230 Influence of Environment on Plants 



been demonstrated that each developmental stage depends upon 

 special external conditions, and in cases where our knowledge is 

 sufficient, a particular stage may be obtained at will. In the Green 

 Algae 1 , as in the case of Fungi, we may classify the stages of develop- 

 ment into purely vegetative growth (growth, cell-division, branching), 

 asexual reproduction (formation of zoospores, conidia) and sexual 

 processes (formation of male and female sexual organs). By modify- 

 ing the external conditions it is possible to induce algae or fungi 

 (Vaucheria, Saprolegnia) to grow continuously for several years or, 

 in the course of a few days, to die after an enormous production of 

 asexual or sexual cells. In some instances even an almost complete 

 stoppage of growth may be caused, reproductive cells being scarcely 

 formed before the organism is again compelled to resort to repro- 

 duction. Thus the sequence of the different stages in development 

 can be modified as we may desire. 



The result of a more thorough investigation of the determining 

 conditions appears to produce at first sight a confused impression of 

 all sorts of possibilities. Even closely allied species exhibit differ- 

 ences in regard to the connection between their development and 

 external conditions. It is especially noteworthy that the same form 

 in development may be produced as the result of very different 

 alterations in the environment. At the same time we can un- 

 doubtedly detect a certain unity in the multiplicity of the individual 

 phenomena. 



If we compare the factors essential for the different stages in de- 

 velopment, we see that the question always resolves itself into one 

 of modification of similar conditions common to all life-processes. We 

 should rather have inferred that there exist specific external stimuli 

 for each developmental stage, for instance, certain chemical agencies. 

 Experiments hitherto made support the conclusion that quantitative 

 alterations in the general conditions of life produce different types 

 of development. An alga or a fungus grows so long as all the con- 

 ditions of nutrition remain at a certain optimum for growth. In 

 order to bring about asexual reproduction, e.g. the formation of zoo- 

 spores, it is sometimes necessary to increase the degree of intensity 

 of external factors ; sometimes, on the other hand, these must be 

 reduced in intensity. In the case of many algae a decrease in light- 

 intensity or in the amount of salts in the culture solution, or in the 

 temperature, induces asexual reproduction, while in others, on the 

 contrary, an increase in regard to each of these factors is required to 

 produce the same result. This holds good for the quantitative vari- 

 ations which induce sexual reproduction in algae. The controlling 



1 See Klebs, Die Bedingung der Fortpflanzimg.,., Jena, 1896 ; also Jahrb.filr Wiss. Bot. 

 1898 and 1900; "Probleme der Entwickelung, in." Biol. Centralbl. 1904, p. 452. 



