540 BOTANY OF THE LIVING PLANT 



Turning next to the products of germination of the spores, that is, 

 to the gametophyte generation, a comparison must be made of the 

 reduced and modified prothalli resulting respectively from the micro- 

 spores and megaspores. 



The microspore of Selaginella produces a single vegetative cell, and 

 an antheridium with a wall and many spermatocytes, each of which 

 gives rise to a freely motile spermatozoid. In the pollen of Pinus 

 two vegetative cells are formed, and obliterated in the developing 

 grain, and three further divisions follow. But there is no cellulose 

 antheridial wall, and only two spermatocytes appear, which remain 

 non-motile. Clearly this is a state further reduced and specialised as 

 compared with Selaginella. But it has been noted in Cycas and 

 Ginkgo that the gametes are motile spermatozoids, and in Microcycas 

 they are numerous. Such instances show that the products of ger- 

 mination of the microspores of Gymnosperms are sometimes motile 

 like those of Selaginella and other Pteridophytes : but they suggest 

 that in other cases the motility of the gametes has been lost in the 

 course of evolution. This change is also seen in Seed-Plants generally. 

 (Compare Figs. 227, 229.) 



The megaspore of Selaginella begins to produce a prothallus while 

 still within the sporangium, and continues its development after it is 

 shed. It bears archegonia which are exposed by rupture of the rugged 

 wall, and are fertilised by spermatozoids moving freely from the 

 germinating microspores. One or several embryos are produced. 

 Similarly in Pinus the female prothallus (endosperm) is formed within 

 the megaspore (embryo-sac), and produces archegonia. But it 

 remains embedded in the sporangium (nucellus), where fertilisation 

 takes place by non-motile gametes conveyed by the pollen-tube to 

 the neck of the archegonium. Several of these may be fertilised, and 

 a plurality of embryos be initiated. It follows that the contents of 

 the megaspore of Selaginella and of the embryo -sac of Pinus are 

 homologous, both being female prothalli, produced by germination of 

 the megaspore. (Compare Figs. 412, IV., and 423.) The only new 

 structure is the integument of the ovule of Pinus : the rest are modifi- 

 cations in accordance with Life on Land, of parts already present. 

 The biological probability of the several steps disclosed by this 

 comparison is such as to justify their acceptance as evolutionary 

 history. Those steps are, the retention of the megaspore upon the 

 parent plant, and the loss of its protective wall : the development of 

 the pollen-tube (perhaps as an extension of a single antheridium) : 

 and the loss of motility of the gametes. 



