ONTOGENY OF THE FLORAL APEX OF MICHELIA FUSCATA

Tucker , Shirley C. (u . Minnesota, Minneapolis.) Ontogeny of the floral apex of Michelia fuscata. Amer. Jour. Bot. 47(4): 266—277. Illus. 1960.–The floral apex of Michelia fuscata, although determinate, has a prolonged period of activity. It undergoes an increase in average height and diameter through the time of initiation of stamens, followed by a decrease in average size during initiation of successive carpels. The floral apex also shows periodic or plastochronic fluctuation in size and configuration between initiation of tiers of appendages at nearly the same level and time. Plastochrons of different duration alternate in an orderly sequence in the floral meristem.     

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

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

DEVELOPMENT AND ORGANIZATION OF THE PRIMARY VASCULAR SYSTEM IN POPULUS DELTOIDES ACCORDING TO PHYLLOTAXY

An actively growing cottonwood bud was embedded in epon-araldite and serially sectioned at 2 μm. The sections were analyzed microscopically with the optical shuttle system of Zimmermann and Tomlinson, and all data were quantitatively recorded relative to the apex and to leaf plastochron index (LPI). Analysis of the sections revealed an acropetally developing procambial system organized according to a precise phyllotaxy. Six procambial strands could be recognized and followed long before the leaf primordia that they would enter were evident at the apex. Origin of these strands coincided with developmental events both in the parent trace and its primordium and in the antecedent leaf on the same orthostichy. Once a primordium and its trace attained a certain stage of development, trace bundles began to develop basipetally from the primordium base. These trace bundles appeared to be the earliest progenitors of wood formation in cottonwood. It was concluded that the concept of residual meristem and its corollary, the hypothesis that acropetally developing procambial strands determine the inception sties of new primordia, apply to the cottonwood apex.     

SHOOT VASCULAR SYSTEM AND PHYLLOTAXIS OF CASUARINA (CASUARINACEAE)

In species of Casuarina with multileaved whorls, each stem vascular bundle divides radially into two at the site of a leaf trace separation, and the same two bundles rejoin acropetally to where the trace supplies a leaf. Such divisions are divisions of a single vascular bundle, and the rejoining of bundles forms a single bundle. Proposals that the extant primary vascular systems of dicotyledons may have been derived as in conifers are incorrect in so far as Casuarina is concerned, or the system has evolved beyond that so far proposed for dicotyledons. Reasons are offered, however, for considering that fernlike leaf gaps are not present. Leaf traces supply leaves at the first nodes distal to their origins. The ways by which an increase or decrease of stem bundles occur are described. Phyllotactic patterns range from helical (rare) to whorled. In the embryo, where leaves occur decussately, of certain species with multileaved whorls, and in the shoot apices of species with tetramerous whorls, slight differences in the levels of leaf attachments and the bending of leaf traces indicate the probable evolution of extant whorled phyllotaxies from one or more helical arrangements. Stages in the evolution are suggested. The leaves in most species with multileaved whorls are in true whorls. The original periderm of branchlets lies internally to the internodal traces and chlorenchyma, but is otherwise external to the vascular system. It is concluded that each leaf originates at its level of separation from the axis despite several structural features suggesting that the leaf bases have become congenitally adnate to the stem.     

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

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

INITIATION OF STAMENS,CARPELS, AND RECEPTACLE IN THE CACTACEAE

Five taxa representing the three tribes of the Cactaceae have similar patterns of stamen and carpel initiation but display differences in early receptacle development. The first ring of stamens and the carpels arise simultaneously from subsurface layers. The bases of carpels are congenitally connate. Additional stamens are initiated centrifugally. The shape of the floral meristem within the ring created by the first stamens varies. In Pereskia corrugata it remains broadly convex; in Opuntia engelmannii it forms a depression with a small convex central region; in Epiphyllum strictum it forms a broad shallow depression; in Echinocereus reichenbachii var. albispinus it develops a deep depression; and in Mammillaria compressa it develops a depression prior to stamen and carpel initiation. Changes in receptacle shape result from cessation of apical growth and activation of an intercalary ring meristem. These two processes occur earlier in ontogeny in the more advanced of these five taxa.     

NERVOUS ORGANIZATION AND THE PROBLEM OF THE SYNAPSE IN ACTINIA EQUINA

The nervous system of Actinia equina was studied by routinehistological methods and by metallic impregnation techniques.Some preliminary results from electron microscopy are included. The organization of the nervous system of this species is morecomplex than that of other anthozoans; it consists of two interconnectednerve plexuses which are developed to differing degrees in variousparts of the body. These are: (1) a superficial (outer) plexuslying in the ectoderm, and (2) a deeper (inner) plexus constitutingthe main nerve net, lying in the mesoderm. The former is composedof bipolar and multipolar nerve cells, and the latter of multipolarcells. Receptor cells in the ectoderm make contact with fibersof the ectodermal plexus. Processes from the mesodermal plexusrun out to the muscle fibers. Connections between the receptor cells and the nerve processesof the superficial plexus and between the processes of the cteeperplexus and the muscle fibers appear to be of the discontinuous(synaptic) type. In the nerve nets themselves, although someconnections resembling synapses have been seen, most of thenerve elements stand in direct connection with one another,so that the system must be regarded as at least partly syncytial.Evidence is given for the growth of the nerve net, in step withthe general growth of the animal, by division of nuclei followedby their movement apart within the syncytium. The distribution of the nerve elements in various parts of thebody, the interconnections between these regions, and the cytologicalcharacteristics of the cells are described. Ways in which excitationcould pass from one part to another are discussed.     

A COMPARATIVE STUDY OF THE PRIMARY VASCULAR SYSTEM OF CONIFERS. I. GENERA WITH HELICAL PHYLLOTAXIS

The primary vascular system of 23 species belonging to 18 genera of conifers with helical phyllotaxis has been investigated with the intent of determining the architecture of the system. Special attention has been given to nodal and subnodal relations of the vascular bundles. The vascular system seems to be composed solely of relatively discrete sympodia, that is, axial vascular bundles from which leaf traces branch unilaterally. Although the discreteness of the sympodia is not immediately apparent because of their undulation and lateral contacts with neighboring ones, close examination, including a statistical analysis of the tangential contacts, seems to reveal that each sympodium maintains its identity throughout. Although two traces may be apparent at nodal levels, the trace supply to a leaf originates, in all species, as a single bundle. An analysis is made of the relationship between the vasculature and the phyllotaxis. It is observed that the direction of trace divergence can be accurately predicted when the direction of the ontogenetic spiral, the angle of divergence of leaf traces, and the number of sympodia are known.