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.     

ONTOGENY AND PHYLLOTAXIS OF THE TERMINAL VEGETATIVE SHOOTS OF MICHELIA FUSCATA

Tucker Shirley C. (Northwestern U., Evanston, Ill.) Ontogeny and phyllotaxis of the terminal vegetative shoots of Michelia fuscata. Amer. Jour. Bot. 49(7): 722–737. Illus. 1962.—Two patterns of symmetry occur in Michelia fuscata In the lead shoots, leaves arise in a 2/5 spiral arrangement which may be either clockwise or counterclockwise. Other shoots are dorsiventrally organized; these shoots produce leaves in a modified ½ phyllotaxis in which the angle between the 2 files of leaves lies between 100° and 150°, according to the particular branch. Both types of shoot have a zonate apical meristem with a biseriate tunica a central initial zone, and a peripheral zone. The apical configuration of cells does not change appreciably during the plastochron. The flat to low-convex outline of the shoot apex is maintained by initiation of the leaves close to the summit of the apex; the diameter of the meristem diminishes greatly after such an initiation. Leaf inception in the subsurface tunica layer is followed by precocious activity of the marginal meristems which extend the stipular flanges completely around the base of the apical meristem. The stipular margins then fuse laterally and form a hood over the apex. A subapical initial meanwhile is active in the leaf blade, where it persists up to the time the leaf is 2 mm high. The most recent primordium is 300 μ high before another leaf is initiated. The vascular system of the stem is a cylindrical network of leaf traces, with 6–12 traces per leaf. The procambium develops acropetally from preexisting vascular strands in the stem below. Elements of the diverse sclereid system differ in shape in different tissues, according to the availability of intercellular space. Goebel's term “Pendelsymmetrie” is discussed with reference to apical activity in Michelia.     

慈姑匍匐茎的起源和球茎的膨大研究

匍匐茎的发生一般见于主茎倒二或倒三叶原基的叶腋部位。在匍匐茎发生区域的主茎一侧,匍匐茎原始细胞的基部形成壳状区;壳状区的形成对匍匐茎原基的外凸起一定作用。匍匐茎无居间分生组织;它的伸长依靠顶端分生组织细胞的横向分裂,使轴向细胞数目增多,并使细胞的轴向延伸。球茎的膨大是通过匍匐茎第8—10节基本分生组织的细胞有丝分裂,增加细胞数目,然后细胞体积的扩大来实现的。球茎中的淀粉一般为单粒淀粉;匍匐茎中的淀粉由单粒和复合两种淀粉粒组成。     

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.     

STIPULES IN SOME MEMBERS OF THE VITACEAE: RELATING PROCESSES OF DEVELOPMENT TO THE MATURE STRUCTURE

The development of stipules especially their spatial and temporal pattern of initiation in relation to the leaf was investigated in Vitis riparia Michx., cv. Concord, Parthenocissus tricuspidata (Sieb. & Zucc.) Planch., Cissus oblonga (Benth.) Planch., Cissus hypoglauca (F.v.M.) A. Gray, and Cissus rhombifolia Vahl. Early initiation is characterized by the occurrence of a single primordium with a wide insertion on the flank of the shoot apex. Distinguishing between stipule primordia and the leaf primordium is impossible at this early stage. Distinct primordia can only be seen in later stages of development. At maturity, the stipules occupy free lateral positions. Developmental processes such as timing of initiation and zonal growth seem to play an important role in early development. In five of the six taxa examined in this study, the early initiation of stipules, their close association with the leaf and also their faster relative rate of growth during early development appear to give them a characteristic protective function. In contrast, C. rhombifolia stipules are initiated later than the leaf and seem to develop at a slower rate than the leaf proper. Consequently, they never enclose their associated leaf but instead cover the next youngest leaf. Many different criteria are used to distinguish the broad category of stipules, and therefore many interpretations have been made depending on the type of approach that is used. This study attempts to look at stipules in terms of developmental processes and demonstrates a more accommodating leaf/stipule concept which provides a clearer comprehension of the nature of the stipule.     

