THE METABOLISM OF PLANTS. 233 



lignification, the conversion of cellulose into an aromatic 

 cellulide ; in the formation of resin-grains, the conversion 

 of starch into an aromatic glucoside. But it must not be 

 forgotten, as Miiller points out, that the simultaneous pre- 

 sence of starch and of tannin or resin in a cell does not prove 

 that the latter are directly derived from the former. It is pos- 

 sible that tannin is a product of the metabolism of the proto- 

 plasm, and that the starch-grains present in a cell in which 

 tannin is being formed become impregnated with it, giving 

 rise to the so-called tannin-grains and eventually to resin- 

 grains. It is true that the starch gradually disappears, but 

 this does not prove the direct conversion of starch into tan- 

 nin ; the tannin may be formed indirectly from the starch 

 through the protoplasm. The fact that tannin is constantly 

 present in the cells of parts in which destructive metabolism 

 is active growing-points, motile organs of leaves, galls, for 

 example tends to prove that it is, in fact, derived from 

 protoplasm. Further, we must not overlook the fact that 

 substances like tyrosin, which contain an aromatic radical, 

 occur in plants, and that they are probably derived more or 

 less directly from protoplasm : it is also quite possible that 

 these substances may take part in the formation of some of 

 the more familiar aromatic bodies which occur in plants. 



It is possible, as Cross and Bevan point out, that the benzoic residue 

 which enters into the composition of the hippuric acid present in the 

 urine of the Herbivora is derived from the aromatic substances present 

 in the lignified cell-walls of the plants consumed. It may be, however, 

 that the aromatic body is also formed from cellulose in the manner 

 suggested by Hoppe-Seyler's observations (p. 228). 



We are already in possession of some information respect- 

 ing the conditions under which the formation of the cellu- 

 lides takes place. It was pointed out in a former lecture (p. 18) 

 that lignification may take place in a cell-wall after the proto- 

 plasm has disappeared from the cell, but that the cell must 

 still form part of a living plant. But it is scarcely possible to 

 make any definite statement as to the chemical nature of the 

 process. Inasmuch as lignin, for example, contains relatively 

 less oxygen than cellulose (taking Erdmann's formula for 



