REPRODUCTION AND DISPERSAL 879 



tion. It is believed that the inception of each of these states depends 

 upon definite external factors, though vegetative activity necessarily must 

 antedate the others, since it is the stage of food accumulation. 



Vegetative activity may be prolonged indefinitely, being favored by 

 the continued uniformity of optimum vegetative conditions. The most 

 important single factor favoring such activity appears to be the constant 

 presence of sufficient water to keep the cell sap dilute, and to facilitate 

 active growth. Another important factor seems to be a uniform and 

 moderately high temperature, chiefly, perhaps, because of its effect upon 

 the absorption of water. Under uniformly high temperatures, Bacillus 

 anthracis and other bacteria have been kept in a state of continued vege- 

 tative activity, with no tendency to develop resting cells ("spores"). 

 Saprolegnia has been kept for six years in a purely vegetative condition, 

 and brewers' yeast probably has been kept essentially vegetative for 

 centuries. 



Both sexual reproduction and asexual reproduction are induced by 

 changes in external conditions, and particularly by changes that are 

 detrimental to optimum vegetative activity. Although species differ 

 quantitatively and qualitatively as to the precise external factors that 

 are involved in the initiation of reproductive activity, it has been shown 

 in many cases that the development of reproductive structures is induced 

 by desiccation, by increased concentration of the medium, by very high 

 and by very low temperatures, by intense illumination, by decreased 

 food supply, and by the presence or absence of specific chemical sub- 

 stances. It is scarcely possible as yet to distinguish sets of factors which 

 commonly initiate sexual reproduction as opposed to asexual reproduc- 

 tion, although in some cases (notably in the molds) the development of 

 asexual spores is favored by those factors which are most opposed to 

 vegetative activity, namely, desiccation, food impoverishment, low tem- 

 perature, high concentration of the medium, and strong illumination; 

 zygospore formation, on the other hand, is favored in the molds by con- 

 ditions which more closely resemble those favoring vegetative activity, 

 namely, moisture, rich food supply, high temperature, low concentration 

 of the medium, and darkness. However, sexual reproduction is favored 

 by strong illumination in Vaucheria, by low temperature, and by food 

 impoverishment in Saprolegnia, and by desiccation in Spirogyra. In 

 some cases it seems as if almost any alteration of previous conditions 

 serves to initiate reproductive activity, and in other cases there seem to 

 be certain individuals or strains predisposed to continued vegetative 



