General Physiology of Plants 189 



pointed out, changes in mass and in form are only 

 secondary symptoms of the real activity of the 

 tissue and must be interpreted with some caution. 

 The activity of the tissue can be studied in a some- 

 what more direct fashion by examining its capaci- 

 ties for fermentation or more especially its re- 

 spiratory activity. This has been done, to date, 

 in only a single piece of work, carried out on callus 

 cultures of Salix capraea by Plantefol (1938, 256, 

 257) in co-operation with Gautheret. Both callus 

 cultures and root cultures, because of their rela- 

 tively reduced dimensions which permit a com- 

 paratively easy gas interchange between culture 

 and nutrient, because of their freedom from 

 trauma, and because of their high growth rates, 

 should provide ideal material for study of proc- 

 esses of this sort (Warburg, 1923, 272, 1926, 273). 



Nothing has been done in the study of photo- 

 synthetic activity using plant tissue cultures. 

 Roots normally do not possess such activity and 

 that of callus cultures is probably low so that they 

 do not appear likely to prove very useful in this 

 respect. 



While these are some of the obvious tissue ac- 

 tivities which suggest themselves as possible sub- 

 jects of investigation, there certainly exist less 

 obvious but no less important ones. One of these 

 turned up unexpectedly in the form of a glandular 



