GROWTH OF THE STEM. 21 



circumstances just named, they ■would be poisoned, 

 because tbey have little power of refusing to take up 

 whatever matter is ' presented to them in a fitting 

 state (35.) But it is by no means certain that the 

 excrementitious matter of all plants is poisonous 

 either to themselves or to others ; and therefore the 

 consequences of roots growing in soil from which 

 they cannot advance are uncertain, and only to be 

 judged of by actual inquiry into the nature of the 

 secretions.* 



41. In general, roots have no buds, and are, there- 

 fore, incapable of multiplying the plant to which they 

 belong. But it constantly happens, in some species, 

 that they have the power of forming what are called 



* There is some dififerenee of opinion among physiologists, 

 ■whether exerementitious matter is thrown off from crops in so con- 

 siderable a quantity as to affect succeeding crops injuriously, or 

 ■whether the difficulty of growing successive crops upon the same 

 spot arises from the exhaustion of certain specific elements from the 

 soil needful to the growth and perfection of one crop, but not neces- 

 sary of another. It is more likely that the incapacity of soils to 

 bear the same crop for a long series of years suspends partly or both 

 these causes ; but the fact that while wheat cannot be raised but 

 four or five times in succession from ordinary soils and that on the 

 other hand in certain deep ajluvial soils of the ■west excellent crops 

 of this grain have been taken twenty or more years in succession 

 ■would go far to prove that the necessity of a change of crops arises 

 more from the exhaustion of the particular food required for that 

 crop than from the injnrious accumulation of exerementitious matter. 

 However this may be, the practice of rotation of crops, the best 

 possible one for keeping the soil of either the farm or garden in 

 good condition, is based upon the fact that plants require change 

 of soil whether our object is to raise the lai^est product, or to do so 

 at the least outlay of manure. 



