ABSTRACT
Roots are an integral component of nearly all plants. Where they are absent, as in certain water plants (Cemtophyllum, Salvinia, Utricularia, and Wolffia spp., for example), this is probably the result of evolutionary loss with the usual root function being performed by other organs. This last statement points to two important areas of biological enquiry which have been differentiated as evolutionary and functional biology (Mayr, 1961). In plants, evolutionary biology deals with problems of population genetics, ecology, and palaeontology, whereas functional biology includes the disciplines of anatomy, biochemistry, and embryology. In the present discussion of cellular patterning within root tissues, it is useful to keep these two avenues of enquiry in mind, for they should lead to an understanding not only of the advantages that cellular patterns confer in enhancing the fitness of the individual and hence of the population of which the individual is a member, but also the means by which such patterns are achieved. However, in considering the evolutionary significance of the cellular structure of roots, the question arises as to how such a property relates to natural selection. The units of living matter on which selection acts are topics of debate in the context of the neo-Darwinian interpretation of evolution. Some scholars suggest that natural selection acts directly on root systems (e.g., Fitter, 1987), whereas others argue that the unit of selection is the whole plant (e.g., Peterson, 1992) on the grounds that it is the whole organism, rather than any particular organ, whose fitness is tested by selective pressures. However, it can also be argued that, on the one hand, populations are the subject of selection or, on the other hand, that it is the hereditary material (genes) which is selected. All arguments have some validity, particularly within the context of a hierarchically organized plant kingdom in which the levels of organization range from the molecular to the individual and the population (Barlow, 1999). Nevertheless, it cannot be denied that the root, like any other organ, contributes to fitness and, thus, is one of the elements of the plant which is offered for natural selection. Likewise, the cellular construction of roots also features somewhere in this general problem of fitness and plant evolution.
