Some life-history consequences of modular construction in plants

Abstract

The nature and life-history consequences of modular construction in plants are discussed with particular reference to growth, reproduction and survival. Plants grow by the iteration of modular units and as a consequence growth can be described in terms of the population dynamics of these structural units. Changes in size, whether positive or negative, depend on the birth and death rates of modules; however, if the births continue to exceed the deaths, plants then have the capability of attaining enormous sizes, especially if they are clonal. The population nature of plant growth also means that plants of the same age may show large variation in individual size if individuals differ in their relative growth rates. Correlations between age and size are often, therefore, very weak. Constraints on the allocation of resources accumulated during growth have important implications for the reproductive schedules of plants, but the analysis of constraint functions has so far revealed little about the actual detail of these schedules. All the meristems of semelparous plants are involved in or die at reproduction and as a consequence death of the genet follows reproduction. For iteroparous plants, however, there are fundamental differences between the reproductive schedules of plants with a single shoot module and those with many shoot modules. The former demonstrate a relatively constant rate of reproduction from year to year following maturity whereas the latter show a continual increase in fecundity with size and age. The reproductive schedules of clonal plants are further discussed in relation to the allocation of meristems to either growth or reproduction. The pattern of mortality is examined at both the level of the module and the genet. Particular attention is focused on the survival and senescence of leaves and shoots; there is no equivalent regular shedding of organs in unitary organisms. Whereas genet senescence and death are coincident with shoot module death in semelparous plants, there is no evident relation between them in iteroparous plants. The life span of the genet reflects the birth and death rates of its modules and both aclonal and clonal plants that are iteroparous may achieve considerable longevity. The longevity of aclonal plants often seems to be restricted by the accumulation of dead material and the problems of being large. Clonal plants are, in contrast, potentially immortal. It is questionable whether the genets of iteroparous plants show senescence as defined for unitary organisms since there is no separation of germ plasm from soma and since apical meristems do not appear to senesce. Insofar as they retain the capacity for rejuvenescence from apical meristems, genets of modular organisms do not senesce; it is only the constituent organs that show senescence, death and decay.