Cross-protection is the phenomenon whereby a plant is protected from infection by earlier or simultaneous exposure to another organism. This chapter will be concerned with protection against fungal and bacterial parasites as a result of infection by related and unrelated fungi and bacteria and of local lesion formation by viruses. It will not be concerned with the interference of the development of viruses by related or unrelated viruses (Matthews, 1970).

For a long time people have been interested in the idea that plants, like animals, can acquire physiological immunity to pathogens. Research, mainly in Europe in the late part of the nineteenth century and in the early decades of this century, was inspired by observations which suggested that some perennial plants were less severely affected by a disease following earlier infection. For example, the varied development of powdery mildew infections on the foliage of oak trees Quercus spp. in successive seasons had intrigued early investigators. Experiments had also been performed which seemed to show that herbaceous plants became more resistant to infection after the plants or the soil bearing them were sprayed with the extracts of fungal cultures. Numerous observations and claims of this type were reviewed by Chester (1933) who criticized the lack of sufficient replication for proper appraisal of the experiments and the frequent inadequacy of controls. Sources of error which had rarely been eliminated before drawing conclusions were losses of virulence in the parasites during experiments, differences in environmental conditions on successive occasions for assessing resistance, and natural increases in resistance as plants aged during the periods of supposed acquisition of resistance.

Plants at all stages of their life-cycles are exposed to many potentially parasitic micro-organisms. Seeds germinate in soils which contain numerous resting parasites awaiting the arrival of roots to stimulate them into activity. Aerial parts of plants are inoculated by fungal spores and bacterial cells carried in air currents and rain-splash droplets. Under favourable conditions of moisture and temperature, plant tissues are thus subjected to attempted infection on numerous occasions. However, these attempts often fail, and most plants remain healthy. Successful establishment of a parasite depends upon a special genetical and physiological relationship so that the cells of the host accept the parasite.

This book is concerned with the processes whereby plants succeed in remaining healthy despite their constant exposure to potential parasites. It leads to a consideration of one of the most interesting problems in biology and biochemistry, namely the molecular basis of the high degree of specialization which is often observed in relationships between parasites and hosts. As will become apparent, there are reasons to believe that the basis of much specialized parasitism rests in the ability of parasites to confound a recognition system linked to other reactions in host cells. Through this linked system, the host normally notices and then fails to accept the intrusion of an alien organism.

It must be clear from the previous chapter that the fate of most potential parasites is decided after they have entered their host plants. It is important, therefore, to know the location of parasites within and between cells, the stages during attempted infection when parasitism succeeds or fails, and the associated responses of host cells as revealed by light and electron microscopy. Based on this knowledge, the significance of the numerous physiological and biochemical changes in infected plants to the processes of resistance and susceptibility can be assessed.

THE CELLULAR LOCATION OF PARASITES IN HOST TISSUES

There have been a limited number of quantitative studies on related cytological changes in host and parasite during the critical stages when resistance is expressed or parasitism is established. Before considering these studies which are confined to a few much analysed host–parasite relationships, it is valuable to review the different ecological niches presented by plants and the ways in which they are exploited by specialized microorganisms. In doing so, it is well to emphasize the types of physiological interaction which might be anticipated between the cells of parasite and host, and to appreciate the rarity of direct contact of the protoplasts of the two organisms. The parasitic protoplast is usually separated by its cell wall from the host, and in many cases a host cell wall also separates the two protoplasts.

The idea that plants produce protective chemical substances after infection was expressed by a number of research workers in the first half of this century, but the concept was formalized by Müller & Börger (1941). As explained in Chapter 4, their observations on hypersensitivity in potato cultivars resistant to the potato blight fungus, and their experiments on the induction of resistance in susceptible cultivars, led them to postulate the existence of phytoalexins. The term phytoalexin meant a warding-off compound produced by the plant, and phytoalexins were thought to form in hypersensitive potato tissue and to prevent further growth of the infection hyphae, but it was also considered that they might be of general occurrence in infected plants.

The first demonstration of the detection of a chemical entity as a phytoalexin was done by Müller (1958) working with the hypersensitive response of bean tissue to the soft-fruit pathogen Monilinia fructicola. Droplets of spore suspension were placed in the cavities of opened bean pods from which the seeds had been removed. The spores were observed to germinate and to cause death of some underlying cells within 24 hours. Infection droplets were collected after different intervals and were tested for their effects on new spores. The droplets became increasingly antifungal after incubation in seed cavities for 14 hours and completely fungistatic after 24 hours. The substance responsible for the antifungal activity could be extracted from combined infection droplets by partition with petroleum spirit, but it was not chemically characterized at the time.

