REPRODUCTION 269 



vegetative activities. This corresponds with Klebs's obser- 

 vations, but puts one part of them in somewhat more defi- 

 nite terms. 



We see, then, that so far as the fresh-water algae and 

 certain fungi are concerned, plants react in reproduction, as 

 in other ways, to external influences. In regions where 

 there are clearly marked seasons, during one of which active 

 vegetation is impossible by reason of extreme cold or dark- 

 ness or dryness, these violent conditions determine the 

 behavior of the organisms living there. In milder seasons 

 and in more constantly temperate regions, more moderate 

 influences determine their behavior. Of the forces which act 

 upon algae, stimulating processes which can be accom- 

 plished without them, light and heat are evidently the most 

 important. A certain minimum amount of heat is a neces- 

 sary condition of life; without it action is impossible; but 

 given this minimum amount, more heat will stimulate to 

 reproduction. A certain minimum amount of light is the 

 necessary source of the energy for food-manufacture, but 

 given this amount, more light will stimulate to some form 

 of reproduction. Neither this larger amount of external 

 heat nor the more intense light is necessary as a means of 

 carrying on reproduction; they only set in operation that 

 succession of processes which terminates in the formation of 

 new individuals. Among the fungi, Klebs* claims that 

 changes in the nutrition furnish the stimulus to a well- 

 nourished mj'celium to develop reproductive organs. 



Vochting's work on the conditions of reproduction in 

 flowering plants t yielded results with which those of Klebs 

 harmonize. Flowers are the visible agents of sexual repro- 

 duction in higher plants. When they are suppressed, or even 

 are imperfect, sexual reproduction does not take place. 

 Sexual reproduction in higher plants consists, as in lower 

 forms, in the union of the microscopically small sexual 

 elements. The forces which institute, stimulate, and favor 



* Klebs. G. Zur Physiologie der Fortpflanzung einiger Pilze. Jahrb. f. 

 wiss. Bot., pp. 146-7, Bd. 35, 1900. 



+ Vochting, H. t'ber den Einfluss des Lichtes auf die Gestaltung und 

 Anlage der Bliithen. Jahrb. f. wlss. Bot.. Bd. 25, 1893. 



