Dixon and A'I'KIns — Osmotic Pressures in Plants. 389 



pressure is responsible for the exhibition of bleeding and root-pressure 

 characteristic of tlie spring. This simultaneously forces much of the air in 

 the tracheae into solution, and raises the carbohydrates to the buds. The 

 observations recorded show that the maximum concentration of the carbo- 

 hydrates is simultaneous with, or just previous to, the expansion of the 

 leaves. Tlie activity in transpiration of the developing leaves is forwarded 

 not only by the opening up of tlie vpater-channels on the removal of air- 

 bubbles, but also by the growth of the leaves themselves, rendered possible 

 by the accession of carbohydrates, &c., 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 tlie carbohydrates, 

 and is responsible largely for the rapid decrease in concentration at the 

 time of the expansion of tlie leaves shown in all the curves. Possibly some- 

 what earlier there is a diminution in the glandular activity of the cells round 

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

 carbohydrates. The secretion, liowever, does not cease at any time of tlie 

 year; and, consequently, even in summer, we find the enormous transpira- 

 tion 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 tlie 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 tlie secretion of sugars remains approxi- 

 mately constant, the concentration of the sap will depend upon the volume of 

 the transpiration stream,, In winter the complete cessation of transpiration 

 allows of a further concentration.' 



Where the wood parenchyma is specially concentrated round the vessels, 

 while the tracheids form comparatively large tracts without living cells 

 intermingled, we may with probability assume that a certain amount of 

 division of labour has come about, and that the vessels function as the 

 principal channels for the transmission of carbohydrates. The traclieids, on 

 the other hand, are chiefly concerned with the upward conveyance of the 

 water. Of course as long as no bubbles are developed in the vessels they will 



1 In view of this continued secretion of carbohydrates into the tracheae, it is advisable to 

 centrifuge the sup from the wood immediately on its .removal from the tree ; otherwise the 

 concentration observed may be greater than that actually obtaining during transpiration. 



SCIENT. PKOC. R.D.S., VOIi. XIV., NO. XXXI. 3 E 



