In considering the apical or primary meristems of the plant body, one of the most perplexing problems is the permanently meristematic condition of these regions, which are somehow spared from the processes of maturation occurring in their derivatives. One might be tempted to relate this property to their terminal position, their three-dimensional mass, or their organization, which is distinct from that of the mature structures they produce. However, the lateral meristems, which share the capacity for continued growth but are strikingly different in every other respect, prevent an easy acquiescence to this temptation. The vascular cambium and the cork cambium, or phellogen, are lateral in position, have the form of cylindrical sheets encircling the plant axis, and are organized in close conformity with the tissues to which they give rise. They initiate only specific tissues rather than whole organs as in the case of the terminal meristems. Furthermore, it must be borne in mind that whereas every vascular plant body must have terminal meristems in order to exist at all, the lateral meristems have a supplemental role and are by no means universal (Barghoorn, 1964).

THE INITIATION OF CAMBIAL ACTIVITY

Nothing emphasizes the differences between primary and secondary meristems more effectively than a consideration of the origin of the vascular cambium. Whereas the shoot and root apical meristems are initiated among the cells of the embryo early in the development of the plant, the cambium has its origin from a partially differentiated vascular tissue, the procambium.

In order to understand developmental processes in a complex system like a plant, it is necessary to analyze parts of the system individually. Thus, in the preceding chapters the shoot apex as the initiating center of the shoot has been examined in detail, as have the initiation and development of the lateral organs to which it gives rise. The full significance of the processes that occur in these parts, however, can only be appreciated in the context of the integrated system in which they occur. It is appropriate now, therefore, to consider the development of the whole shoot system. Reflection upon the enormous diversity of shoot forms might seem to make this an impossible task, but fortunately the emergence of concepts of shoot architecture has established a framework for the analysis of varied patterns of shoot ontogeny. A relatively small number of developmental processes, occurring in various combinations, provide interpretations of widely divergent shoot forms.

This chapter will consider first the sequence of events by which the individual shoot is elaborated to its final form. It will then examine different developmental potentialities that may be expressed by shoots, often within the same shoot system. The ways in which different shoot expressions fit together into integrated shoot systems will be examined in terms of architectural concepts. Finally, these concepts will be used to interpret some of the major plant growth forms, such as trees, shrubs, and herbs.

In the development of the plant body, one cell, the zygote, is able to express the full genetic potentialities of the organism; that is, it normally gives rise to the whole plant. All other cells express their potentialities less completely, and such expression is progressively restricted as one proceeds through the later stages of tissue and cell differentiation. Ultimately an individual cell differentiates as a highly specific entity such as a tracheid or a sieve element, which clearly expresses only a small portion of the total genetic capacity of the organism. Where differentiation brings about a drastic change in the morphology of the cell, such as the death of the protoplast in a tracheid, or the loss of the nucleus in a sieve element, no further expression of genetic potentiality is possible and differentiation is irreversible. In most cells, however, no such irrevocable loss occurs, and if differentiation is a manifestation of differential gene action rather than of mutational changes in the nucleus, reactivation of these cells by the appropriate stimuli might be expected to result in further and perhaps different expressions of their potentialities. This expectation is amply realized under many conditions, both natural and artificial, in which differentiated plant cells undergo further development and realize different and more complete expressions of their potentialities than in their original differentiation. In fact, there is today such widespread exploitation of this phenomenon in applied research, and even commercially, that its fundamental significance for the understanding of plant development may be insufficiently appreciated.

The particular conditions under which development of a vascular plant begins in the embryo obviously hold great interest for the student of morphogenesis. The question that clearly requires investigation is whether the sequential pattern that emerges during embryogeny is to be regarded as an expression of the inherent capacity of the zygote, as the result of specific regulation from the environment, or as the manifestation of subtle interaction between the two. Although descriptive accounts of embryogenesis are helpful in exploring this problem, experimental and analytical techniques can provide a different and more penetrating analysis of these possibilities. Experimental embryology has been an extremely valuable discipline in elucidating problems of animal morphogenesis, but the plant counterpart of this field has played a limited role in the understanding of plant morphogenesis. A major factor contributing to this deficiency has been the relative inaccessibility of the plant embryo at the formative stages, with the result that the botanical work that most closely corresponds to experimental animal embryology has been done with the apical meristerns of the adult plant. Nonetheless, there are several techniques by which the development of the embryo may be probed, and these have led to the acquisition of a body of information from which a meaningful interpretation of embryogeny is beginning to emerge.

