170 COOK 



egonium of an aposporous prothallus, the only essential differ- 

 ence being that mitapsis has been postponed until shortly before 

 the formation of the egg-cells. 



The divisions of the pollen nucleus may be homologous with 

 the subdivision of a microspore to form a prothallus, but the fact 

 that some of these nuclei may enter into conjugation renders 

 them also homologous with the antherozoids to which the 

 moss-plant or the fern-prothallus eventually gives rise. In a 

 similar way, the egg-cell and its immediate predecessors may 

 be compared directly to those of the archegoniates, but they 

 need not be supposed to have the double homologies enjoyed 

 by the pollen nuclei. The complete female homologue of the 

 pollen-grain or microspore would, in this system, no longer exist, 

 the female prothallus being supplied by aposporous development 

 before mitapsis takes place. The aposporous prothallus antici- 

 pates and supplies the period of life-history previously attained 

 by the independent development of the detached macrospore. 



A prothallus which arose b}'- apospory, by maintaining an 

 intimate contact with the parent plant, would be able to nourish 

 and protect its progeny much better than the prothallus devel- 

 oped from a detached macrospore. The relation of the young 

 plant to its prothallial parent would be quite the same as that of 

 a moss-plant to its capsule or a fern plant to its prothallus, as 

 already stated. The difference lies in the special relation of 

 the aposporous prothallus to its vegetative parent. 



ASSOCIATION OF TWO GENERATIONS BY APOSPORY. 



Under the doctrine of alternation of generations it is often said 

 that the moss-capsule develops as a parasite on the parent plant. 

 In the angiosperms there would be, by parity of reasoning, a 

 double parasitism, the embryo is a parasite on the nucellus, and 

 the nucellus upon the parent plant. Parasitism is, however, hardly 

 the correct word, for the moss-capsule, the fern-frond and the 

 angiosperm-embryo are all developments from fertilized egg- 

 cells. The difference is merely that of the stage at which the 

 gametes or their products shall part company with their parent or- 

 ganisms. By continuing to assist the fertilized egg-cell, and 

 allowing it to develop into a capsule, the moss-plant is able to pro- 



