88o ECOLOGY 



activity, while other individuals or strains appear to develop reproduc- 

 tive organs, almost regardless of external conditions. From the repro- 

 ductive standpoint one of the most plastic of plants is Vaucheria (figs. 

 94-100), which in poorly illuminated running water may be kept in a 

 vegetative condition for several years, while in standing water it pro- 

 duces zoospores if weakly illuminated, and sex organs if well-illuminated 

 or if grown in media poor in food l ; zoospores may be formed also when 

 the food is scanty, and desiccation may result in the formation of non- 

 motile, thick- walled resting spores (aplanospores). 



In the lichens shade and moisture favor the formation of the soredia, while light 

 and desiccation favor the development of the organs concerned in asexual repro- 

 duction (apothecia). In Botrydium, zoospores develop in water, but when the plants 

 are desiccated, there develop aplanospores comparable to those of Vaucheria. 

 Saprolegnia is quite as plastic as is Vaucheria, vegetating indefinitely if well nour- 

 ished, but developing zoospores if grown in distilled water ; the development of 

 sex organs is favored by growth on solid substrata, by low temperatures, by food 

 impoverishment, and by the addition of specific salts to the media. In Spirogyra 

 zygospore formation is facilitated by high temperature as well as by desiccation; 

 there is a striking contrast between the dark green vegetative filaments of dilute 

 fresh water and the yellowish reproductive filaments of ponds that are drying up. 

 In Oedogonium, zoospore production is favored by depriving the media of nitrates 

 and phosphates, by growth in darkened distilled water, and by transfer from a rich 

 to a poor nutrient solution. In Botrytis there is a reciprocal relation between the 

 sclerotia and the conidia, the former being favored by good vegetative conditions, 

 while spore formation is favored by desiccation, by poor nutrition, and by high 

 concentration of the medium. Species differ widely as to the effect of increased 

 concentration of the medium; in Stigeoclonium, and perhaps in most forms, low 

 concentrations favor zoospore production, but in Tetraspora, zoospores continue to 

 develop at high concentrations, and in Vaucheria, concentration seems to make 

 but little difference. In Basidiobolus low concentrations favor zygospore production, 

 and high concentrations facilitate the development of thick-walled resting spores. 

 A reduced supply of oxygen appears to induce reproduction in Vlothrix. Monas, 

 one of the infusorians, reproduces vegetatively or sexually at 20 C., but by asexual 

 spores at temperatures between i C. and 4 C. 



Comparatively little is known concerning reproductive reactions to external con- 

 ditions among the higher fungi, though in Coprinus, Stereum, and Xylaria reproduc- 

 tive activity is favored by illumination, by poor nutrition, and by partial desiccation 

 In the rusts the development of teleutospores is hastened by refrigeration, as in alpine 

 cultures. It has been found also that in Uromyces Veratri, similar aecidiospores 

 produce the uredo generation if sown on young leaves, and the teleuto generation 

 if sown on old or wounded leaves, suggesting that the kind of spore that is formed 

 may be related to nutrition. In some cases external factors not only initiate periods 



1 In H ' ydrodictyon intense light favors zoospore production, and in Ulothrix light seems 

 to be without influence in this connection. 