The Structure of the Plumule of Nelumbo Nucifera Gaertn. and the Nature of its Scale

The structure of the plumule of Nelumbo nucifera Gaertn. and its feature covered with scale are seldom seen in dicotyledon. The fact that the plumule possesses scale is even more uncommon. This particular phenomenon is investigated by observing the differentiation of the plumule apex and the development of the leaf organs. After the seed is formed, the embryo has two young leaves and a terminal bud covered with scale. In the bud it has already differentiated the 3rd and the 4th leaf primordium and a shoot apex, the differentiation of which is very complex. So the structure of the plumule passes through 4 plastochrons altogether. It is made clear through observation and analysis that, before the 4th leaf primordium is formed, the transforma- tions of the shoot apex of the embryo in each plastochron are fundamentally alike. After the 4th leaf primordium is developed, the shoot apex becomes complex and there appear 3 different active cell regions which become the bases of vegetative bud of the seeding apex. The development of these 3 active cell regions will be stated in “The Structure of the Vegetative Bud of Nelumbo nucifera Gaertn. and the Nature of its Scales.” The apices of the plumule are almost slightly domed in structure. As a rule, their width is from 95 to 107 μ. Their height is from 17 to 20 μ during one plastochron. Before the 3rd leaf initiation, the anatomical structure of apices is examined and the fol- lowing zones may be delimited: zone of tunica initials, zone of corpus initials, peripheral zone, and zone of rib meristems. It is frequently observed that the cell of corpus in subapical peripheral zone develops periclinal division, which is the initial cell of leaf primordium; Procambium will appear before the stage of the appearance of leaf buttress. The apex of the plumule is in an apical position, but when the seedling is formed, as the developing leaves are alternate, the directions of the shoot apex are changed, simultaneously the base part of the leaf encloses the axis, and the adaxial meristem also differentiates the scale which encloses the terminal bud, thus placing the bud in axillary of the leaf and forming a zigzag phenomenon of the axis of the seedling. Above the basal adaxial side of the leaf primordium develops the scale of the plumule with meristem periclinal division of closely attached protoderm as its base. So the scale of the plumule of Nelumbo nucifera Gaertn. and the axillary stipule are of the same origin. To sum up, the scale of the embryo of Nelumbo nucifera Gaertn. is differentiated from the adaxial meristem of the basal part of the leaf primordium, and is the derivative part of the leaf. It has the same function as the coleoptile of the monocotyledon. Whether they are homologous organs or not is still to be investigated.     

PHYLLODE THEORY IN RELATION TO LEAF ONTOGENY IN SANSEVIERIA TRIFASCIATA

Shoot apices of Sansevieria trifasciata have a three-layered mantle, a zone of subapical initials, a central meristem, and a peripheral meristem. Leaf initiation begins with periclinal divisions in L-3 and is followed by periclinal divisions in L-2 and anticlinal divisions in L-l. At first, the primordium is a mound of tissue at one point on the flank, but it soon takes the form of a low ridge encircling the apex. An ephemeral adaxial meristem differentiates in L-2 of the primordium when it is about 50 μ high and is active until the primordium is about 450 μ high. Then it ceases basipetally and is not observable after the primordium is about 600μ high. As the adaxial meristem ceases at the base of the radial tip, its two lateral regions become the submarginal meristems of the expanded portion. Marginal meristems differentiate from the protoderm, and oblique-anticlinal divisions of the marginal initials result in the formation of an abaxial and adaxial epidermis. These derivatives undergo a few anticlinal divisions, increasing marginal width, and then they divide periclinally, increasing marginal thickness. After the primordium is about 600-700 μ high it continues to grow in length by a diffuse basal intercalary meristem. When the leaf is 3 dm long, an adaxial rounding meristem differentiates in the region just above the sheath. Leaf vasculature consists of parallel bundles which anastomose acropetally. Vascular bundles are arranged in a semicircle in the expanded portion and in a circle in the radial tip. There is one centrally located bundle at the apex as a result of lateral anastomoses. Present evidence from leaf ontogeny and mature vasculature in S. trifasciata is interpreted as supporting the concept that the liliaceous leaf is homologous with the phyllodes of A corns and Acacia.     

