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’, 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 z00- 
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 Fortpflanzung..., Jena, 1896; also Jahrb. fiir Wiss. Bot. 
1898 and 1900; ‘‘ Probleme der Entwickelung, 11.” Biol. Centralbi, 1904, p. 452. 
