42 FOOD OF THE VEGETATING PLANT. CHAP. K 



the proportion of about four to one ; carbonic acid 

 gas in the proportion of about one part in J 00 ; 

 and vapour in a proportion still less.* Such then 

 are the component principles of the soil and at- 

 mosphere, and sources of vegetable nourishment. 

 Byseleo But the whole of the ingredients of the soil and 

 atmosphere are not taken up indiscriminately by 

 the plant and converted into vegetable food, be- 

 cause plants do not thrive indiscriminately in all 

 varieties of soil. Part only of the ingredients are 

 selected, and in certain proportions ; as is evident 

 from the analysis of the vegetable substance given 

 in the foregoing book, in which it was found that 

 carbon, hydrogene, oxygene, and nitrogene, are the 

 principal ingredients of plants ; while the other in- 

 gredients contained in them occur but in very small 

 proportions. It does not, however, follow that these 

 ingredients enter the plant in an uncombined and 

 insulated state, because they do not always so exist 

 in the soil and atmosphere ; it follows only that 

 they are inhaled or absorbed by the vegetating 

 plant under one modification or another. The plant 

 then does not select such principles as are the most 

 abundant in the soil and atmosphere ; nor in the 

 proportions in which they exist ; nor in an uncom- 

 bined and insulated state. But what are the sub- 

 stances actually selected ; in what state are they 

 taken up ; and in what proportions ? In order to 

 give arrangement and elucidation to the subject, I 

 * De Luc on Evaporation. 



