46 VEGETATION. 



-:oast, as was observed by Duhamel, will not thrive in a. 

 ^oil that does not contain rauriat of soda. 



It has been thought, however, that the salts are not 

 actually taken up by the root, though converted to 

 purposes of utility by acting as astringents in stopping 

 up the orifices of the vessels of the plant, and prevent- 

 ing the admission of too much water ; but it is to be 

 recollected that the salts in question are found by anal- 

 ysis in the very substance of the plant, and must con- 

 sequently have entered in solution. It has been also 

 thought that salts are favourable to vegetation only in 

 proportion as they hasten the putrefaction of vegetable 

 substances contained in the soil, or attract the humidity 

 of the atmosphere. But gypsum is not deliquescent, 

 and if its action consists merely in accelerating putre- 

 faction, why is its beneficial effect confined but to a 

 small number of plants ? 



Some writers have contended that the salts which are 

 found in vegetables are merely accidental in their oc- 

 currence, and not necessary to the health or perfection 

 of the individual ; because they are found to exist in 

 but a very small proportion, both in the soil and plant ; 

 but as there are many species in which some salts are 

 to be met with constantly and uniformly, at least if they 

 have vegetated in a soil in which they are found to 

 thrive, we can scarcely regard their occurrence as 

 being merely accidental, or as producing no beneficial 

 effect upon the plant. But the proportion of salts 

 lodged in the soil, is not so small as is generally be- 

 lieved ; and it is abundantly sufficient for all the pur- 

 poses of vegetation. It may even in some cases con- 

 tain too much ; for it is to be recollected that saline 

 substances are beneficial to vegetation only when ap- 



