8i8 ECOLOGY 



Of chief ecological interest in connection with heterogamy are the 

 factors concerned in facilitating the fusion of gametes and particularly 

 the fusion of gametes of different immediate ancestry. In the algae as a 

 group, fusion is comparatively easy, since the eggs are immersed and the 

 sperms capable of locomotion. In the bryophytes and pteridophytes the 

 difficulty is greater, because usually they are land plants. In the liver- 

 worts and ferns the gametophytic generation commonly grows close to 

 the moist soil, where there often is sufficient moisture for the swimming 

 of such minute bodies as sperms. In many cases the female organs con- 

 tain substances which occasion prochemotactic reactions in sperms, thus 

 greatly facilitating the fusion of gametes; among such substances are 

 cane sugar (as in mosses) and maUc acid (as in ferns). 



Monoecism and dioecism. — When the same individual bears correl- 

 ative kinds of reproductive organs (e.^. antheridia and archegonia, or 

 stamens and pistils), the species is called monoecious if the organs are 

 borne on separate branches, and hermaphroditic if the organs are borne 

 together on a common branch ; if the two kinds of organs are borne on 

 separate individuals, the species is called dioecious. In the heteros- 

 porous pteridophytes dioecism occurs regularly, male gametophytes 

 developing from microspores and female gametophytes from mega- 

 spores. In most homosporous pteridophytes the gametophytes are 

 monoecious, but they are chiefly dioecious in Equisetum, and there are 

 many dioecious species among the algae and bryophytes. Obviously 

 the movement of sperms to the female organs is easier in monoecious 

 than in dioecious species. It has been supposed that the chief advantage 

 of dioecism is that it prevents close inbreeding {i.e. fusion between closely 

 related sex cells), it being believed oftentimes that certain advantages 

 are associated with the fusion of gametes of difierent immediate ancestry 

 (see p. 820). In the homosporous pteridophytes the fusion of related 

 gametes often is impossible, since many species are dichogamous, that 

 is, with the correlative organs on the same individual maturing con- 

 secutively; commonly the male organs develop first. 



In some dioecious species there are features that facilitate the germination of 

 male and female plants in close proximity; for example, the elaters of Equisetum 

 (figs. 337, 338) often cause a group of spores to become intertangled and thus to 

 fall and germinate together, and in Azolla the microspores cohere in masses and 

 often have hooks, the so-called glochidia, which become caught in the projecting 

 filaments of the megaspores (fig. 403). However, in many cases dioecism doubtless 

 is disadvantageous because of the difficulties in the way of fusion between male and 

 female gametes. In many mosses sporophytic generations rarely are seen, partly, 



