INFLUENCE OF THE SUBSTRATUM. 497 



Under these circumstances it is a matter of indifference whether 10 per cent 

 or only traces of lime or silica can be demonstrated in the soil, and the hypothesis 

 that plant-species which grow on limestone fail to grow on slate because they 

 are not able to supply their need of calcium, or that the plants growing on slate 

 cannot flourish on limestone mountains because they cannot obtain the necessary 

 amount of silica, must be abandoned, as well as the assumption that these substances 

 when absorbed as food serve as a stimulus to change of form. 



I strongly supported this latter hypothesis at the time, and thought I should be 

 able to strengthen and confirm it by careful cultural experiments. Seeds of se\eral 

 species which demand lime were sown in soil containing hardly perceptible quanti- 

 ties of lime, and the seedlings were watered with w^ater devoid of calcium; in 

 another place seeds of species demanding a silica-containing substratum were placed 

 in soil which contained much limestone, and the seedlings were watered with lime- 

 water. At first it seemed as if an alteration of form had actually taken place in 

 some individuals. But this was a mistake, or rather, the alteration only consisted 

 in the greater or less luxuriance of the foliage, lengthening or shortening of the 

 stem, abundant or scanty development of flowers and the like. But no actual 

 change of form which would be retained by their descendants could be obtained. 

 The species of plants accustomed to lime, grown on a soil devoid of lime, presented 

 a miserable appearance, with scanty flowers which ripened only a few seeds, 

 whilst the silica-demanding species grown on lime-containing soil soon withered 

 and died without flowering at all. The change of form, indeed the actual inter- 

 change I had anticipated between the closely allied species which grow on the 

 two rocky substrata in a state of nature, did not occur at all. 



If we still take the case of siliceous and calcareous plants, and regard the soil 

 as the source of free inorganic substances which influence the plants, we are 

 forced to assume that greater quantities of one substance will be injurious to one 

 or other of them. The absorbent cells have the capacity of choosing between the 

 substances at their disposal, but this capacity has a definite limit in every species. 

 The cells can absorb as much as they require from a very weak solution of 

 common salt, soda, gypsum, calcium bicarbonate, &c., but a concentrated solution 

 of these salts may injure and destroy their structure and function. If it 

 is allowed to act for any length of time on the cells whose function is to absorb 

 inorganic nutriment, the death of the whole plant will inevitably result. If the 

 Moss which grows on blocks of granite is watered with a saturated solution of 

 gypsum; if the soil into which our Meadow-grasses send their roots is watered 

 with a saturated solution of common salt; or if the humus in which the plants of an 

 upland moor grow is mixed with sodium carbonate or calcium bicarbonate, the plants 

 invariably perish, and the same mineral substances, which in a very weak solution 

 are needful, or at any rate harmless, become poisonous when the solutions are 

 concentrated. The fact that one species of plant prefers this and another that 

 mineral substance (see vol. i. p. 7.3), however, renders it probable that the injurious 

 effect of materials in large quantity in the soil varies, that a large quantity of 



Vol. II. 



82 



