INTRODUCTION. Internal Organization of the Plant Body. Summary of Types of Cells and Tissues. General References. DEVELOPMENT OF THE SEED PLANT. The Embryo. From embryo to the Adult Plant. Apical Meristems and Their Derivatives. Differentiation, Specialization, and Morphogenesis. References. THE CELL. Cytoplasm. Nucleus. Plastids. Mitochondria. Microbodies. Vacuoles. Paramural Bodies. Ribosomes. Dictyosomes. Endoplasmic Reticulum. Lipid Globules. Microtubules. Ergastic Substances. References. CELL WALL. Macromolecular Components and Their Organization in the Wall. Cell Wall Layers. Intercellular Spaces. Pits, Primary Pit--Fields, and Plasmodesmata. Origin of Cell Wall During Cell Division. Growth of Cell Wall. References. PARENCHYMA AND COLLENCHYMA. Parenchyma. Collenchyma. References. SCLERENCHYMA. Sclereids. Fibers. Development of Sclereids and Fibers. References. EPIDERMIS. Composition. Developmental Aspects. Cell Wall. Stomata. Trichomes. References. XYLEM: GENERAL STRUCTURE AND CELL TYPES. Gross Structure of Secondary Xylem. Cell Types in the Secondary Xylem. Primary Xylem. Differentiation of Tracheary Elements. References. XYLEM: VARIATION IN WOOD STRUCTURE. Conifer Wood. Dicotyledon Wood. Some Factors in Development of Secondary Xylem. Identification of Wood. References. VASCULAR CAMBIUM. Organization of Cambium. Developmental Changes in the Initial Layer. Patterns and Causal Relations in Cambial Activity. References. PHLOEM. Cell Types. Primary Phloem. Secondary Phloem. References. PERIDERM. Structure of Periderm and Related Tissues. Development of Periderm. Outer Aspect of Bark in Relation to Structure. Lenticels. References. SECRETORY STRUCTURES. External Secretory Structures. Internal Secretory Structures. References. THE ROOT: PRIMARY STATE OF GROWTH. Types of Roots. Primary Structure. Development. References. THE ROOT: SECONDARY STATE OF GROWTH AND ADVENTITIOUS ROOTS. Common Types of Secondary Growth. Variations in Secondary Growths. Physiologic Aspects of Secondary Growth in Roots. Adventitious Roots. References. THE STEM: PRIMARY STATE OF GROWTH. External Morphology. Primary Structure. Development. References. THE STEM: SECONDARY GROWTH AND STRUCTURAL TYPES. Secondary Growth. Types of Stems. References. THE LEAF: BASIC STRUCTURE AND DEVELOPMENT. Morphology. Histology of Angiosperm Leaf. Development. Abscission. References. THE LEAF: VARIATIONS IN STRUCTURE. Leaf Structure and Environment. Dicotyledon Leaves. Monocotyledon Leaves. Gymnosperm Leaves. References. THE FLOWER: STRUCTURE AND DEVELOPMENT. Concept. Structure. Development. References. THE FLOWER: REPRODUCTIVE CYCLE. Microsporogenesis. Pollen. Male Gametophyte. Megasporogenesis. Female Gametophyte. Fertilization. References. THE FRUIT. Concept and Classification. The Fruit Wall. Fruit Types. Fruit Growths. Fruit Abscission. References. THE SEED. Concept and Morphology. Seed Development. Seed Coat. Nutrient Storage Tissues. References. EMBRYO AND SEEDLING. Mature Embryo. Development of Embryo. Classification of Embryos. Seedling. References. Glossary. Index.

Anatomy of Seed Plants

Publisher Summary This chapter discusses the interactions and internal programmes that result in the patterned differentiation of vascular tissues. The term “pattern” is used to refer to the high degree of order, or predictability, of the system at all levels— the orientation and contacts of the strands forming a coherent system supplying all the organs of the plant, the organization of the transporting cells in files that are functional vessels and sieve tubes, and the relations among the different vascular tissues (phloem, cambium and xylem) and their component cells. The stress is, thus, on relations between cells of the same and of different types and not on the processes of differentiation and controlled gene expression. It is shown that all aspects of the development and maintenance of the vascular tissues might depend on one control system, which is at least partially hormonal. The way this control could operate is in the working hypothesis that together with the problems and possibilities it poses is the central conclusion of this work. A major characteristic of the hypothesis considered is that a gradual, feedback relation between differentiation and signal distribution creates a reliable pattern. Controls of this type may not result in very precise patterns, but they would be simple and dependable in avoiding mistakes of functional significance. They could, thus, account for the vascular patterns found in plants, where no two halves of the same leaf are exactly the same. These controls also have the virtue of depending on known signals and cellular responses. It is, thus, possible that the principles on which these controls are based have a wide relevance, both in plants and in other cases of biological development. The results considered in the chapter suggest that plant hormones, and especially auxin, play an essential role both in the relation among plant organs and in the relations among neighboring cells.

The Control of the Patterned Differentiation of Vascular Tissues

In plants, the developmental mechanisms that regulate the positioning of lateral organs along the primary root are currently unknown. We present evidence on how lateral root initiation is controlled in a spatiotemporal manner in the model plant Arabidopsis thaliana. First, lateral roots are spaced along the main axis in a regular left-right alternating pattern that correlates with gravity-induced waving and depends on AUX1, an auxin influx carrier essential for gravitropic response. Second, we found evidence that the priming of pericycle cells for lateral root initiation might take place in the basal meristem, correlating with elevated auxin sensitivity in this part of the root. This local auxin responsiveness oscillates with peaks of expression at regular intervals of 15 hours. Each peak in the auxin-reporter maximum correlates with the formation of a consecutive lateral root. Third, auxin signaling in the basal meristem triggers pericycle cells for lateral root initiation prior to the action of INDOLE-3-ACETIC ACID14 (SOLITARY ROOT).

