides initiate the first and the second petiolar buds, respectively. The initial cells are usually contiguous to the leaf apical cell complex, constructing the abaxial and adaxial flanks of the very young leaf primordium. However, the first petiolar bud sometimes develops from cells located farther from the leaf apical cell complex. These cells are derived from those originally situated in the peripheral region of the shoot apical meristem. This study was supported by a Grant-in-Aid for Encouragement of Young Scientists by the Ministry of Education, Science and Culture, of Japan No. 474322 in 1979.     

Anatomy and development of the fern sporophyte

Recent research on the developmental anatomy and morphology of the fern sporophyte is reviewed. Detailed histological and experimental studies of the organization of the fern shoot apical meristem have reconfirmed the recently controversial role of the shoot apical cell as the single apical initial of the meristem. The shoot apical meristem is nevertheless an anatomically and functionally complex structure with a strongly zoned cytohistological organization. Fern shoot apex organization can be compared with that of seed plants. The control of leaf initiation and phyllotaxy remains poorly understood. Studies differ as to whether leaf initiation in ferns involves one leaf mother cell or a multicellular region of the shoot apex. The concept of non-appendicular fronds is refuted for living ferns. The later developmental changes in the determinate leaf apical and marginal meristems of the leaf primordium form an area that is still largely unexplored but could be investigated by methods similar to those used to study shoot and root apices. Branching in ferns is morphologiclaly and developmentally diverse. There is apparently more than one developmental mode of dichotomous branching, and several modes of lateral bud formation have been described, including the phyllogenous initiation of branches at the base of leaf primordia. Developmental changes in bud meristems related to apical dominance, inhibition, and bud activation is another major area for continued study. The traditional concept of the role of the root apical cell has been reestablished by studies similar to those made of the shoot apex. Detailed ultrastructural investigations of the root ofAzolla have given a sophisticated new picture of developmental processes in that organ. Fern roots show remarkably precise patterns of histogenesis in relation to apical segmentation. The formation of secondary vascular tissue inBotrychium suggests that the Ophioglossales may be related to the seed plants. The causal relationship of leaf (and branch and root) formation and the initiation of vascular tissue in the shoot needs more study. Although still poorly understood, protoxylem systems in ferns are variable and may have morphological and systematic significance. Recent investigations of hydraulic conductance in fern stems have found possible correlations of conductance levels with growth forms. The anatomical diversity of ferns makes comparative functional anatomy a promising field for future study.     

SYMMETRY AND DEVELOPMENT OF BUTOMUS UMBELLATUS (BUTOMACEAE) AND LIMNOCHARIS FLAVA (LIMNOCHARITACEAE)

The prostrate rhizome of Butomus umbellatus produces branch primordia of two sorts, inflorescence primordia and nonprecocious vegetative lateral buds. The inflorescence primordia form precociously by the bifurcation of the apical meristem of the rhizome, whereas the non-precocious vegetative buds are formed away from the apical meristem. The rhizome normally produces a branch in the axial of each foliage leaf. However, it is unclear whether the rhizome is a monopodial or a sympodial structure. Lateral buds are produced on the inflorescence of B. umbellatus either by the bifurcation or trifurcation of apical meristems. The inflorescence consists of monochasial units as well as units of greater complexity, and certain of the flower buds lack subtending bracts. The upright vegetative axis of Limnocharis flava has sympodial growth and produces evicted branch primordia solely by meristematic bifurcation. Only certain leaves of the axis are associated with evicted branch primordia and each such primordium gives rise to an inflorescence. The flowers of L. flava are borne in a cincinnus and, although the inflorescence is simpler than that of Butomus umbellatus, the two inflorescences appear to conform to a fundamental body plan. The ultimate bud on the inflorescence of Limnocharis flava always forms a vegetative shoot, and the inflorescence may also produce supernumerary vegetative buds. Butomus umbellatus and Limnocharis flava exhibit a high degree of mirror image symmetry.     

