Pollen Walls as Adaptive Systems

Abstract

The structural variation of angiosperm pollen is primarily related to the wall, for the male gametophyte itself is simple in organization and more or less uniform in character throughout the group. The division of the wall into an outer exine and an inner intine is clearly a fundamental feature, as it is in the pollen grains and spores of other vascular groups, but the relative development of the strata differs widely among the families. The exine, usually the more structurally complex of the wall layers, is strikingly diverse in the types of wall sculpturing it can express, as well as in the numbers, distribution, and architecture of apertures and internal cavities. Faced with such variation, the first impulse is assuredly to attempt to bring some order and understanding by classification and ordination, and this has been the aim of palynologists since the days of von Mohl, a centuryand-a-half ago. As this present symposium has shown, the task is by no means complete; but what has been achieved is already impressive, not least in the contribution pollen taxonomy has made to the advance of angiosperm taxonomy in general. If one's inclination is to ask for explanations, however, classification is not enough. The instinct is to seek for physiological meaning in the pollen grain wall-to attempt to understand what functions the structural features fulfill in the general biology of plants, and to search for some comprehension of the variation in form in evolutionary terms, seeking for evidence of adaptive diversification. The essential function of the pollen grain and the tube that emerges from it is, of course, to deliver a pair of gametes to the embryo sac. The haploid male gametophyte with its single vegetative cell is structurally simple, but the attainment of its functional objective, the double fertilization, demands considerable physiological sophistication. This is expressed in a whole sequence of interlocking adaptations associated with dispersal, interaction with the stigma and style, nutrition, growth, and target finding. We now know that in the journey between the anther and the receptive stigma a major role is played by the pollen grain wall, which is concerned not only with protection and dispersal, but with the hydrodynamics of the gametophyte within it, and also in various subtle ways in the interactions on the stigma. The wall, in fact,-serves many functions; and this must mean that in the course of angiosperm evolution it has been the target of many kinds of selective pressure. To it, as to any adaptive structure, a familiar truism must apply, namely, that the result of evolution under manifold selective forces must in each of the surviving lineages represent some sort of a compro-