50 



The Plant World. 



those cf another are not affected at all. Also, as a plant devel- 

 ops, its internal conditions, — i. e., its vital nature, — are contin- 

 ually altered, so that one and the same set of environmental 

 conditions may produce quite different effects upon the same 

 plant at different times in its life cycle. 



The environment is an exceedingly complex thing, and one 

 very difficult of analysis. As already pointed out, it consists 

 of a number of factors, such as temperature, wave-length and 

 intensity of light, evaporating power of the air, nature of the 

 soil, etc. Each one of these component factors may play an 

 important role in causing the premature death of plants or in 

 producing unusual phenomena of development. Some of the 

 ways in which the soil may affect plant behavior will be briefly 

 considered here. 



The presence cf some sort of soil is essential to the growth 

 of most higher plants, on account of the mechanical need of 

 an anchorage. This anchorage is effected by the growth of the 

 root system, progressing hand in hand with the growth of the 

 upper parts of the plant. The growth of the roots is determined 

 by several soil factors besides that of mechanical penetrability, 

 which is, of course, an essential feature. The oxygen for root 

 respiration must mainly come from the soil and diffuse directly 

 into the growing roots. Water and the nutrient mineral salts 

 must also diffuse into the roots as they are removed by the plant 

 activities. These are necessary, not only for the formation of 

 the underground parts of the plant, but also for that of the 

 portions above the soil. But it is not primarily the amount of 

 oxygen, nutrient salts and water, which determines the behavior 

 cf the plant, rather is it the rate at which it is possible for these 

 substances to diffuse into the roots. And this rate does not 

 necessarily depend directly upon the amount of these substances 

 existing in the neighborhood of the root system, but is modified 

 greatly by the physics of the soil itself. Soil temperature is 

 also very important. It determines whether or not roots can 

 live and whether they may grow. It also affects the activities 

 of the whole organism profoundly, through its effects upon the 

 possible rates of diffusion through the soil and from the soil 

 into the roots. 



The most important of the soil factors is undoubtedly that 

 of the water relation; in higher plants the water used is practi- 



