TRANSLOCATION OF NUTRITIVE MATERIALS 231 



living substance, becomes continually weaker in that con- 

 stituent, and hence more and more diffuses into them as 

 the construction goes on. The utilisation or consumption 

 of the sugar so acts as an attracting force, directing the 

 stream to the points where it is required. The same 

 principle applies to the consideration of the deposition of 

 the large reserves of carbohydrates in seeds, tubers, or 

 other organs. The withdrawal of it from the travelling 

 stream which is the result of the formation of the quantities 

 of starch or cellulose which those reservoirs contain, leads 

 to fresh quantities being transported slowly but con- 

 tinuously to those cells, owing to the physical process of 

 diffusion already described. The stream passes in fact in 

 both cases exactly in proportion as the consumption takes 

 place, whether the consumption takes the form of construc- 

 tion of new protoplasm, or the transformation of the travel- 

 ling carbohydrates into the insoluble resting forms. 



This passage of the sugar about the plant need not 

 demand a coincident transport of water, so that the old 

 idea that there was an actual stream of fluid along the 

 bast, or in the old nomenclature a stream of descending 

 sap, need not have any foundation in fact. The principle 

 of diffusion alone will suffice to explain the passage of the 

 sugar. Disturbances of the fluid contents of the cells do 

 no doubt occur, as osmosis is continually taking place 

 in both directions between the contiguous cells. A defi- 

 nite flow of water need not, however, coincide in either 

 magnitude or direction with the passage of the stream 

 of sugar. 



The translocation of the sugar, we see, thus varies in 

 direction and in magnitude according to the varying pro- 

 cesses which are from time to time proceeding. As the 

 variations in these processes, particularly those of growth 

 and nutrition, are often sudden and considerable, we find 

 the translocation is generally accompanied by changes of 

 the carbohydrate from the labile to the storage forms, and 

 vice versa. It is very usual to find temporary accumulations 



