324 BOTANY OF THE LIVING PLANT 



the universal adoption of tiiis derivative state by Seed-bearing 

 Plants. 



The germination of the microspore of Selaginella compares with 

 the maturing of the pollen-grains of Gymnosperms in the formation 

 first of the prothallial cell, which corresponds to the cells which are 

 obliterated in them. The further development in Selaginella is more 

 elaborate, for there is a protective layer of cells of the wall, and a 

 considerable mass of spermatocytes, each giving rise to a sperma- 

 tozoid. In Pitiiis the gametes are non-motile, and only tvv'o in 

 number. But it has been noted in Cycas and Ginkgo that they are 

 motile, and in Microcycas they are numerous. Such instances show 

 that the products of development within the microspores of Gymno- 

 sperms and of Selaginella are comparable. In both cases they act as 

 male gametes. 



The product of germination of the megaspore in Selaginella is a 

 prothallus, bearing archegonia which on fertilisation produce an 

 embryo, or several. Similarly, in Pinus the embryo-sac develops 

 the endosperm before fertiUsation. It also bears archegonia corre- 

 sponding in essentials to those of Selaginella, which on fertihsation 

 produce embryos. It may therefore be concluded that the megaspore 

 of Selaginella and its contents are comparable to the embryo-sac of 

 Gymnosperms, and its contents. When parts of different organisms 

 are thus found to be similar in origin they are said to he homologous one 

 with another. We conclude from their development that the micro- 

 spore of Selaginella is homologous with the pollen-grain of Pinus, and 

 of Seed-Plants at large ; and that the megaspore of Selaginella is 

 homologous with the embryo-sac of Pinus, and of Seed-Plants at 

 large. And this holds notwithstanding that m Selaginella both 

 megaspores and microspores are shed from the parent plant, but in 

 Seed-Plants only the microspores are shed, while the megaspore is 

 retained. 



If further evidence were reqaired of the correctness of these conclus'ons, 

 it is found in the fact that in certain fossils belonging to the Lycopodiales 

 (Miadesmia and Lepidocarpon) a seed-hke structure did actually exist. It 

 resulted from the retention of the megaspore in the sporangium, just as it is 

 seen as a permanent and constant feature in Seed Plants. The conclusion is 

 that Selaginella stands in this respect on a more primitive looting than these 

 fossils, or than the whole of the successful Series of Seed-bearing Plants. 



It has been seen that in Selaginella the megasporangia and micro - 

 sporangia are alike in their origin, and development up to the stage 

 of the spore-mother-cells. It follows that these sporangia also arc 