LEAF DEVELOPMENT IN DOXANTHA UNGUIS-CATI (BIGNONIACEAE)

Leaf structure in Doxantha unguis-cati is polymorphic. The usual mature compound leaf is composed of two lanceolate leaflets and a terminal tripartite spine-tendril. Leaf primordia are initiated simultaneously in pairs on opposite flanks of the shoot apical meristem by periclinal cell divisions in the third subsurface layer of the peripheral flank meristem. Two leaflet primordia are the first lateral appendages of the compound leaf. Initiation of these leaflet primordia occurs on the adaxial side of a compound leaf primordium 63–70 μm long. Lamina formation is initiated at the base of a leaflet primordium 70–90 μm long and continues acropetally. Mesophyll differentiation occurs in later stages of development of leaflets. The second pair of lateral appendages of the leaf primordium differentiate as prongs of the tendril. Initiation of the second pair of lateral appendages occurs on the adaxial side of a primordium approximately 168 μm long. Acropetal procambialization and vacuolation of cells extend to the apex of tendrils about 112 μm long, restricting the tendril meristem to the adaxial side of the primordium and resulting in curvature of the tendril. The tendril meristem is gradually limited to a more basipetal position as elongation of apical cells continues. Initiatory divisions and early ontogenetic stages of leaflets and tendrils are similar. Their ontogeny differs when the lateral primordia are approximately 70 μm long. Marginal and submarginal initials differentiate within leaflets but not in tendrils. Apical growth of tendrils ceases very early in ontogeny as compared with leaflets.     

CELL DIVISION STUDIES OF THE SHOOT APEX OF DATURA STRAMONIUM DURING TRANSITION TO FLOWERING

Seedlings of Datura stramonium L., although not photoperiodically sensitive, are useful for floral transition studies when raised in a growth chamber at a constant temperature of 25 C with a photoperiod of 8 hr of light (1,600-2,000 ft-c) and 16 hr of darkness. A terminal flower is formed after the seventh or eighth leaf primordium is produced. A constant rate of leaf initiation up to the time of flowering enables specific apical stages to be obtained and studied. Changes in the mitotic index, substantiated with calculated rates of cell division (measured by the accumulation of metaphases following treatment with colchicine) were studied in shoot apical zones during transition to flowering. Fluctuations in the mitotic index of each zone in the vegetative and transition apex with respect to apical stage as well as time of day were not statistically significant. The mitotic index of the summit zone of the vegetative apex was significantly lower than in the other zones whose mitotic indices were not significantly different from one another. During floral transition the mitotic index of the summit zone as well as the central zone (just below the summit zone) significantly increased while no significant changes were detected in the flank zones. It was shown that the mitotic index could be considered representative of the rates of cell division in Datura.     

LEAF-OPPOSED BUDS IN MUSA: THEIR DEVELOPMENT AND A COMPARISON WITH ALLIED MONOCOTYLEDONS

A single, lateral, vegetative bud which is positioned 180° from the axil of a leaf is a generic feature of Musa (Musaceae). Such leaf-opposed buds occur in all ten species and five cultivars examined, representing all four sections of the genus and all groups of cultivated bananas and plantains. The bud arises relatively late and is first visible as a vascular-free “clear zone” in the axis directly below the future bud meristem site. It is first associated with the fifth or sixth leaf primordium from the apex. A defined superficial meristem develops on the stem directly above the insertion of the leaf margins one or more plastochrons later. Normal, basically axillary, vegetative buds occur in the closely related genera: Orchidantha (Lowiaceae), Heliconia (Heliconiaceae), Strelitzia, and Ravenala (Strelitziaceae). These buds arise in the axil of the first to the third leaf primordium in a manner similar to most other monocotyledons. Axillary vegetative buds also occur in the remaining families of the Zingiberales: Cannaceae, Costaceae, Marantaceae, and Zingiberaceae.     

INDETERMINATE GROWTH AND RAMIFICATION OF THE CLIMBING LEAVES OF LYGODIUM JAPONICUM (SCHIZAEACEAE)

Morphological and anatomical specializations of the climbing leaves (CL) of Lygodium japonicum were investigated. Examination of growth relationships between the rachis and pinnae of the circumnutating CL revealed a close relationship to the “searcher” morphology of twining shoots. The CL has resting pinna apices (leafbuds) capable of replacing a damaged leaf apex or ramifying the foliar axis. Their structure and growth is similar to the main leaf apex. CL growth is indeterminate and occurs at a steady rate. Crozier uncoiling and rachis elongation occurs by a mechanism of unequal rates of cell division and elongation. The adaptations of the CL are interpreted as specializations within the basic principles of fern leaf morphogenesis.     

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.     

