884 ECOLOGY 



sexuality have recently been discovered in the rusts and smuts and in 

 certain other groups. Even the formation of asexual spores appears to 

 have ceased in some fungi, as in the internal fungus of Lolium, which 

 probably is a smut, and it is rare in others, as in most mycorhiza fungi. 

 It commonly has been supposed that the reduced or modified sexuality 

 of the fungi is in some way associated with their saprophytic or para- 

 sitic mode of life; since well-nourished plants reproduce sexually less 

 than do poorly nourished plants, it is possible that the good nutritive 

 conditions of the group in part account for the character of its sexual 

 development. 



Reproductive variations in the bryophytes and pteridophytes. — Light 

 favors the development of sex organs in liverworts, mosses, and ferns. 

 In Marchantia, weak light or an excess of moisture favors ordinary veg- 

 etative reproduction; an increase of illumination favors the development 

 of gemmae, and strong illumination favors the development of sex organs. 

 In weak light, fern gametophytes develop into filaments resembling 

 moss protonemata instead of producing sex organs. If in Salvinia the 

 sperms and eggs fail to fuse, the female gametophyte, whose growth 

 commonly is checked at such fusion, continues to grow vegetatively, 

 producing new female organs; thus embryo development seems in some 

 way to check gametophytic vegetative activity. Similarly, the gameto- 

 phytes of Osmunda are long-lived, if fusion does not take place. While 

 most fern gametophytes are monoecious, producing first male organs 

 and then female organs, gametophytes that are poorly nourished (having, 

 for example, a small supply of nitrogen) or are exposed to strong illumina- 

 tion, may produce male organs only, as though the food supply were 

 insuflicient for complete development; in rare instances, vigorous, well- 

 nourished gametophytes bear only archegonia. In the ostrich fern 

 {Onodea Struthiopteris) the gametophytes commonly are dioecious, the 

 larger plants being female, and the smaller plants being male; in or- 

 dinary cultures one to twelve per cent of the plants are monoecious. 

 Under certain culture conditions, as when female plants are transferred 

 to rich nutrient media, at least fifty per cent of the plants may become 

 monoecious. Similar phenomena occur in some monoecious mosses, 

 antheridia being the only sex organs developed when the nutritive con- 

 ditions are poor ; in some dioecious forms (as in Dicranum) the male 

 plants are smaller than the female plants. In the ferns it often is easy 

 to induce apogamy and apospory, especially where the illumination is 

 weak, or where the soil is dry or poor in food materials. 



