REPRODUCTION. 6oi a 



X. 



of the sporangium as being an asexual reproductive organ. In 

 the latter case the spores are not formed from the protoplasm 

 of a single reproductive organ, but from the fused protoplasm 

 of two distinct reproductive organs. A reproductive organ'} 

 which is incapable of producing spores from its own proto-\ 

 plasm is a sexual reproductive organ ; and -the fusion of the 

 protoplasm of two such organs which leads to the forma- 

 tion of spores constitutes what is known as the sexual 

 process. 



The mode in which the sexual process is effected is by no 

 means the same in all plants. Without entering at present 

 into minute details, it may be pointed out that it is usually 

 preceded by a process of cell-formation in one or both of the 

 organs concerned. The cells thus formed are, like the spores, 

 reproductive cells, but unlike the spores they are not capable, 

 each by itself, of giving rise to a new individual ; it is only 

 when two of these imperfect reproductive cells, sexual repro- 

 ductive cells, or gametes, as they are termed, coalesce, that a 

 fertile cell, a sexually produced spore, is formed. 



Before entering into detail with regard to these two modes 

 of spore-formation, we may note the fact that a suppression 

 of spore-formation, either sexual or asexual, may occur, and 

 vegetative reproduction be substituted. For instance, you are 

 aware that, in the case of Ferns, the fern-plant produces 

 spores which give rise to prothallia on germination. Now 

 Bower has ascertained in the Fern Athyrium Filix famina 

 var. clarissima, that the sporangia do not produce spores, 

 but that some of them grow out directly into prothallia; 

 that there is a substitution of vegetative reproduction for 

 reproduction by means of asexually produced spores, a sub- 

 stitution which is termed apospory. Again, you are aware 

 that the fern-plant is normally developed from the sexually 

 produced spore (oospore) formed by means of the sexual 

 reproductive organs of the prothallium. Now Farlow, de 

 Bary, and Sadebeck, have observed cases (Pteris cretica, Aspi- 

 dium Filix mas> var. cristatum, Aspidium falcatum, Todca 

 africand] in which -the fern-plant is developed as a bud 

 from the prothallium without the intervention of the sexual 



