SPOROPHYTE, Evolution of. Some botanists are still disputing in regard to those earliest stages in the evolution of the sporophyte which are found in the alga and fungi; but there is complete agreement that in all plants from the liverworts and mosses to the sunflowers and orchids the sporophyte be gins with the fertilized egg. (See ALTERNATION OP GENERATIONS). Leaving out the alga and fungi and beginning with the liverworts, the evolution of the sporophyte can be traced with more or less certainty. Before considering the details of development, it may be said that, in such early stages of evolution as are seen in the liverwort and mosses, the sporophyte has no leaves or roots and is dependent upon the gametophyte throughout its lifetime. In the ferns and their allies, the sporophvte is de pendent upon the gametophyte during its early embryology, but soon develops leaves and roots and becomes completely independent of the gametophyte, which then dies and disappears. In the seed plants, the gametophyte is the parasitic generation and contributes very little to the sporophyte. except during the first few divisions of the fertilized egg. Several of the most fundamental features in the evolution of the sporophyte are seen in the liverworts. In Riccia, one of the lower liverworts, the sporophyte is a small, globular body about one-sixteenth of an inch in diameter, and con sists of a central mass of spores surrounded by a single layer of cells (Fig. 1. A). The signifi cant fact is that the entire product of the fertilized egg, except a single, protective outer layer of cells, produces spores. Theoretically, the earliest sporophyte should be one, all of whose cells produce spores. In Riccia. a single outer layer of cells has been diverted from the spore producing function, to serve as a pro tective layer. Botanists speak of this diversion as the 'sterilization of sporogenous tissue.' The evolution of the sporophyte is marked by more and more sterilization of sporogenous tissue, so that less and less of the product of the fertilized egg is devoted to the production of spores. In some liverworts, all the spores are derived from one-half of the fertilized egg, all the cells derived from the other half being diverted to a vegetative function. This is shown, diagrammatically, in Fig. 1. B, where s indicates the spore tissue and v the diverted, or vegetative tissue. In most liverworts, even less of the product of the fertilized egg develops into spores, not only one-half, but a large part of the other half being diverted to a vegetative function. This diverted portion
forms a °foot° which acts as an absorbing organ, and a stalk, which becomes much elongated and places the spores in a more favorable position for dispersal when they are shed (Fig. 1, C-f, foot; s, stalk; sp, spore tissue). In many forms, even in the spore region, some of the cells become diverted to a nutritive function or aid in the dispersal of spores. In Anthoceros, a much studied liver wort, there is a still further sterilization of sporogenous tissue, a central column being di verted to vegetative purposes, so that the spore tissue forms only a thimble-shaped mass, the lower part of which is shown, in longitudinal section, in Fig. 1, D. In this case, the youngest spore tissue is at the bottom and the mature spores higher up, a continuous growth in the lower portion resulting in somewhat prolonged period of spore production. In the mosses, there is still further sterilization, so that a comparatively small portion of the sporophyte produces spores. In all these sporophytcs, there is more or less green tissue, so that there is some advance toward independence; but there are no leaves and the sporophyte lives and dies with its foot attached to the gatneto phyte. The simplest form among the ferns and fern allies has a root, stem and leaf, and so is independent. There is no form intermediate be tween a sporophyte without a leaf and a sporophyte with a fairly well developed leaf. Consequently, the origin of the leaf is still a problem. In the ferns, the spore tissue is not only greatly restricted, appearing only as the °fruit dots' on the underside of the leaf, but it often appears late in the life history of the plant. In the flowering plants, there is still less spore tissue and it may appear still later in the development of the plant. In some of the cycads, the plant may reach an age of 25 years, or even more, .before any spore tissue is produced. There is such a tre mendous difference between a flowering plant and the small, simple sporophyte of Riccia, that it hardly seems possible that the large, complex form could represent a final stage in an evolution resulting from a pro gressive sterilization of sporogenous tissue. However, this is the most generally accepted theory of the evolution of the sporophyte. Con sult Bower, F, 0., The Origin of a Land Flora' ; Campbell, D. H., 'Mosses and Ferns.'