This chapter is devoted to the quantitative description of a number of shoot-apical systems. It does this in considerable detail, perhaps with tedious detail. Nevertheless, it supplies the bulk of the evidence upon which the thesis of the book is built, and all but the sections on wheat and subterranean clover are published for the first time. Many will be content to treat the chapter as resource material, and use it to check the claims made elsewhere. Others with a special interest in the test plants will perhaps persist, and still others will find it helpful to the planning of work on other systems.

Some attempt has been made to reduce the tedium by relegating the description of methods and procedures to the Appendix. The systems described have been studied over a period of some fifteen years so there is an unavoidable unevenness of purpose and treatment. The wheat apex was the first to be studied, followed by that for clover – a dicotyledon. Flax followed because of a growing interest in phyllotaxis for its own sake. The same may be said for Eucalyptus, as an example of the decussate condition, and the less extended studies of tobacco, lupin and cauliflower aimed to fill in some obvious gaps in our knowledge of spiral systems. Finally, Ficus was selected as an extreme example of a tightly packed apex, which could be expected to be subject to physical constraint during its long period of development.

Controlled environments are almost essential for studies of growth rate in plants, and the temperature and light regimes used in these studies were selected to give near-optimal rates of growth in each case.

Long acquaintance with the wheat plant suggested that the genesis and growth of tiller buds might provide a suitable test system for the supposed effects of constraint on growth rates and on the genesis of form. Detailed accounts of collaborative work on this problem are currently being published (Williams et al. 1975; Williams and Langer, 1975), and a preliminary experiment involving both the removal of constraint and the application of additional constraint will be published by Williams and Metcalf (1975).

Drawings illustrating the initiation and early growth of the bud in the axil of leaf 2 are given in Fig. 4.7.3, and later stages for similar tillers appear as Figs. 7.1 and 7.2. The external shape of these early-stage tillers is entirely determined by their location. In Fig. 7.1 the tiller is shown in place and also removed at the level of its half-junction with the axis, thus revealing the snug cavity in which it is growing. In Fig. 7.2 the moulding is even more obvious, with a vertical dent to the right marking the position of the midrib of leaf 3. Note also that the first tiller leaf has the appearance of being extruded from between the margins of the prophyll, and a small piece of the inner margin of the leaf is actually caught between the outer margin and the prophyll.

The three-dimensional drawings are a little misleading visually because the removal of the relatively massive and slightly more mature tissues of the outer leaf sheath gives a false impression of freedom. The transverse sections of Fig. 7.3 (also Fig. 4.7.6 above) will counter such an impression; they also illustrate some additional features of the system.

This chapter attempts to draw the threads together, and will be rather more speculative in character than those that have gone before. Not only does the presentation of masses of descriptive matter require the cement of integrative speculation; the very emergence of a truly quantitative biology would seem to depend upon it. As pointed out in the introduction, it was found impractical to present the facts about growth and development of a range of shoot-apical systems (Chapter 4) without drawing attention to situations in which physical constraint seemed likely to be relevant to the interpretation of events. We can now take a closer look at those situations to see if they are consistent with the proposition that physical constraint is indeed a significant element in the genesis of form and the determination of rates of growth of various primordia. The proposition has already been developed by presenting the essential results of a study of the initiation and growth of tiller buds in wheat. This system is seen as a valuable test system for these ideas.

This short chapter is concerned solely with the growth and development of the ear of wheat, Triticum aestivum. At the outset it is freely admitted that the work was undertaken largely in response to the challenge of describing such a complex biological system with precision. There was also the hope that such work would throw light on the nature of the transition from vegetative to reproductive development in the wheat plant. The chapter draws heavily upon the descriptive paper by Williams (1966 a) and to a lesser extent on the more general statement on the inflorescence in Gramineae (Williams, 19666). Studies of the developmental morphology of the wheat spike have been made by Bonnett (1936) and Barnard (1955), and the latter's histogenetic classification of its members into foliar and cauline types is adopted.

The unit of the gramineous inflorescence is the spikelet, a group of one or more flowers – usually called florets – with a number of associated bracts. These are collected into inflorescences with a tremendous range of form, from the open panicle of Avena to very compact structures such as the spike in Triticum. A common form of the spikelet consists of an axis (the rachilla) bearing two sterile glumes at the base and an indefinite number of lemmas, each with a floret in its axil. In its mature state (at anthesis) the wheat spikelet is as depicted in Fig. 6.4E but its basic structure is more easily grasped at the developmental stage shown in Figs. 6.1 and 6.6. Fig. 6.1 shows two lateral spikelets in which the seventh (possibly the last) floret has just been initiated, and in which the glumes and other bracts have not yet grown up to conceal the inner members.

This appendix is primarily concerned with procedures which were developed for the quantitative studies of shoot-apical systems. It includes some suggestions on data processing, but it cannot be said too often that further studies of the kind will demand flexibility of approach and an awareness of the necessity for precision.

The final item is a set of tables for the conversion of relative growth rates (R day−1) to doubling times.