ANATOMY AND ASPECTS OF DEVELOPMENT OF THE STAMINATE INFLORESCENCES AND FLORETS OF SEVEN SPECIES OF ALLOCASUARINA (CASUARINACEAE)

The morphology, ontogeny, and vascular anatomy of the staminate inflorescences and florets of seven species of Allocasuarina are described. The generally terminal but open-ended inflorescences occur on monoecious or staminate dioecious trees and consist of whorls of bracts, each subtending a sessile axillary floret. Each floret consists of one terminal stamen with a bilobed, tetrasporangiate anther enclosed typically by cuculliform appendages, commonly considered bracteoles, an inner median pair and an outer lateral pair. The mature stamen is exerted, the anther is basifixed and is extrorsely dehiscent. In early development of a male inflorescence very little internodal elongation occurs and enclosing cataphylls appear. The inflorescence apex is a low dome with a uniseriate tunica and a small group of central corpus cells. Bract primordia are initiated by periclinal divisions of C₁ followed by further divisions of the corpus and anticlinal divisions in the tunica. The bracts are epinastic and become gamophyllous except apically by cell divisions in both sides of each primordium. Stomata are restricted to the axis furrows and the abaxial tips of the bracts. The axillary florets arise in acropetal succession initiated by periclinal divisions in C₁ accompanied by anticlinal divisions in the tunica. The lateral floral appendages are also initiated by C₁ followed by anticlinal divisions in the tunica. They become adnate basally later with the subtending bract. The median sterile appendages are initiated in a manner similar to the initiation of the outer appendages. The stamen is initiated by divisions in the outer layers of the corpus and in the tunica, and then develops first by apical growth followed by intercalary growth. The vascular system of the inflorescence is identical to that of the vegetative stem. Each floret is supplied by a single bundle that has its source in a branch from each of the two traces supplying a bract. Six bundles arise from the floral bundle; four of these terminate in the base of the stamen and two form an amphicribal bundle that supplies the anther. Pollen is binucleate, 3- to 7-porate. The exine is tegillate.     

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.     

Some Aspects of Floral Histogenesis in Bajra (Pennisetum typhoides S. & H.)

The young reproductive apex in Bajra (Pennisetum typhoides S.& H.) possesses a biseriate tunica and a massive corpus.The cells of three or four peripheral layers and six to eightlayers at the summit of the apex are eumeristematic. The centralregion consists of elongated, highly vacuolated, and lightlystained cells arranged in files. The initiation of the spikeletbud is by periclinal divisions first in the corpus and laterin T₂ cells. Similarly the longer bristle or the extension ofthe fascicular axis develops from the corpus and T₂ cells. Theother bristles develop from the tunica layers. The chaff membersare initiated and develop like a leaf. The development of thestamen resembles that of a spikelet or an axillary bud. Thedevelopment of the carpel is similar to that of the leaf primordium.The origin and development of the male flower is like that ofan axillary bud.     

EARLY HISTOGENESIS OF THE ADULT LEAVES OF DARLINGTONIA CALIFORNICA (SARRACENIACEAE) AND ITS BEARING ON THE NATURE OF EPIASCIDIATE FOLIAR APPENDAGES

In order to assess the validity of various interpretations of tubular leaves of angiosperms, a histogenetic study of the ontogeny of adult leaves of Darlingtonia californica was undertaken. The adult leaf of Darlingtonia is characterized by a sheathing leaf base, an elongate ascidium, an overarching hood, and two “fishtail” appendages which arise near the leaf apex. A keellike growth, with two rows of alternate vascular bundles, traverses the tube from base to mouth. Ontogenetic studies show that the primordium arises by a monopodial rather than a sympodial mode of growth as previously reported. After the formation of a small, erect primordium, a restricted adaxial meristem is initiated that expands both adaxially and upwards. This “querzone” serves, in effect, to congenitally combine the two primordial margins during its extension. Growth and maturation of the subjacent portions cause tubular elongation in the leaf. Primordial apical divisions are later replaced by more general intercalary growth with acropetal and centrifugal maturation. The hood and fishtails are established early in ontogeny by adaxial growth of the primordial apex and subsequent activation of juxtaposed localized meristems. Comparative morphology has established that the epiascidiate leaf is a foliar appendage that undergoes certain specific morphogenetic modifications. It has a structural relationship to ensiform appendages of Acacia and Acorus as well as to peltate foliar organs. The early ontogeny of Darlingtonia leaves is considered to be homologous with other epiascidiate foliar organs, including some supposedly primitive carpels.     

