38 THE PHYSIOLOGY OF PLANTS 



sometimes accelerating, sometimes retarding, will depend upon 

 the nature of the plant, and vary at different periods ; for one 

 species will be unaffected by a large amount of a certain sub- 

 stance, the same amount of which would be injurious to 

 another species or stop its growth entirely. If, therefore, we 

 hear of plants the ash of which is shown by chemical analysis 

 to contain a strikingly large amount of such a substance, we 

 must not suppose that this species needs so much of that 

 substance for its actual growth, but all that we can logically 

 conclude is, that this species can absorb without harm to itself 

 the stated amount of that substance. Plants rich in salt or 

 saltpetre, in silica or zinc, are therefore not plants which need 

 these substances in such large quantities, but which have the 

 power of resisting the over-loading of their tissues with these 

 waste products. The same may be said of the plants rich in 

 lime. 



If, therefore, a plot of land, the soil of which has a special 

 character owing to the preponderance of one of these substances, 

 is to be planted afresh, those plants should have the preference 

 which can endure the largest amount of the substance in 

 question. What is said here of the several elements of the 

 soil is equally applicable to the other properties of the soil 

 (the facility of absorbing heat, the power of retaining water, 

 &c.). The constant occurrence of certain kinds of plants in 

 localities characterised by the preponderance of certain sub- 

 stances is explained by the greater power some plants have to 

 adapt themselves to the given chemical and physical conditions 

 of the soil. 



For practical purposes the occurrence of lime-, silica-, salt- 

 petre-, or salt-loving plants should be noted by horticulturists 

 and agriculturists, as it affords a clue to the condition of the 

 soil. Such soils, characterised by the preponderance of one 

 substance, have the disadvantage that a number of cultivated 

 plants do not thrive on them on account of the excessive 

 richness in this one substance. If, therefore, a crop should 

 fail in such soil, we know where, in the first place, to look for 

 the cause. 