Auxin-dependent regulation of lateral root positioning in the basal meristem of Arabidopsis.

The first somatic single cells of carrot hypocotyl explants having the competence to form embryos in the presence of 2,4-dichlorophenoxyacetic acid (2,4-D) were identified using semi-automatic cell tracking. These competent cells are present as a small subpopulation of enlarged and vacuolated cells derived from cytoplasm-rich and rapidly proliferating non-embryogenic cells that originate from the provascular elements of the hypocotyl. A search for marker genes to monitor the transition of somatic into competent and embryogenic cells in established suspension cell cultures resulted in the identification of a gene transiently expressed in a small subpopulation of the same enlarged single cells that are formed during the initiation of the embryogenic cultures from hypocotyl explants. The predicted amino acid sequence and in vitro kinase assays show that this gene encodes a leucine-rich repeat containing receptor-like kinase protein, designated Somatic Embryogenesis Receptor-like Kinase (SERK). Somatic embryos formed from cells expressing a SERK promoter-luciferase reporter gene. During somatic embryogenesis, SERK expression ceased after the globular stage. In plants, SERK mRNA could only be detected transiently in the zygotic embryo up to the early globular stage but not in unpollinated flowers nor in any other plant tissue. These results suggest that somatic cells competent to form embryos and early globular somatic embryos share a highly specific signal transduction chain with the zygotic embryo from shortly after fertilization to the early globular embryo.

A leucine-rich repeat containing receptor-like kinase marks somatic plant cells competent to form embryos

Summary
Although plants devote a large proportion of their resources to roots, we have a poor understanding of the constraints under which root systems function. Roots are much less variable morphologically than leaves and it is likely that root systems rather than individual roots are the focus of natural selection. In other words, architecture is more important than morphology. Existing classifications of root systems, based on the developmental model, have failed to provide much insight into their functioning and an alternative, topological model is outlined, in which the link is the basic unit of classification. Other components of the architecture of root systems, including link lengths, branching angles and diameters, are considered and the ecological implications of variation in each is discussed.

Simulation models of transport and space exploration are discussed and it is shown that resource cost, transport efficiency and exploration efficiency cannot be simultaneously minimized and that optimum form may vary with the mobility of the resource. In general, a ‘herringbone’ structure seems to be the most efficient at exploration of space but the least transport-efficient and the most expensive.

An architectural approach to the comparative ecology of plant root systems

III. Primary Organization of the Shoot Apex .. V Terow hi n dologiyhe Sh o - -. .-... ...... ...... ..----.... is III. Prim~~~a r y Og en eztic n o h ho A sp e ct .... ........ ..-. ----... 10 IV. G owth and Form in the Shoot .15 (a) Genetic Aspects 1.. . . .. ......5.... ..... . 15 (b) Ontogenetic Changes 18 (c) Factors A ffecting Shoot Form 2.....4 ........ 24 (d) Seasonal Changes ........ ... .. ... 31 (e) C on clu sion s 33 ..... 33

Recent experimental studies of the shoot apex and shoot morphogenesis

Morphological and histological effects of trifluralin on root development

A B S T R A C T The proximal-distal distribution of the lateral roots of five species was studied. A detailed investigation was carried out on two of the five species, Ceratopteris thalictroides and Cucurbita maxima. A definite pattern of lateral root arrangement, with a degree of variability related to the number of protoxylem poles, was found in all of the species studied. In the fern Ceratopteris, lateral root initiation was found to be related to the segmentation of the apical cell, which in turn determines the distribution of the laterals. In this species the lateral roots occur in a predictable sequence and they are grouped in pairs. In the angiosperms studied, the pattern of lateral root distribution seemed to depend primarily upon a rather strict longitudinal relationship between the lateral root primordia formed opposite any one protoxylem pole. In Cucurbita maxima, 93.7 i 5.02% of the lateral root primordia observed were in a specific sequence. The laterals of this species are also arranged in groups. In the other plants studied, Arachis hypogaea, Victoria trickeri, and Eichhornia crassipes, the laterals were not as regularly arranged, but nevertheless they were found to be arranged in groups along the main root axis and not randomly dispersed. Factors controlling the spacing of lateral root primordia include their relationship with the developing vascular system, a direct effect of the parent root apex, and an effect of older lateral root pri

Sequence and pattern of lateral root formation in five selected species

Embryoids which developed from callus obtained from leaves ofMedicago sativa L. (alfalfa) were studied with the light and scanning electron microscopes; embryoids are not common in leguminous plants. Sections showed that in organised callus embryoids apparently originated from groups of embryonic cells, whereas in friable callus, and in the epidermis of cotyledons, hypocotyls and roots of callus-derived plantlets, they often originated from single cells. Many early stages of embryogenesis (1, 2, 3- and 4-celled stages) were observed, particularly in epidermal sites. Stages in somatic embryogenesis, though variable, resembled those described for the normal zygotic embryos of alfalfa. A suspensor-like structure was sometimes present in the embryoids. Numbers of cotyledons, up to six, were observed. The SEM provided confirmation of the many stages of development and the very large number of embryoids present, as well as evidence of the range of form resulting from differential growth of regions of the embryos.

Elizabeth G. Cutter

Papers

Recent experimental studies of the shoot apex and shoot morphogenesis

Morphological and histological effects of trifluralin on root development

Sequence and pattern of lateral root formation in five selected species

Origin and development of somatic embryos inMedicago sativa L. (alfalfa)

Angle Meristem Origin and Development in Selaginella martensii