124 LECTURE VIII. 



substances which they form are readily decomposed and thus 

 energy is set free; this will be fully considered in treating of 

 the destructive metabolism of the plant. 



With regard to the other elements, it appears that they 

 do not enter into the composition of the substances of which 

 the tissues of plants consist; their importance being of this 

 nature, that they promote the metabolism of the plant. It is 

 true that in the analysis of protoplasm or of cell-wall an ash 

 containing some or all of these elements is always obtained, 

 and further that proteids and carbohydrates form chemical 

 compounds with the alkalies and alkaline earths, but there 

 is no evidence to shew that the elements in question occur in 

 the living plant as integral constituents of the chemical mole- 

 cules of the substances which compose its organised structure. 



We will now proceed to consider in detail the forms in 

 which these elements are absorbed by plants, and the part 

 which they respectively play in the economy. 



Carbon. The form in which this element is absorbed by 

 plants depends upon the nature of the plant. Plants which 

 contain chlorophyll obtain their carbon by the absorption of 

 carbon dioxide, whereas plants which do not contain chlo- 

 rophyll obtain their carbon by the absorption of organic 

 substances in which C is directly combined with H. 



It must not be concluded, however, that plants containing chlorophyll 

 are incapable of absorbing complex carbon compounds. Thus it has 

 been ascertained by the researches of Darwin and others that the "insec- 

 tivorous" plants absorb such compounds by their modified leaves, and 

 that these compounds are of importance in their nutrition. Further, it is 

 well known that a number of green plants, such as the Mistletoe, Thesium, 

 Melampyrum, Rhinanthus, Euphrasia and others, live parasitically on 

 other plants : it is possible, however, that these plants do not absorb 

 complex carbon compounds from their hosts, but simply water holding 

 inorganic salts in solution. Finally, it has been proved by direct experi- 

 ment that green plants take up complex carbonaceous substances, such 

 as Urea (Cameron), Glycocoll, Asparagin, Leucin, Tyrosin (Knop and 

 Wolf), when supplied to their roots. These remarks apply also to H, 

 O, and N, inasmuch as these elements are present in combination with 

 C in the substances mentioned. 



In order to ascertain whether or not organic acids (oxalic, tartaric, 

 succinic) can serve to supply green plants with carbon, Stutzer grew a 



