190 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



bursting the cells, had contaminated the sap of the tracheae 

 both with electrolytes and non-electrolytes. This con- 

 tamination was further shown by the fact that, while the 

 sap pressed from the wood became more or less deeply 

 coloured brown owing to the presence of a chromogen and 

 an oxidase, that centrifuged from it remained almost 

 colourless, indicating that either one or both of these 

 bodies were retained in the substance of the wood, or, 

 more precisely, in the living cells. Moreover, the centri- 

 fuged sap is neutral to litmus, whereas that obtained by 

 pressure is acid. 



At the same time the interesting fact was made clear 

 that even in the month of August the carbohydrates present 

 in the tracheae are sufficiently concentrated to produce a 

 depression of freezing-point amounting to 0'030 and 

 0'038 in stem and root respectively. It thus appears that 

 the solution of carbohydrates passing up through the 

 tracheae in the transpiration stream has the same freezing- 

 point as a 0'25 per cent, solution of glucose, or a 0'50 per 

 cent, solution of sucrose. The concentrations of the carbo- 

 hydrates of the root approximated to those of 0'3 per 

 cent, solution of glucose, or of - 7 per cent, of sucrose. 



These observations strikingly negative Sachs's view that 

 the stream rising from the roots to the leaves during 

 transpiration is to be regarded as a very dilute solution of 

 salts only. 



It is interesting to note how much more concentrated 

 the sap of the vacuoles of the cells of the leaves was at the 

 same time. This was extracted by pressure from leaves 

 which had been treated with liquid air to render their 

 protoplasm permeable. 



Contemporaneously with the experiments on Acer 

 pseudoplatanus similar measurements were made on the 

 sap of Populus alba, and are recorded in Table XLIX. 