THREE-DIMENSIONAL RECONSTRUCTION

The three-dimensional drawings which are used throughout this book are based on serial sections of the structures they represent. They have been very helpful in portraying changes in form and in identifying critical events in the genesis of form. One of my colleagues has labelled them the Michelin-type drawings, for rather obvious reasons, and others have thought the contour lines were too prominent. However, the lines do contribute greatly to one's awareness of form, as will be seen in Fig. A.2E and F. The procedure has been published in detail (Williams, 1970), but the essentials are repeated here for convenience.

Fig. A.1 illustrates the principles by reference to a sphere of radius 34 μm. Let us suppose that this sphere has been serially sectioned by horizontal planes 10 μm apart, and such that one plane passes through the centre of the sphere. The contour lines of the sphere are shown in plan view in Fig. A.1A superimposed on a square grid whose individual members have sides of 20 μm. The three lower contours are, of course, identical with the three upper ones, and all lie within the equatorial contour.

This salutary statement from D'Arcy Thompson's chapter on phyllotaxis comes immediately after an assertion that Church (1904) saw in phyllotaxis an organic mystery, a something for which we are unable to suggest any precise cause. It could also be reassuring for those who have experienced difficulty in following the erudite mathematical analysis of the subject by Richards (1951). It is amusing to note that, in another place, Richards (1948) says that Church's theoretical treatment of the subject stood little chance of sympathetic hearing, or even understanding, in a day when the average botanist prided himself on his incompetence in even the most elementary mathematics.

While it is to be hoped that botanists no longer pride themselves on such ignorance, it is a moot point whether many of us are well qualified even today, and it remains a fact that the study of the phenomena of phyllotaxis is much neglected. Those interested in the history of the subject should consult the papers already mentioned, and others by Snow and Snow (1931), Wardlaw (1949), R. Snow (1955) and Richards and Schwabe (1969). In his chapter entitled ‘The Succession of Parts’, Dormer (1972) also provides a valuable general commentary on the subject.

The immediate purpose of this chapter is to set out Richards's objective procedures for the quantitative description of shoot apical systems, and to use them as a basis for geometric modelling of such systems.

At the time of his death, Petrie had completed early drafts of two or three chapters of a book which was to have been called ‘The Developmental Physiology of Plants’, and the above quotation from Plato had been placed at the beginning of Chapter 2, ‘The Change in Dry Weight and Leaf Area, and First Steps in the Analysis of Growth Rate’. Unfortunately the book had not reached a stage from which it could have been completed by any of his colleagues, and we had to content ourselves with placing the quotation on the title page of a bound volume of Petrie's papers in plant physiology for the Library of the Waite Institute, Adelaide.

The second quotation is from one of Petrie's published papers, and epitomizes his thinking and general approach to the study of growth, though more extended statements along the same lines had in fact appeared a year earlier (Ballard and Petrie, 1936).

Although it is commonly acknowledged that we owe to Blackman the first clear statement of the mathematical principles underlying the law of exponential growth, his ‘efficiency index’ has had an extraordinary history of criticism and rejection. The efficiency index is, of course, none other than the relative growth rate – a concept which has always been eminently respectable. No doubt the nature of the analogy – that of compound interest – and reference to it as a physiological constant are the bases of the misunderstandings. His earliest critics were Kidd et al. (1920) and they were effectively answered by Blackman (1920) in the same volume of the New Phytologist. Of special interest is the contention of Kidd et al. that the only way in which plants can be compared is by the comparison of the whole series of efficiency indices throughout their life-cycles.

Much has already been said about leaf growth, because this is essential to the definition of the patterns of growth within shoot-apical systems. For two of those systems, however, Williams and Rijven (1965, 1970) have provided descriptions of growth and chemical change which relate to leaf growth as such.

Leaves are organs of limited growth arising laterally on the shoot apex, and they exhibit a steady progression from the meristematic condition of the primordium through to maturity and senescence. It is scarcely surprising, therefore, to find strong similarities in this progression even for such different plants as subterranean clover and wheat. Both studies happen to be on the fourth leaf of the primary shoot, and they are concerned mainly with changes in DNA, RNA, proteins and cell wall materials. The plants were grown in controlled environments, and the work achieved a high level of precision through sampling methods and chemical procedures which are fully described in the original papers. This chapter summarizes the main results.

SUBTERRANEAN CLOVER

The fourth leaf of subterranean clover was chosen because it is the first on the main axis to attain the characteristic form and size of the adult trifoliate leaf, and because its development could scarcely have been influenced by cotyledonary reserves. Fig. 5.1 illustrates the changing size and form of the fourth leaf from day 7, the earliest stage for which cell counts are available. The first chemical data relate to day 10, when the primordia are still only 0.66 mm long. The drawings of Fig. 5.1 include a thin disc of stem tissue, the disc of leaf attachment, but this was not in the samples.