78 NUTRITION. 



essential for all ; but besides carbonic acid and water, the 

 existence of nitrogen, in some combination or other, from 

 which the plant may derive it, is highly necessary to 



38. Besides water, carbonic acid, and azotised mate- 

 rial, which may be looked upon as the chief sources of 

 the nutrition of plants, it is well known that for the 

 perfect development, and even for the mere existence of 

 certain species, something else is required to exist in 

 the soil in which they grow ; and from late experiments 

 we are at liberty to conclude that the plant by its roots 

 possesses a power of selecting the material useful for it. 



39. That inorganic substances existing in the soil 

 are taken in by the plant, is evident enough from daily 

 observation, but the difficulty is to determine what part 

 as regards nutrition they are called upon to perform : 

 some seeds of a Sun-flower were sown in a sandy soil 

 which was destitute of saltpetre, and the plants that 

 grew in it gave none of this salt on analysis, whilst 

 some others grown in the same soil, but watered with 

 the salt in solution, presented it in their intimate 

 structure. That the saltpetre was not essential to the 

 existence of the Sun-flowers, although absorbed by 

 some of them, is plain ; and experiments of Saussure 

 prove that plants will to a certain extent absorb sub- 

 stances, whether useful or not, or even detrimental to 

 them. 



40. We know whole hosts of plants, however, that 

 only thrive in a particular soil, the necessity of which to 

 them arises from its containing some ingredient neces- 

 sary for their support or vigour, and which ingredient 

 we find both in the soil and plants grown in it when 

 submitted to analysis ; such, for instance, as maritime 

 plants growing in situations where the salts of Potassa 

 and Soda exist in the soil, and which plants are em- 

 ployed to yield them again for commercial purposes, 

 and to the welfare of which the presence of these salts 

 in considerable quantity seems necessary. 



41. It is sometimes difficult also to trace the relation 



