122 LECTURE VIII. 



inaccurately, termed the organic constituents of plants, the 

 other elements being the inorganic constituents. 



In illustration of the ultimate chemical composition of various parts of 

 plants the following analyses, taken from Johnson, may be given. 



Dry solid Wheat Wheat Potato Peas Clover 



contains per cent. Grain Straw Tubers Grain Hay 



Carbon 46*1 48*4 44*0 46*5 47*4 



Hydrogen 5'8 5*3 5*8 6*2 5*0 



Oxygen 43-4 38-9 447 40-0 37-8 



Nitrogen 2 '3 0*4 1*5 4-2 2'i 



Ash, including \ 



Sulphur and Phosphorus \ 24 7 4 3' , 



lOO'O lOO'O ICO'O lOO'O ICO'O 



An enumeration of the constituent elements of a plant is, 

 then, an enumeration of the substances which it absorbed 

 during its life. But some only of these substances can be 

 regarded as constituents of its food, if by "food" we mean 

 those substances absorbed by the plant which go to build up 

 its organic substances, which supply it with energy, or which 

 exert some beneficial influence upon its metabolic processes; 

 for a plant, like an animal, takes up not only such substances 

 as these, but others as well which are of no nutritive value, or 

 are even injurious. That this is the case has been already 

 shewn in the fourth lecture (p. 65) in treating of the absorption 

 by the roots. Any substance present in the soil is taken up by 

 the roots, provided that the physical conditions of absorption 

 are fulfilled, quite independently of its beneficial or injurious 

 action upon the plant; and this holds good also with refer- 

 ence to the absorption of gases by the leaves. 



An important illustration of this point is afforded by Phillip's recent 

 observations upon the absorption of metallic oxides by the roots of plants. 

 He has found that healthy plants, grown under favourable conditions, 

 may absorb small quantities of lead, zinc, copper, and arsenic. The lead 

 and the zinc appear to be deposited in the tissues without causing any 

 disturbance of the functions of the plant, but when copper and arsenic 

 are present in rather large quantity in the soil they exert a distinctly 

 poisonous influence. 



We will now proceed to enquire which are the chemical 

 elements which serve as food to plants. It was pointed out 



