The significance of apical meristems in the phylogeny of land plants

Abstract

Two principal types of shoot apex are recognized, (1) the classical pyramidal apical cell of pteridophytes which does not divide periclinally, and so never contributes directly to the inner tissues of the axis, and (2) that of seed plants, in which there are one or more superposed initials of which the innermost contributes directly to inner tissues by periclinal divisions. The occurrence of these types in the major groups of living land plants is reviewed. The first type is present in ferns,Equisetum, Psilotum, Tmesipteris, Selaginella, and bryophytes (when apical growth occurs in them). The second type occurs in all angiosperms and gymnosperms, and also inLycopodium, Phylloglossum, Isoetes, andStylites. No exceptions nor intermediate conditions occur. Both types of apex are known in Carboniferous fossils, the first type in sphenopsids and the second in pteridosperms and cordaitean gymnosperms. These apices define two evolutionary lines of land plants which may have originated separately as sporophytes from ancestors with no diploid generation. Correlation of these types with the following features is considered: sieve-element inclusions; initiation of leaves and sporangia; the primary vascular system; embryogeny, the vascular cambium; initiation of lateral roots and the type of branching. Seed plants and pteropsids differ in respect of all these features, giving support to the hypothesis that the sporophytes of these two groups arose independently. The position of sphenopsids and lycopsids is also considered.