8i6 ECOLOGY 



other vascular plants equal homosporous pteridophytes in their capacity 

 for dispersal; the great wealth of ferns on oceanic islands commonly is 

 explained by the easy dissemination of their spores by wind. 



In Salvinia there is no true dehiscence, the whole sporangium being shed and the 

 spores germinating within. In Azolla the sporangia! wall slowly decays, setting free 

 the spores. In Marsilea the spores are contained within a hard-walled structure, 

 the sporocarp; when moistened the internal mucilaginous tissue absorbs water and 

 it swells to such an extent as to burst the sporocarp wall and protrude into the water, 

 carrying with it the attached sporangial masses (&g. 411). In heterosporous pteri- 

 dophytes the microspores have the mobility characteristic of the spores of homo- 

 sporous forms, but the megaspores are much less mobile; indeed, in some species 

 of Selaginella mobility is entirely absent, and the megaspore no longer is a dis- 

 seminule (fig. 308). With regard to protection and endurance, spores may vary 

 from the relatively delicate chlorophyll-containing spores of Equisetum, which die 

 unless germination occurs at once, to the remarkably protected spores of Selaginella 

 (fig. 303), which commonly germinate only after a long resting period. Remarkable 

 resistance to severe conditions is shown by the spores of Marsilea, which have 

 been known to germinate when sporocarps that had been kept dry for eighteen years 

 were placed in water; much of this capacity for endurance is due to the impermea- 

 bility of the sporocarp wall, as is shown by the fact that the spores in sporocarps that 

 have been kept in alcohol for three years may still remain capable of germination. 



In the seed plants there is an extremely complicated situation (p. 256 ff.). Het- 

 erospory is there universal, and the microspores (better known as pollen grains) 

 are scattered by various agents. The megaspores, however, always are retained, 

 having no longer the character of disseminules. The ecological features of these 

 organs will be considered in connection with flowers (p. 825 ff.). 



Sexual reproduction. — Significant features. — The chief feature of 

 sexual reproduction is the union or fusion of two cells, known as gametes, 

 resulting in the production of a sexually formed spore. Usually the two 

 gametes may be distinguished as male cells or sperms, and female cells 

 or eggs. The spore resulting from fusion, upon germination, develops 

 into a structure called the embryo. 



Isogamy. — In those thallophytes in which sexuality seems to be just 

 beginning {e.g. Ulothrix, figs. 1133, 1134), the two gametes are similar 

 in size and in structure and usually in activity; such a condition is called 

 isogamy; the spore resulting from the fusion of equal gametes is 

 called a zygospore (figs. 49, 50). Isogamous gametes may be ciliated 

 and actively motile (as in Ulothrix), non-ciliated but somewhat motile (as 

 in the diatoms), or almost immotile, that is, not leaving the plant body 

 (as in Spirogyra, fig. 109) . Although isogamous gametes exhibit no struc- 

 tural differences, there is some evidence of unlikeness; for example, in 

 Ulothrix and in Acetabularia, fusion takes place only between gametes 



