Vascular Organization of Angiosperms

A New Vision

Jean-Pierre Andre

• Publisher: CRC Press
• Year: 2019
• eBook ISBN: 9780429530227
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This study of plant anatomy is based on newly available data on the structure and spatial organization of the vascular system of plants. For the first time, by means of a new technique of intracellular moulding, the vascular system can be observed in its length. Many examples are chosen from among the major groups of the plant kingdom to illustrate

Part I
Potential Applications of Microcasting

Histogenesis, Structure and Function of the Xylem

OVERVIEW

The cells of terrestrial plants are differentiated¹ into tissues, each with differing morphology and function. Differentiation is absent or weak in Thallophytes and is accentuated in the higher evolutionary plant forms to reach its most advanced stage in the Angiosperms (Table 1). With differentiation, the vital functions of the plant have been progressively allocated to different tissues and, consequently, exchanges of matter and signals have been developed between its various parts. The capture of solar energy and the fixing of atmospheric carbon, for example, have become the specific functions of the photosynthetic tissue and the foliar mesophyll, the absorption of water and dissolved minerals from the soil has become that of the young root tissues at the other end of the plant, and the distribution of absorbed and metabolized substances is that of the vascular tissues, the xylem and phloem, in the species endowed with those tissues, the Tracheophytes.

It has been at least three centuries since the circulation of water and matter in the plant was foreseen from simple observations (Grew, 1682; Malpighi, 1686) and about a century and a half since the vascular tissues were located and named (Hartig, 1837, 1854; Naegeli, 1858; Sanio, 1873, 1874). Research has progressed steadily since, and histogenesis² and the structure and functions of the xylem and phloem continue to be fascinating aspects of the study of plant anatomy and physiology.

The xylem and phloem, which were named by the botanist Naegeli, are the pathways of circulation of two flows of matter. The greater flow in terms of mass is the hydric flow of raw sap through the xylem, rising from the soil and supplying all the tissues; most of the water is transpired by leaves and evaporates into the atmosphere. The latter flow, slower and slighter in volume, is the transport of concentrated solutions of carbon metabolites produced by photosynthesis to growing and reserve organs via the phloem.

Table 1. Classification of the plant kingdom

* The seed is a fertilized ovule that maintains exchanges with the mother plant until maturity. A parallel has been established with ovipary and vivipary in the Animal Kingdom (Emberger, 1960).

The divisions of the Plant Kingdom are dichotomic at the level of major groups: they are based on the identification of major characters of the anatomy and mode of reproduction of species. Since the appearance of the Cormophytes, or higher plants, the acquisition of these characters over the course of evolution seems to have played a critical role in the extension and spread of plants out of the original aquatic environment.

In the edification of a complex vegetative structure, according to Robert and Catesson (2000), cell differentiation leads to the formation of tissues, the most specialized of which, ensuring the rapid transfer of nutrients and signal molecules from one end of the plant to another, have greatly contributed to the establishment of plants in the terrestrial environment.

The phylum Tracheophytes, or vascular plants, is made up of two groups: (1) Pteridophytes, comprising about 10,000 species, principally the ferns, all survivors of primitive families that are slowly becoming extinct (Emberger, 1960), and (2) Spermatophytes, themselves divided into two sub-groups, the Gymnosperms, a plant group of less than 1000 species, also declining (Emberger, 1960), and the Angiosperms, which constitute the majority of present plants (around 250,000 species). The Tracheophytes present a wide diversity of characters and are the subject of constant phylogenetic reclassification on many points, depending on the importance of new parameters taken into account, particularly genetic ones (Soltis and Soltis, 2000). All these species have the common characteristic of having two kinds of conducting tissue, xylem and phloem. The phylum is named for the resemblance between animal tracheids and the conducting elements of the xylem.

The two tissues form two closely associated networks, which extend as the plant grows and which draw its architecture with the finest details from the roots to the leaves, flowers, and fruits. Their histogenesis explains their perfect superposition at every point: their cells are in fact derived from divisions of the same meristem, the cambial meristem,³ which stretches between them like a thin tissue.

