HORTICULTURAL SOCIETY OP LONDON. 



331 



enter into the system. These investigations have been recently 

 borne out by the experiments of Lampadius, but the data which 

 he has supplied are hardly complete enough to allow of very 

 accurate conclusions being drawn from his results, and the whole 

 subject is still involved in uncertainty. 



Although the sources whence plants derive their food are now 

 well known, yet the precise mode in w hich the assimilation of 

 the elements of organised matter is carried on, are as yet very little 

 understood. The connection which exists between the assimila- 

 tion of carbon, oxygen, and hydrogen, and the formation of 

 organised structure is w^holly unknown, whilst even the chemical 

 laws which govern many of the more simple changes in the for- 

 mation of the various proximate vegetable principles, have 

 scarcely been investigated. In some measure this is a necessary 

 consequence of the want of more distinct information, regard- 

 ing the chemical powers of the rays of light, and till this is sup- 

 plied some parts of the chemistry of vegetation must remain in- 

 complete. 



Equally enveloped in uncertainty are the causes of the deterio- 

 rating' influence which some plants exert on the soil. It must be 

 evident that if any plants are cultivated year after year upon the 

 same land, they must gradually diminish the quantity of some of 

 the inorganic constituents of the soil, and in fact that in time the 

 soil will be unable to support any more of those plants unless 

 some means be adopted for supplying to the soil those inorganic 

 compounds which the crops have removed. In practice this is 

 effected sometimes by manure, and sometimes by fallowing. In 

 the latter case, by exposure to the air and moisture for some 

 time, the disintegration of stony substances in the soil is effected, 

 and thus various saline and earthy compounds are added to the 

 soil. Sometimes the necessity of some arrangement of this kind 

 is obviated by varying the nature of the plants cultivated, sub- 

 stituting for the plants which have exhausted the soil of some of 

 its constituents, other plants which do not require them, but which 

 absorb another kind of inoreranic matter. This effect of plants 

 has been known from very early time, and led to the adoption of 

 various systems of rotations of crops ; it has, however, been alleged 

 that there exists another cause of deterioration perfectly different 

 in nature from the one just adverted to. Certain experiments 

 have been quoted to prove that plants excrete^ by their roots 

 all useless or noxious matters, whether they are formed in their 

 vessels, or absorbed along with other substances from the soil. 

 It has been supposed that the presence of these excreted matters 

 in the soil must render it unfit for the cultivation of the same 

 species of plant, although it might not interfere with the growth 



