204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



are, then, certainly not intermediary products in the building up of 

 the living protoplasm. Search must be made elsewhere than in a 

 process of assimilation for the origin of these compounds which, 

 without nutritive value for the plant are, however, often produced 

 by it in considerable quantities. What then is their origin and their 

 signification ? 



Some years ago in connection with this subject I advanced an 

 hypothesis relating specially to alkaloids. 



This hypothesis having been accepted w^ith some favor, I extend 

 it to-day to all compounds of the same character. I admit that, far 

 from being products of assimilation, they are products of denutri- 

 tion. They represent the losses of vegetal metabolism. They cor- 

 respond to what among animals are urea, uric acid, glycocoll, biliary 

 pigments, etc. It is, in fact, not conceivable that the biological syn- 

 thesis of proteins, any more than synthetic operations in vitro gen- 

 erally, could be made with a theoretical yield, without leaving some 

 secondary products, some residues which could no longer be utilized. 

 Conversely, using the tissues, all the phenomena of assimilation and 

 of combustion must produce in plants as well as in animals some 

 corresponding losses, nitrogenous or otherwise. 



All these i3roducts are not simply useless, but they are injurious 

 to the maintenance of life. They represent poisons from which the 

 organisms of both kingdoms must be freed at any cost under penalty 

 of toxication. The animal can do this by expelling them; but the 

 plant, deprived of excretory organs, can do this only very imper- 

 fectly. It must be content to retain them and is restricted to render- 

 ing them inoffensive by keei^ing them outside of the vital circulation 

 and preventing them from reentering the living cell from which they 

 have been expelled and from exercising their toxic influence on the 

 protoplasm. And we find that it does this, for the compounds in 

 question are never found actually present in the interior of such cells. 

 The cell wall thus becomes a sorting place of useful and poisonous 

 substances; it is permeable to the first, impermeable to the second. 

 Can an explanation be given of the mechanism that regulates this 

 sorting? 



No physical characteristic (such as solubility, ionization, the col- 

 loidal or crystalline state) distinguishes the two kinds of substances 

 from each other. No difference in chemical composition exists be- 

 tween them ; they are formed of the same elements, which are those of 

 protoplasm itself. It clearly follows, then, in my opinion, that only 

 a difference of molecular structure can explain their opposite be- 

 havior. Let us now see what is loiown of their constitution. 



Eesearches in this subject have led to the remarkable result, the 

 final consequences of which are not yet known, that all these products 

 are cyclic compounds. The carbon atoms of the turpentines, of 