In this book, which is devoted to certain aspects of the morphology and spatial organization of the xylem, the presence and role of the phloem are mentioned only incidentally. The overall concepts of histogenesis and histology of the xylem are reviewed in this section so that they can be used to relate the activity of the cambial meristem to xylem structures described in the following sections.

HISTOGENESIS, STRUCTURE AND FUNCTION OF THE XYLEM

The histogenesis of conducting tissues and of the xylem in particular is a difficult process to capture in its spatio-temporal dynamics. To give a brief description, at the tissue scale to start with, let us follow the growth of a shoot of a ligneous Dicotyledon from an apical bud of the preceding year (Fig. 1.1A). From its apex to its lower part, this shoot will comprise at any given moment all the stages of xylem formation.

• Primary and secondary xylem of dicotyledons

At stage 1 of its growth, all the cells of the shoot grow in length. The procambial meristem, partly preformed in the bud, extends the interconnected network of its strands up to the apex. The primary xylem and phloem are differentiated from these strands, the xylem from their inner cells and the phloem from their outer cells (Fig. 1.1, section S1). The bundles, formed of these three tissues, are extended into the leaf petioles.

At stage 2, most of the basal cells of the shoot have completed their elongation, while the shoot continues to lengthen by division and growth of the younger cells. The meristem is now composed of a basal part, the cambium, and an elongating distal part formed of strands of the procambium. While the latter continue to produce primary xylem and phloem, the cambium laterally extends into the interfascicular regions and forms a double sheath of secondary xylem and phloem. After the elongation, the shoot grows in diameter (Fig. 1.1, section S2). The secondary tissues extend towards the apex (stage 3) until they reach the base of the new bud, when the growth of the entire organ is completed (stage 4).

Fig. 1.1. Shoot growth in a ligneous Dicotyledon. A. Successive growth stages between the apical bud of the branch (stage 0) and the end of growth (stage 4). (On the diagrams, the leaves are sectioned at the base of the petioles. The shoot is shown with bark stripped.) The longitudinal growth (curve C) results from the cumulative elongation of all the cells after their formation by division close to the apex (ap). The spaced-out vertical lines at stages 1, 2, and 3 represent the primary bundles (b) in the growing part of the shoot; the horizontal line d delimits the basal part of stage 3, in which the growth is completed: the primary xylem (x1) is progressively ensheathed from the base to the apex by the secondary xylem (x2), represented by short lines close together. The lower part of the figure shows two cross-sections (SI and S2) respectively above and below the limit d of the two parts: only the meristematic continuum (procambium pcb + cambium cb) and the primary and secondary xylem (x) and phloem (ph) are illustrated. B. A branch is composed of units of successive growth. Seen in axial section, each annual deposit of wood covers and extends the preceding one; seen in cross-section (S3), it forms a supplementary ring (A0, A1, etc.: years of growth).

Fig. 1.2. Arrangement of the meristematic procambium-cambium (pcb-cb) continuum and the primary and secondary xylem (x1, x2) in a maple shoot; A. Diagram of the shoot in which the leaves are sectioned. The dottted line marks the present lower limit of the part that is still growing and thus the upper limit of the cambium and wood. In diagrams B, C, and D, the diameter-length ratio of the shoot is dilated by a factor of 8. B. Procambium-cambium continuum: the procambial network produces cauline bundles (cau) the branches of twhich form foliar traces (ft). Lower down, the interfascicular regions (ir) are progressively occupied by the extension of the cambium (arrows); C. The network of the primary xylem (x1), copy of the pcb network, extends from the base to the apex of the shoot; D. The wood or secondary xylem (x2), produced by the cb, progressively ensheaths x1, except at the points of emergence of the primary xylem of leaves (and of non-represented buds) at the “gaps” (foliar gap and bud gap, fg and bg); E. Diagram, in radial section along a cauline bundle, of the superposition of meristems (pcb, cb), primary and secondary phloem (phi, ph2), primary xylem (x1), itself made up of protoxylem, then metaxylem (px, mx)...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Dedication
• Foreword
• Preface and Acknowledgements
• Introduction
• I Potential Applications of Microcasting
• II The Technique of Microcasting
• References
• Index