A morphometric comparison of Ecklonia kurome (Laminariales,Phaeophyta) from Japan*

A morphological comparison was made of Ecklonia kurome Okamura collected from seven localities, including a lectotype locality (Shirahama), in Japan. Morphometric characters and their size ranges were recorded as follows: the central lamina width (5.1–67.8 cm), the median fascia thickness (0.9–3.3 mm), the primary pinna width (2.2–24.0 cm), the ratio of width to length of the primary pinna (0.07–0.56) and the primary pinna number index (PPNI; 0.24–2.21). The results suggested that the morphological variations observed within this species were related to their habitat. A thick and distinct median fascia and an undulate central lamina margin were found on plants from moderate wave-exposed locations. Plants having narrow primary pinnae with indistinct ruga were observed on shores facing open sea. Wide and thin plants grew in wave-sheltered habitats. Flat plants were found on a shore exposed to a continuous strong current throughout the year. There were noticeable differences in morphology among the Tateyama, Tsuno, Aburatsubo and Tonoshima populations. The yellow and crisp primary pinna edge was a major characteristic of the Tateyama population. Narrow plants in which the central lamina and primary pinnae were half as wide as those of the Shirahama population were found in Tsuno. Wide and thin plants were observed in Aburatsubo. In this locality, the central lamina and primary pinnae were nearly three times wider but half the thickness of those of the Shirahama population. Flat plants grew in Tonoshima. The Muroto and Oki populations exhibited morphologically similar characteristics to the Shirahama population. A hollow stipe and slightly high PPNI value were found in the Muroto population, and a slightly low PPNI value was found on the Oki population.     

Model for the role of auxin polar transport in patterning of the leaf adaxial–abaxial axis

Leaf adaxial–abaxial polarity refers to the two leaf faces, which have different types of cells performing distinct biological functions. In 1951, Ian Sussex reported that when an incipient leaf primordium was surgically isolated by an incision across the vegetative shoot apical meristem (SAM), a radialized structure without an adaxial domain would form. This led to the proposal that a signal, now called the Sussex signal, is transported from the SAM to emerging primordia to direct leaf adaxial–abaxial patterning. It was recently proposed that instead of the Sussex signal, polar transport of the plant hormone auxin is critical in leaf polarity formation. However, how auxin polar transport functions in the process is unknown. Through live imaging, we established a profile of auxin polar transport in and around young leaf primordia. Here we show that auxin polar transport in lateral regions of an incipient primordium forms auxin convergence points. We demonstrated that blocking auxin polar transport in the lateral regions of the incipient primordium by incisions abolished the auxin convergence points and caused abaxialized leaves to form. The lateral incisions also blocked the formation of leaf middle domain and margins and disrupted expression of the middle domain/margin‐associated marker gene WUSCHEL‐RELATED HOMEOBOX 1 (SlWOX1). Based on these results we propose that the auxin convergence points are required for the formation of leaf middle domain and margins, and the functional middle domain and margins ensure leaf adaxial–abaxial polarity. How middle domain and margins function in the process is discussed.     

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.     

The Relationship Between the Distribution of Periclinal Cell Divisions in the Shoot Apex and Leaf Initiation

Periclinal cell divisions in vegetative shoot apices of Pisumand Silene were recorded from serial thin sections by mappingall the periclinal cell walls formed less than one cell cyclepreviously. The distribution of periclinal divisions in theapical domes corresponded to the distributions subsequentlyoccurring in the apices when the young leaf primordia were forming.In Pisum, periclinal divisions were almost entirely absent fromthe I₁ region of the apical dome for half a plastochron justafter the formation of a leaf primordium and appeared, simultaneouslyover the whole of the next potential leaf site, about half aplastochron before the primordium formed. In Silene periclinaldivisions seemed to always present in the apical dome at thepotential leaf sites and also round the sides of the dome wherethe ensheathing leaf bases were to form. Periclinal divisionstherefore anticipated the formation of leaf primordia by occuring,in Pisum about one cell cycle and in Silene two or more cellcycles, before the change in the direction of growth or deformationof the surface associated with primordial initiation. Pisum, Silene, planes of cell division, orientation of cell walls, leaf primordia, shoot apical meristem, plastochron     

OBSERVATIONS ON LEAF AND BUD FORMATION IN HYDROCHARIS MORSUS-RANAE

Regular sequences of leaf and bud formation occur in several members of the Hydrocharitaceae, including Hydrocharis, in which buds are normally formed in the axil of every second leaf of the phyllotactic spiral. Leaf inception begins by periclinal divisions of the inner cells of the 2-layered tunica. Bud formation, which occurs in the apical meristem itself, immediately following the inception of the subtending leaf primordium, begins by divisions in various planes in the corpus, the 2 tunica layers remaining continuous throughout. The young bud meristem soon gives rise to a lateral bud, before leaf formation begins upon it. Because of these and other features, this species is one of considerable morphogenetic interest. Morphogenesis of the whole plant, and in particular the factors controlling the regular sequence of leaf and bud formation, have been and are being investigated experimentally.