352 ECOLOGY. 



large female plants, though in some cases the latter bear also antheridla. It 

 has been found that when the spores are given but little nutriment they form 

 male prothallia, and the spores supplied with abundant nutriment form 

 female prothallia. 



643. Permanent separation of sexes by different amounts of nutriment sup- 

 plied the spores. — This separation of the sexualorgansof different prothallia, 

 which in most of the ferns, and in equisetum, is dependent on the chance 

 supply of nutriment to the germinating spores, is made certain when we come 

 to such plants as isoetes and selaginella. Here certain of the spores receive 

 more nutriment while they are forming than others. In the large sporangia 

 (macrosporangia) only a few of the cells of the spore-producing tissue form 

 spores, the remaining cells being dissolved to nourish the growing macro- 

 spores, which are few in number. In the small sporangia (microsporangia) 

 all the cells of the spore-producing tissue form spores. Consequently each 

 one has a less amount of nutriment, and it is very much smaller, a micro- 

 spore. The sexual nature of the prothallium in selaginella and isoetes, then, is 

 predetermined in the spores while they are forming on the sporophyte. The 

 microspores are to produce male prothallia, while the macrospores are to 

 produce female prothallia. 



644. Heterospory. — This production of two kinds of spores by isoetes, 

 selaginella, and some of the other fern plants is heterospory, or such plants 

 are said to be heterosporous. Heterospory, then, so far as we know from liv- 

 ing forms, has originated in the fern group. In all the higher plants, in the 

 gymnosperms and angiosperms, it has been perpetuated, the microspores being 

 represented by the pollen, while the macrospores are represented by the em- 

 bryo sac; the male organ of the gymnosperms and angiosperms being the 

 antherid cell in the pollen or pollen tube, or in some cases perhaps the pollen 

 grain itself, and the female organ in the angiosperms perhaps reduced to 

 the egg cell of the embryo sac. 



645. In the pteridophytes water serves as the medium for conveying the 

 sperm cell to the female organ. — In the ferns and their allies, as well as in 

 the liverworts and mosses, surface water is a necessary medium through 

 which the generative or sperm cell of the male organ, the spermatozoid, may 

 reach the germ cell of the female organ. The sperm cell is here motile. 

 This is true in a large number of cases in the algse, which are mostly aquatic 

 plants, while in other cases currents of water float the sperm cell to the 

 female organ. 



646. In the higher plants a modification of the prothallium is necessary. 

 — As we pass to the gymnosperms and angiosperms, however, where tlie 

 primitive phase (the gametcjphyte) of the plants has become dependent solely 

 on the modern phase (the sporophyte) of the plant, surface waiter no longer 

 serves as the medium through wliich a motile sperm cell reaches the egg cell 

 to fertilize it. The female prothallium, or macrospore, is, in nearly all 



