186 OBSERVATIONS ON THE DISEASES OF PLANTS 



The sum of what lias been said may be stated thus : — The more 

 phosphates are relatively increased in any soil in consequence of 

 its mode of formation or cultivation, the more will the plants 

 which it sustains have a tendency to deviate from their original 

 type, to form sub-species and varieties, and finally to be attacked 

 and destroyed by internal diseases. 



5. But to apply the general principles which have been ad- 

 duced to a particular case. 



The potato is a fleshy tuber, which, together with a great pro- 

 portion of water, contains a marked quantity of starcli, with an 

 extraordinarily small amount of proteinous matters. All combi- 

 nations of protein, it is well known, are greatly subject to decom- 

 position when in solution ; and, indeed, as soon as this takes jilace, 

 affect in a similar manner even such substances as starch and 

 cellulose, which slowly, and with difficulty, yield to the decom- 

 posing power of atmospheric influences. 



What an effect, then, must the condition which our cultivated 

 fields present have upon a plant subjected to such treatment ? 

 The answer to this question leads to very striking results. 



I will, however, first remark, that every natural soil that is 

 formed exclusively of shattered and decomposed I'ocks contains 

 very little phosphorus ; and though sufficiently so for most wild 

 plants, is by no means over rich in alkalies. The first arising 

 from the general rarity of phosphates, the second from the soluble 

 alkalies being washed out by the rain and carried to the springs 

 and rivers, and thence to the sea. A quantity of phosphates are, 

 however, brought to the soil with manure, which, in proportion 

 to the alkalies, is far greater than is needed by any plant in a 

 normal condition. 



In examining the inorganic matters of plants and soil, attention 

 has hitherto been directed almost exclusively to the circumstance, 

 whether the inorganic constituents of the latter are actually in 

 sufficient abundance. This method is far from satisfactory, and 

 does not embrace every necessary consideration. For a plant is 

 not endowed with sense to choose what is the most proper 

 amongst the several nutritious matters which surround it. The 

 reception of nutriment, as appears by the most careful experi- 

 ments, depends exclusively on the attractive power by which two 

 different matters strive to combine with each other — a power 

 which Dutrochet, in the particular case which has the greatest 

 resemblance with the circumstances of the absorption of nutri- 

 ment by plants and animals, has distinguished by the name of 

 endosmose, which is wrong and inapposite in the case before us. 

 According to the laws of this species of attraction any substance 

 dissolved in water is always attracted by another fluid, if such 

 fluids in general (as water and albumen, gum and sugar) are 