INFLORESCENCE AND FLOWER DEVELOPMENT IN THE PIPERACEAE. I. PEPEROMIA

Flowers of Peperomia species are the simplest structurally of any of the members of the Piperaceae. The spicate inflorescences form terminally and in axillary position; in each, the apex first is zonate in configuration with a two-layered tunica while 3-4 leaves are initiated. Later, when the inflorescence apical meristem begins bract initiation, the biseriate tunica persists, but zonal distinctions diminish and the apex can be described in terms of a simple tunicacorpus configuration. The inflorescence apex aborts after producing 30-40 bracts in acropetal succession an abscission layer forms across the base of the apex, and the meristem dries and drops off. Bracts are produced by periclinal divisions in T₂ (and occasionally also in the third layer as well); the later-formed floral apices arise by periclinal divisions in T₂ and the third layer. Each floral apex is at first a long transverse ridge in the axil, perpendicular to the long axis of the inflorescence. This establishes bilateral symmetry in the flower, which persists throughout subsequent growth. The floral meristem becomes saddle-shaped, and two stamen primordia are delimited, one at either end and lower than the central floral apex. A solitary carpel is initiated abaxially, and soon forms a circular rim which heightens as a tube with an apical pore. Within the open carpel, a solitary ovule is initiated from the entire remains of the floral apical meristem; it, hence, is terminal in the flower, and its placentation is basal. Carpellary closure in P. metallica results from accelerated growth of the abaxial lip, and the two margins become appressed. Species differ greatly as to whether the abaxial or the adaxial lobe predominates in late stages of carpel development. In P. metallica, the receptive portion of the stigma forms from the shorter lobe which is overtopped. Stigmatoid tissue forms internal to the receptive stigma. The prevailing bilateral floral symmetry, absence of a perianth, and the spicate inflorescence are features which distinguish Peperomia (and Piperaceae) from the magnolialian line of angiosperms.     

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.     

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.     

FLORAL DEVELOPMENT OF POTAMOGETON RICHARDSONII

The inception and development of the sterile floral appendages of Potamogeton richardsonii have been re-investigated with a refined dissection technique (Sattler, 1968) and improved microtechnical methods (Feder and O'Brien, 1968). The results obtained by Sattler (1965) are confirmed, i.e., the sterile appendages are initiated at the flanks of the floral apex before the stamen primordia are formed. Consequently, they may be homologized with tepals or perianth members, although in the mature flower they are inserted at the stamen connective, due to growth between and at the base of each developing tepal and stamen. Each carpel arises as a radial primordium which becomes peltate immediately after its inception. One ovule primordium is initiated at the cross-zone. The stigma becomes bilobed. A slight outgrowth develops at the abaxial side of the style. The floral apex has a two-layered tunica. The primordia of the tepals, carpels, and ovules arise by periclinal divisions in the second tunica layer, whereas the stamen primordia are initiated by periclinal divisions in the corpus and second tunica layer. Variation in floral pattern, especially with regard to the number of appendages, has been observed in flowers near the tip of the inflorescence axis.     

