242 BOTANY PART I 



the case in many water plants in which the whole external surface is 

 of use in absorption. Since the salts only exist in very dilute solution 

 in the water, the need of an extended surface for this purpose is 

 readily understood; this in part explains the frequent occurrence of 

 finely divided leaves in water plants. The salts dissolved in the water 

 are not, however, sufficient for all aquatic plants; many absorb 

 substances from the soil underlying the water by means of their roots, 

 and do not succeed when deprived of roots. 



As a rule in the higher plants the salts are absorbed from the soil. 

 The salts contained in the nutrient solution described above, or similar 

 compounds, are constantly present in the water of the soil ; some of 

 them, however, in such small amount as only to suffice for the growth 

 of plants for a short period. Other sources of supply of the food-salts 

 must exist when such growth continues. In fact, the amount of salts 

 dissolved in the soil-water is no measure of the fertility of the soil. 

 The soil always contains food-salts, partly in an absorbed condition, 

 and partly in mineral form which the plant has to render accessible. 

 This is effected mainly by the excretion of carbonic acid from the root- 

 hairs. Many substances are much more readily soluble in water con- 

 taining carbonic acid than in pure water. 



The solution of solid rock by the plant may most readily be shown by allowing 

 the roots to grow against smooth polished slabs of marble ; the course of the roots 

 is indicated by the etching of the surface. 



There are other cases in which stronger acids than carbonic acid excreted by 

 the plant are concerned in bringing minerals into a soluble form. This can hardly 

 be doubted when felspar and mica are dissolved by certain Lichens ( 24 ). Fungi 

 and Bacteria also frequently produce and excrete solvents of this kind during their 

 metabolism, and may have a similar effect on insoluble substances in the soil. 



Some soils, especially those containing much clay, lime, or humus, have the 

 property of retaining potassium and ammonium salts, and in less degree salts of 

 calcium and magnesium, as well as phosphates ; these substances are not easily 

 washed out of the soil but can be obtained by plants. This is spoken of as the 

 power of absorption of the soil for the substances in question. This does not hold 

 for all salts ; thus, for instance, sulphates and nitrates are not absorbed. Absorp- 

 tion is completely wanting in a pure sandy soil. 



When the substratum contains, in addition to water and nutrient 

 salts, dissolved organic substances, these may be absorbed in the same 

 way. Water cultures show, however, that at least the typical green 

 plant is not dependent on such substances. It is otherwise with the 

 Fungi and other plants which resemble them in metabolism (p. 255).^ 



In addition to water and nutrient salts dissolved gases may also be 

 absorbed by the roots. As a rule only oxygen need be considered. 

 The main source from which gases are absorbed is the atmosphere. 



Transport of the Nutrient Salts. The salts do not remain in 

 the epidermal cells of the root or shoot but pass from the place of 

 absorption through the whole plant. This takes place in two ways, 



