202 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



up of the water-channels on the removal of air-bubbles, 

 but also by the growth of the leaves themselves, rendered 

 possible by the accession of sugars, etc., carried up in the 

 sap. The increase in the volume of the transpiration 

 stream from these causes, and more favourable external 

 conditions, leads to a dilution of the sugars, and is re- 

 sponsible largely for the rapid decrease in concentration 

 at the time of the expansion of the leaves, as shown in all 

 the curves. Possibly somewhat earlier there is a diminution 

 in the enzymic activity of the cells round the vessels, and 

 this initiates, and later contributes to, the dilution of the 

 sugars. The secretion, however, does not cease at any 

 time of the year; and consequently even in summer we 

 find the enormous transpiration stream possessing a very 

 noticeable concentration of sugars, which is, indeed, 

 greater than that in which the same substances are present 

 in the human blood. To make good this expenditure the 

 glandular cells must be constantly replenished by supplies 

 forwarded from the organs of carbon assimilation through 

 the bark, medullary rays, and wood parenchyma. The 

 rise in concentration of the sap in the tracheae towards the 

 end of the year is to be ascribed, in all probability, to the 

 more or less uniform continuance of this secretion, coupled 

 with the reduction of transpiration, entailing a diminution 

 in the rate of the current past the secreting cells prior to 

 leaf fall. For it is evident that, if the rate of the secretion 

 of sugars remains approximately constant, the concentra- 

 tion of the sap will depend upon the volume of the tran- 

 spiration stream. In winter the complete cessation of 

 transpiration allows of a further concentration.* 



* In view of this continued secretion of carbohydrates into the trachese, 

 it is advisable to centrifuge the sap from the wood immediately on its 

 removal from the tree; otherwise the concentration observed may be 

 greater than that actually obtaining during transpiration. 



