PHYSIOLOGY 215 



these substances vary in different species, so that what may be 

 true for one is not necessarily true for all. 



It is, therefore, not surprising to find that several ideas 

 have been put forward. 



With regard to the origin of tannins practically nothing of 

 fundamental importance is known. 



According to the investigations of Kraus, tannin, although 

 not a direct photosynthetic product — as is indicated by the 

 fact that the tannin does not increase in the leaves of plants 

 which are able to photosynthesize in dull light — is not formed 

 unless carbon dioxide and light are available. He found that 

 etiolated leaves produced no tannin, and that the amount of 

 this substance in shaded leaves was less than that contained 

 in the leaves of the same plant fully exposed to the sun. 

 The tannin thus formed is translocated to the stem and root. 



This, however, is not the only origin for tannin, for if 

 tannin-containing seeds, e.g. the oak, be germinated in dark- 

 ness, there is an increase in the amount of tannin ; further, the 

 production of various aromatic compounds may be a stage in 

 the synthesis of proteins, and some of these may eventually 

 give rise to tannin. 



The facts regarding the distribution of tannin have an im- 

 portant bearing on the subject. It is abundant in leaves ; in 

 parts in which growth is very active, such as growing points ; 

 in galls and other pathological growths ; also it is found in 

 association with secretory organs, such as gland cells of Sar- 

 racenia and Utricularia, and in parts in whiqh the protoplasm 

 is especially irritable, such as pulvini. Pfeffer found that in 

 young fully formed pulvini no tannin occurs, but it appears 

 soon after movements commence and gradually increases in 

 quantity until the leaf dies. 



In the case of Robifita pseudacacia the pulvini of the leaf- 

 lets contain less tannin than the main pulvinus, which is much 

 less sensitive than are the secondary pulvini. 



The consideration of these facts supports the conclusion 

 arrived at by Sachs that tannin results from intense meta- 

 bolism such as occurs in active leaves ; in rapid tissue forma- 

 tion, as in galls and vegetative apices ; during germination 

 and secretion ; and as a consequence of particular stimulation, 

 as in mobile pulvini. 