Throughout the embryophytes, fertilization and embryogenesis occur in specialized structures that appear to provide a distinctive environment.

Any attempt to study the apical meristem of the shoot, even a simple dissection to expose it for observation, leads immediately to a consideration of the initiation and development of lateral appendages, particularly leaf primordia. It is evident that the formation of leaf primordia is a major activity of the shoot meristem and that the early development of these primordia in such close proximity to the meristem must result in important developmental interactions between the two. This intimate relationship is reflected in the unity of the mature shoot system in which any attempt to isolate stem and leaf either structurally or functionally is artificial. The student of phylogeny finds an interpretation of this relationship, at least in the ferns and seed plants, in the evolution of stem and leaf from a primitively undifferentiated branching system. The object of developmental analysis, however, must be to understand how, in the individual living plant, structures that originate as outgrowths of the meristem acquire distinctive characteristics and interact with one another and with the meristem that produced them. This analysis is further complicated by the fact that the same meristem frequently gives rise to other appendages that develop as replicas of the original shoot.

Whereas the shoot is characterized by potentially unlimited or indeterminate growth and this feature is retained by lateral branches, the leaf is an organ of transient, although in some cases extensive, growth.

Most of the work reviewed thus far has consisted of observations of various types of shoot apices that are for the most part normal or at least intact. It is also possible to investigate the organization of the meristem by subjecting it to experimental treatments and analyzing its reaction to these manipulations. The most widely used and the most successful of the experimental procedures applied to shoot apices has been microsurgery, in which delicate instruments are used to make punctures, incisions, and excisions in various regions of the meristem. Experiments of this sort have also been carried out in sterile culture so that the surgically isolated region of the meristem can be grown in isolation from the remainder of the plant. By these methods it is possible to obtain information about the roles played by various portions of the meristem and about interactions between parts of the meristem and between the meristem and other parts of the plant. One limitation of the surgical method is that it cannot be assumed that the effects of operations are just the isolation or removal of particular regions of meristematic tissue. Wounding may itself produce its own responses that are difficult to evaluate in terms of the normal apex, for example, cell proliferation. Thus, methods involving surgical manipulations tell a great deal about the potentialities of portions of a meristem, information that is extremely valuable, but experiments must be controlled carefully and the results screened rigorously if the information is to be applied validly to the interpretation of the organization of the intact meristem.

In the previous chapter four questions about leaf origin and development were posed. The first of these, which asked why any outgrowths of the shoot meristem occur at all, was left essentially unanswered. The second, which dealt with the location of outgrowth, was dealt with in that chapter. The last two, which questioned the nature of the influences that cause an outgrowth to become a leaf and the response of the outgrowth to these influences, could be phrased in another way: If outgrowths are initiated, why do some become leaves and others branch shoots? The reason for phrasing the question in this way is that in addition to the regular formation of leaves, it is characteristic of the shoots of all but a few vascular plants to give rise to a succession of branches such that the whole shoot becomes a ramifying system. Clearly the difference between a determinate and dorsiventral leaf and a branch that is a replica of the main axis is a striking one, and it is important to seek an explanation for this difference in the initiation or early development of both types of appendages. This chapter is devoted to a consideration of these questions.

LEAF DETERMINATION

In many ways, one of the most revealing approaches to the study of leaf development is that in which the partially developed organ is removed from the plant and allowed to continue its development on a culture medium of known composition in complete isolation from the parent organism.

Previous chapters have considered how the basic plan of the vascular plant shoot is initiated and elaborated. It will be recalled that shoot development occurs in two relatively distinct phases. An initial phase involves terminal meristem activity in which the tissues and organs are laid down. There follows a phase of expansion growth in the subapical part of the shoot during which the previously formed structures enlarge and mature. Chapter 11 will examine how variations in the extent of the expansion phase could produce shoots of widely differing morphology. However, there are other developmental variations in the basic body plan of the shoot in which the phase of terminal meristem growth is principally involved, and these are the subject of this chapter.

