92 METABOLISM 



however, teaches us that plants of high organization which make considerable 

 demands on the soil, such as oats and maize, never occur on such soils, but only 

 plants which are unassuming in their requirements. In mountainous regions 

 lichens are always the first plants to appear, and these organisms, although we 

 must admit that qualitatively they require the same materials as Algae and 

 Fungi (p. 82), are content with much smaller quantities, simply because they 

 grow so very slowly. A higher plant, actively metabolic, would under such circum- 

 stances grow itself to death. As soon as lichens have effected the first settle- 

 ment on such a primitive soil, mosses, ferns, and finally flowering plants quickly 

 follow. In consequence, the substratum becomes less and less a natural soil of 

 disintegration and more and more of the nature of an arable soil, for each gene- 

 ration of plants makes the soil fitter for its successors, notwithstanding the fact 

 that it withdraws food materials from it. This follows from the fact that, in 

 the first place, every plant gives off carbon-dioxide, which, as we have seen, has 

 a disintegrating effect on the rock, and, secondly, that the dead parts of the 

 plant, not only the roots in the soil, but also the leaves, twigs, branches, and stems 

 formed above ground, ultimately reach the soil once more and decompose there. 

 In consequence, their organic constituents will be either entirely destroyed and, 

 amongst other things, carbon-dioxide will be produced, or such compounds as are 

 more resistent to decomposition will be transformed into humus, to which the 

 brown-black colour of the soil is due. Owing to the formation of carbon-dioxide 

 in the process of decomposition of plant debris the air in the soil is always very 

 rich in this gas ; Wollny (1897, 145) states that a minimum of 0-7 per cent, is 

 present in the soil in winter time, and a maximum of 4-8 per cent, in summer, so 

 that, in this respect also, vegetation has a marked effect on the rock constituents 

 of the soil. The humus has in addition the power of altering the characters 

 of the soil to a remarkable degree, both physically and chemically. From 

 a physical point of view the humus particles, deposited between the mineral 

 constituents, effect a loosening of the soil and, at the same time, increase its 

 capacity for retaining moisture (p. 26). Chemically, certain special humin con- 

 stituents are added. Since most of these are not considered as nutrients to the 

 majority of plants, we need only say that they consist of little-known compounds 

 of hydrogen, oxygen, nitrogen, and carbon, in part neutral in chemical reaction, 

 in part showing the characteristics of acids. We shall have something to say 

 afterwards as to the latter. In addition to the humin compounds humus contains, 

 as well, the ash constituents of plants from which the humus has been derived, 

 but in a form difficult to extract with water, without diminishing the absorption 

 by the plant. 



We have now to consider a phenomenon of the greatest importance to 

 plant life, known as soil absorption, viz. the capacity of the soil to extract 

 substances from their aqueous solutions. 



In order to demonstrate this absorptive process we permit the filtration 

 of a solution of indigo-carmine or yellow liquid manure through a layer of earth 

 of a certain thickness ; we shall find that the fluids which escape are quite 

 colourless. Were this power of absorption limited to colouring matters, the 

 importance of the process, so far as plants are concerned, would be insignificant ; 

 but inorganic salts are also firmly retained by the soil, so that solutions of such 

 bodies, after they have passed through the soil, have lost in concentration, and 

 have also lost entirely certain constituents which they previously contained. 

 Suppose we add to the soil one of the nutritive solutions mentioned on p. 81, 

 we shall find that the potassium, calcium, magnesium, and phosphoric acid are 

 retained, as well as ammonia and the less important plant nutrient sodium, but 

 that the acids, other than phosphoric, i. e. nitric, sulphuric, and hydrochloric, 

 are not. Absorption of the metals above mentioned belonging to the series of 

 alkalis and alkaline earths takes place in different ways according as they occur 



