1910.] Forms of Plants as a Result of Environment. 551 



the different stages of development are multifarious. But a result of 

 general importance is obtained from experimental researches. The different 

 types of development are produced by quantitative alterations of those 

 factors of nature which are necessary to all life, such as light, temperature, 

 moisture, chemical composition of food, etc. Just as, in the case of the 

 Saprolegnia, the quantitative decrease of nitrogenous substances is essential 

 for the formation of zoospores, so, with the green algte, the same process 

 takes place on a decrease of the intensity of light, of the concentration of 

 inorganic salts, or of the temperature. In other cases, an increase in 

 regard to each of these factors is required to bring about the same result. 

 In view of our further discussion, it is important to bear in mind that 

 the sexual reproduction of the green algee also follows this general rule. 

 The controlling factors are found to be a reduction in the supply of nutritive 

 salts (especially the nitrogenous ones) and an increase in the intensity of 

 light, the efficiency of the illumination being responsible for the formation 

 of organic substances, such as carbohydrates. We shall find that these 

 alterations of environment even play a considerable part with the higher 

 plants. 



Whilst with the lower plants the deciding influence of environment is 

 beyond doubt, it looks very much as if, with the higher plants (and similarly 

 with animals), development is independent of it. In the first case, the 

 external influences operate on the simple cells in a comparatively direct 

 manner and in a short time. But the higher plants are composed of 

 different members, such as roots, stems, leaves and flowers, which stand in 

 very close relation to one another and affect one another reciprocally. 

 Each alteration of an external factor has an influence upon all the different 

 organs. Those parts of the plant which really produce all new formations, 

 such as the young embryo in the seed or the germinating point of the buds, 

 are always enclosed and protected, so that all changes of environment are 

 transmitted to them along exceedingly complex lines. Further, it must be 

 added that the special internal state of the plant susceptible to any reaction 

 accounted for by environment is the result of conditions of life which existed 

 long ago. These original influences frequently gave the plant a definite 

 tendency, which it continues to follow even though the external conditions 

 vary considerably. The development of flowering plants, carried out for 

 innumerable generations under relatively equal conditions of life, is generally 

 far more clearly defined than in the case of the lower plants. 



In spite of all these and other difficulties, we must try to grasp the 

 fundamental problem by means of experiments. Our experience hitherto 

 encourages us to advance, teaching us that the higher plants too are subject. 