It should be expected that if terminal meristem activity is modified there might be cases of extreme modification in the kind of organs produced and in the extent and pattern of their subsequent growth and development, and indeed this is so, as any student of plant taxonomy or morphology knows. These modifications have been the subject of extensive researches in which the question has been the degree of homology between the modified organs and more usual organs of the shoot. However, in this chapter attention will be confined to some examples that have proved to be especially amenable to developmental analysis and about which relatively recent information is available.

We saw in the previous chapter that it is characteristic for morphogenetic events to continue throughout the life-span of most plants. This is in marked contrast to animal development, in which there is a concentration of morphogenetic phenomena in the embryonic stages. Nonetheless, like the animal, the vascular plant begins life as a single cell, the fertilized egg, and passes through an embryonic phase during which the fundamental body plan is laid down. Although it may be argued that all plants that develop from a single cell into a multicellular state pass through an embryonic phase, historically the term embryo has been restricted to those groups in which the early stages are enclosed within parental tissue and are presumed to be nutritionally dependent upon the parent organism. On this basis the bryophytes and the vascular plants often are designated the Embryophyta. In the bryophytes and the lower vascular plants there is no interruption of growth to mark the end of the embryonic phase, which is therefore rather ill defined. On the other hand, in the seed plants, embryonic development is considered to be terminated at the maturation of the seed, and this leads to a sharp distinction between the embryo and all postgermination stages.

Throughout the Embryophyta, as well as in some lower groups, plant development from a zygote alternates in the life cycle with development of a second plant body from a single-celled spore.

Recent advances in modelling plant stands have emphasised the importance of the structural and functional properties of plant canopies, as distinct from those of the constituent parts. In response to proposals made following the 1984 meeting on the ‘control of leaf growth’, which resulted in Seminar Series Publication 27, the Environmental Physiology Group held a series of sessions on plant canopies during the March 1986 meeting of the Society for Experimental Biology at Nottingham. All the invited speakers at these sessions have contributed chapters to this volume either individually or with collaborators.

Chapters have been included on all the major processes occurring in canopies, although there has been space neither for consideration of the manipulation of canopies by chemical or genetical means, nor for discussion of the canopy as habitat for micro-organisms, insects or vertebrates. A policy decision was made at an early stage of planning to encourage authors to look at a diverse range of canopy types and geographical distribution in order to avoid any bias introduced by, for example, considering only temperate zone cereal crops. The reader can decide how successful this policy has been. Some omissions represent genuine areas of ignorance, but it is a matter of regret that space was not available to allow consideration of stands of mixed species either in agricultural intercropping systems or in natural communities.

It is a pleasure to acknowledge the financial and other support of the Environmental Physiology Group, the Association of Applied Biologists and the British Ecological Society.

I would like to record the contributors’ co-operation during the meeting and to thank them for all the time they and their collaborators devoted to preparing and revising their manuscripts.

Introduction

A synthesis of canopy processes can be accomplished at various levels of detail. If historical data are available, then a statistical analysis of that data may provide a kind of synthesis; however, in this case the synthesis is implicit in the statistical tool used, yielding limited insight to us. Alternatively, a mechanistic approach can be used and each relevant process described by appropriate, state-of-the-art, quantitative relations with explicit integration (or synthesis) to achieve an ‘integrated whole’. Clearly, statistical and mechanistic approaches represent extremes of a continuum where all intermediate states are possible. Thus a clear statement of objectives, guiding rules for pursuing these objectives, definition of the system, and evaluation criteria are prerequisites for beginning an orderly synthesis of canopy processes.

This chapter represents an attempt at an orderly synthesis of canopy processes with a reasonably mechanistic approach. The plant-environment model entitled Cupid (Norman & Campbell, 1983; Norman, 1979; Norman, 1982) is used as an example.

Rules for constructing a model

A system of rules for pursuing a synthesis of processes can aid one in resisting the temptation to ‘over sell’ and thus avoid having either to resort to short-term expediency when failure is in sight, or to justify the means deceptively with an end result that was essentially known before the modelling was begun.

