210 BOTANY part i 



Other species. Plant-geography ("*), however, shows that the composi- 

 tion of the soil exerts a great influence on the distribution of plants. 

 This depends, on the one hand, on the fact that different plants make 

 different demands on the amount and solubility of the essential food- 

 materials, and, on the other, upon the presence in the soil of substances 

 other than the indispensable salts ; the influence of these non-essential 

 substances is different upon different species of plants. For example, 

 CaCog has a poisonous effect on some plants, and NaCl upon others, 

 while other plants can endure large doses of these substances. 



The effect of the soil upon the distribution of plants does not depend merely 

 upon its chemical nature. The physical properties of soils play an important role. 

 Further, a plant may be absent from a locality, which, so far as the nature of the 

 soil is concerned, would be suitable, because its seeds have never been brought to 

 the spot. 



Since the plant always obtains nutrient salts from the soil in 

 very dilute solution, the epidermal cells which absorb this will also 

 contain only small quantities of the salts. The salts must, however, 

 reach all parts of the plant, and especially the foliage leaves. For 

 this purpose diffusion would be too slow a process, and we therefore 

 find that the nutrient salts are conveyed in the plant by the 

 transpiration current. It is thus not merely water but a very dilute 

 food-solution that is conducted by the vascular bundles, and the use 

 of transpiration is, in the first place, to concentrate this nutrient 

 solution and, in the second, to bring it quickly to the proper parts 

 of the plant. Apart from this result it would be difficult to under- 

 stand the process of transpiration, and the plant would certainly 

 have found means of limiting it. When it is actually checked (cf. 

 p. 197), we have to do with jjlants which grow slowly on account of 

 the poor supply of salts, and also it is true of carbonic acid. 



Besides water and salts dissolved gases are also taken up by the 

 root ; as a rule only oxygen is of importance. The main source of the 

 gases absorbed by the plant is, however, the atmosphere. 



When the substratum on which a plant grows contains, in addition 

 to water and food-salts, other substances, e.g. dissolved organic sub- 

 stances, these may be absorbed and transported in the same way. 

 Water-cultures show, however, that the typical green plant at least is 

 not dependent upon such substances. It is otherwise with fungi and 

 other plants which resemble the fungi in their metabolism. 



(e) Gases 



While water and salts are, as has been seen, as a rule absorbed from 

 the soil, the air contains substances which are necessary to the success- 

 ful existence of the plant, and must l)e termed food-materials. These 

 are carbon dioxide and oxygen. They are, as a rule, obtained from 

 the atmosphere. Only submerged water plants ol)tain them from the 



