25O THE PTERIDOPHYTA 



small microspores, about the same size as ordinary 

 spores (Fig. 39, C). These are formed in separate 

 sporangia (Fig. 39, B). In these heterosporous species the 

 ordinary kind of free-living prothallus, bearing both 

 male and female sexual organs, is not produced. The 

 megaspore on germination produces a small mass of 

 cells at its apex, the bottom of the spore being some- 

 times afterwards filled with cells (Fig. 39, G), and on 

 this small prothallus one or more archegonia are 

 formed. The microspore produces a prothallus of even 

 fewer cells, in most cases inside the microspore wall 

 (Fig. 39, D), and with the exception of one or two, the 

 whole of these form a single antheridium (Fig. 39, D). 

 The spore absorbs water and the wall bursts, setting 

 free the sperms (E, F). Fertilisation takes place as in 

 the ordinary (homosporous) Pteridophytes. The ferti- 

 lised egg obtains the food which enables it to grow 

 into a self-supporting plant mainly or entirely from 

 the organic food supplies stored in the megaspore (G). 

 The far-reaching reduction of the prothallus and the 

 suppression of its powers of independent growth and 

 nutrition that we see in these heterosporous Pteri- 

 dophytes involves the direct dependence of the young 

 sporophyte produced from the fertilised egg not only on 

 the gametophyte but on the spore (megaspore} which 

 produced it, instead of on a free-living sexual genera- 

 tion, as in the homosporous Pteridophyte. This opens 

 the door, so to speak, to a further adaptation to land 

 life, in which the megaspore is retained in the 

 sporangium, and the sexual generation need no longer 



FIG. 39. Selaginella, a heterosporous cone-bearing Pteridophyte. 

 A, end of leafy branch, with three cones (natural size) . B, portion 

 of stem, of cone with megasporangium (left) opened showing 

 megaspores, and (right) microsporangium setting free microspores. 

 C, microspore (mi.) and megaspore (me.) showing thick wall 