Shoot apex,leaf development and unifacial tip inAgave wightii drumm. et prain (agavaceae)

The shoot apex has one tunica layer enclosing a mass of corpus which is differentiated cytohistologically into central mother cell zone, flank zone, rib zone and a ‘cambium-like’ zone. Occurrence of ‘cambium-like’ zone during minimal phase is considered as an expression of nodal region. Agave wightii shows spirodistichous arrangement of leaves which have an expanded photosynthetic surface with a reduced unifacial tip. Leaves are initiated by periclinal divisions in the second layer. Vertical growth in the leaves is by subapical initials and lateral growth is by marginal and submarginal initials in their early stages of development. The unifacial tip is formed by the extension of adaxial meristematic activity. The derivatives thus formed are pushed to the abaxial side of the primordiuj. Hence the unifacial part of the leaf is regarded as equivalent to a phyllode.     

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.     

Planes of Cell Division and Growth in the Shoot Apex of Pisum

The planes of cell division and growth were examined in thecourse of a single plastochron in the shoot apical meristemby observing the orientations of mitotic spindles. In the I₁region of the apical dome, cell divisions were at first anticlinalbut 30 h before a leaf primordium emerged at this site 20 percent of the cell divisions became periclinal. These periclinaldivisions were found only in the corpus. Periclinal divisionsin the tunica were coincident with the appearance of the primordiumas a bulge. The change in the direction of growth in I₁ at thesite of the incipient leaf primordium occurred without any changein the rate of growth in this region of the meristem.     

Peltate Leaf Development in Brasenta schreberi Gruel

The present investigation is a report with a detailed account of peltate leaf development in Brasenia schreberi. The preliminary indication of leaf initiation is seen in periclinal divisions of the subepidrmal cells on the flank of the shoot apex. By the time a leaf primordium becomes 73 μm high, the apical growth is present, and it has undergone repeated periclinal and anticlinal divisions and introduces aew cells into the body of the primordium. The procambial strand is evident very early in ontogeny. Vacuolation of cells proceeds acropetally along the primordium. At 109 μm longitudinal sections reveal a ventral outgrowth. This ventral growth, termed a lateral zone or Querzone, is initiated by subepidermal cells on the ventral (adaxial) side of primordium. The Querzone is considered to represent the congenital fused laminar margins of the leaf. Transection analysis of a primordium of 160 μm indicates that the Querzone growth is initiated on the midbasal portion of primordium. Below the midbasal portion of primordittm which later forms the petiole of the leaf. The marginal 'growth occurs at a time when the primordinm is approximately 160 μm high. Initiation of marginal growth takes place in 42 μm from the base of a primordium. Marginal growth at first can be seen while the primordium exhibits unilateral. By anticlinal divisions, the marginal initials produce the upper and lower protoderm, which give rise to the upper and lower epidermis respectively, and submarginal initiates divide in anticlinal plane, giving rise to the adaxial and abaxial layers. Subsequent divisions of the adaxial layer are anticlinal only so that it forms a single layer which later develops as the palisade parenchyma. The cells of the abaxial layer, by divisions in all planes, form the tissues of the spongy parenchyma and all of the lateral venation system. At maturity, the mesophyll is much less compact and there are mumerous large intercellular spaces in the spongy region and between the palisade cells. The veins are collateral: There is a reduction in the amount of xylem and the ultimate veinlets terminate as single spiral tracheids. The glandular bairs are mumerous on the lower epidermis and are epidermal in origin, each consisting two basel cells and one termined cell. On the basis of morphological examination we suggest that the peltate leaf of Brasenia schreberi is clearly distinct from the foliage leaf in the development of adaxial meristem and marginal growth. We found the primordimn of Brasenia schreberi as an erect organ and arises.by monopodial growth. In the development of leaf and the form of venation the Brasenia schreberi shows marked resemblance and relationship to the Nelumbo nucifera.     

Magnolia tiepii sp. nov. from Vietnam

A new species of Magnolia sect. Gwillimia DC., Magnoliaceae from Vietnam is described and illustrated. Magnolia tiepii is recognized from southern Vietnam, where it occurs in Khanh Vinh slope, Khanh Vinh District, Khanh Hoa Province. It is distinguished from the closely related M. henryi by having narrowly cuneate leaf blade, short petiole, stipular scar at the base of the leaf blade, short peduncle and ellipsoid fruit.