336 PHYSIOLOGY OF NUTRITION. 



their stalks dipping into spring water contained in small glasses, 

 and these are then put into a glass receptacle, which may, if 

 necessary, be covered with a sheet of glass. Thus arranged, we 

 expose the material to the light in the slight shade afforded, e.g., 

 by birch trees. After a few days we treat these halves like those 

 already dealt with, cutting out pieces with the help of our 

 templets, from exactly corresponding places of course, and de- 

 termining the dry weight of these pieces and the amount of 

 tannin they contain. Leaves of Quercus and Viburnum may be 

 used for similar experiments. We shall find that our leaves 

 exposed to the light gain in dry weight and in tannin ; that is 

 not the case if the leaves are kept for a few days in the dark. 

 Making similar experiments with leaves of Saxifraga, with the 

 modification, however, that the leaves are exposed to the light 

 under a large bell-glass in an atmosphere devoid of Carbon 

 dioxide, it is found that no increase in the quantity of tannin 

 takes place. We have therefore made out that tannin can only 

 be produced in green leaves under the influence of light and in 

 presence of Carbon dioxide (indirect influence of assimilation).* 



In the evening of a warm summer day we cut off half the leaf 

 as above from ten leaves of Alnus glutinosa, the other half being 

 left attached to the plant. We then determine the tannin in 

 150 sq. cm. of the separated halves. Next morning the other 

 halves are tested. Loss of tannin has taken place. The searching- 

 investigations of Kraus (p. 9 of his cited work) teach that the 

 tannin does not undergo decomposition in the leaves, but migrates 

 out of them. It travels along the leaf-nerves into the stems or 

 branches, proceeds by the cortex, and ultimately is deposited 

 according to the kind of plant in the rhizomes, the wood, or else- 

 where. This primary tannin formed in the leaves does not again 

 enter into metabolism. It serves particularly to protect th& 

 organs in which it is deposited, as a protection against being 

 eaten by animals, and against decay. 



Besides the primary tannins, however, there are the so-called 

 secondary tannins, which in many plants are formed in darkness 

 as a result of metabolic processes. If we test seeds of Vicia Faba 

 we find them to be devoid of tannin. If now we examine seed- 



* For the material used and under the conditions obtaining in our experi- 

 ment, this is certainly correct. Under modified conditions, however, the result 

 may be somewhat different. See Biisgen, Beubachtungen uber das Verlialten 

 des Gerbstoffes in den PJlanzen, Jena, 1889, p. 28. 