Introduction

Green plants utilise the sun's energy to synthesise organic compounds from carbon dioxide and water. The pioneering work, concurrently carried out by Liebig in Germany and Lawes & Gilbert in the UK more than a century ago, conclusively showed that plants must take up inorganic nutrients from the soil to produce these organic components. Since that discovery it has been established that many elements are necessary for optimum functioning of the biochemical machinery of the plant. Most of these are necessary in such small amounts, however, that the supply from the seed, or from natural sources suffices. In agriculture the situation is often different for the macro-elements nitrogen, phosphorus and potassium that are needed in such large quantities, especially where crop management practices aim at very high yields, that the supply from natural sources falls far short of the demand. Fertiliser experiments show that, up to a certain level, addition of these elements from a fertiliser bag leads to higher yields. Unfortunately, interpretation of these fertiliser experiments seldom exceeds the derivation of the optimum nutrient application rate for the conditions of the experiment, either in physical or in economic terms. The lack of explanatory conclusions hinders the use of such results for predictive purposes, for example, in the formulation of fertiliser recommendations for the farmer.

Introduction

Plant canopies modify their own microclimate. The heat and vapour released into the atmosphere at plant surfaces changes the temperature and humidity of the air in contact with those surfaces. These changes in temperature and humidity, in their turn, modulate the fluxes of heat and vapour from the vegetation. The importance of this ‘atmospheric feedback’ depends, amongst other things, on the area of the plant canopy (Jarvis & McNaughton, 1986). Small areas of vegetation modify shallow layers of the atmosphere, and local changes in microclimate are small. The influence of a single field extends upwards for perhaps 10 metres. The gradients of temperature and humidity through this layer have been studied in detail by canopy meteorologists.

If a uniform canopy covers an area of some hundreds of square kilometres then the effect of the vegetation will be felt throughout the whole of the turbulent planetary boundary layer, up to a kilometre or so above the ground. On this regional scale, processes affecting the surface energy balance have received very little scientific attention. This situation is now changing under pressure from hydrologists, who want methods for estimating regional evaporation, and climatologists, who must model the surface energy balance to improve predictions from their models of the global circulation of the atmosphere.

The purpose of this chapter is to review efforts to extend canopy energy balance models to the regional scale. First is a brief descriptive account of atmospheric transport processes in the whole planetary boundary layer, to set the scene.

Introduction

This chapter is about turbulent transfer between a plant community and the atmosphere, especially the transfer of heat, water vapour, CO2 and other scalar entities. We consider the way in which turbulent transfer influences the microclimate within the plant community, in particular the mean scalar concentrations, including temperature and humidity. The second section provides a brief, qualitative overview, to establish the connections between the turbulent transfer process in a plant canopy and exchange processes at both smaller scales, those of individual leaves, and larger scales, those of the entire planetary boundary layer of the atmosphere. Then, with frequent reference to the observed properties of turbulence in plant canopies, the third and fourth sections review two kinds of theory for describing turbulent transfer. The more common Eulerian theories consider the behaviour of the turbulent transfer process at a grid of points fixed in space. Less common, but of increasing importance, are the Lagrangian theories: these describe turbulent transfer by considering the statistical behaviour of the wandering blobs of fluid which actually carry the transferred entity.

Overview

Common experience shows that the air motion within a plant canopy is highly erratic and intermittent. The origin of this behaviour lies in the planetary boundary layer (PBL), the turbulent layer of the atmosphere which extends from the ground to a height of the order of a kilometre (within a factor of three or so).

Introduction: modules and architecture

The shape of the canopy influences many important aspects of the growth and development of plants and such effects are felt at many levels. Differences in canopy form may affect not only how much photosynthetically active radiation is intercepted by plants but may also regulate the spectral composition of radiation that filters to lower levels in the canopy and thus have photomorphogenetic consequences. The extent of shading both by and from close neighbours will also be affected by canopy shape, as will the degree of presentation to, or concealment from, consumers of nutritious foods such as fruits, leaves and buds. In a more agricultural context, canopy arrangement influences the extent to which disease spores or the droplets of a chemical designed to kill them (or prevent their development) can enter infectable zones.

The above ecological repertoire of plants is linked directly to their gross form and invites an obvious question concerning their evolution, namely: does the architectural ‘type’ of a plant have a rôle in the (Darwinian) fitness of an individual or, in other words, have certain whole plant forms been selected during evolution while others have been less successful?

This question forms the major theme of this chapter although Fisher (1984) has recently considered a similar topic. In addition, in order to better understand the mechanisms behind the magnificent variability in plant form that we see, some recent experimental data indicating the rôle of genomic changes in determining plant shape will be presented.