62 Scientific Proceedings, Royal Dublin Society. 



of the stem have, however, slightly greater pressure than the intervening 

 portions. 



3. During the early spring the sap is enriched by the addition of large 

 quantities of sugars from the storage cells of the wood-parenchyma and of 

 the medullary rays. Accordingly the osmotic pressure rises in a very marked 

 degree from root to summit, the increase being particularly great in the 

 upper regions. 



4. During the late spring the concentration of sugars is still considerable, 

 being roughly half of the earlier value. The electrolytes of the sap are, 

 however, present in much greater concentration than in the early spring. 



5. In Acer macrophyllum reducing sugars are never found in the wood- 

 sap, except in traces, whereas sucrose is present in quantity. In the other 

 trees examined both reducing sugars and sucrose are present, the latter 

 predominating as a rule. During the vernal mobilization of reserves the 

 reducing sugars consist of the hexoses and maltose ; at other times the latter 

 is absent. 



6. In evergreens and sub-evergreens the seasonal changes are not very 

 striking, nor are the gradients of osmotic pressures from root to summit as 

 regular as in deciduous trees. The osmotic pressure of the transpiration sap 

 in the root exceeds that in the stem at certain seasons. 



Bibliography. 



1. Dixon, H. H., and Atkins, W. E. G. — Osmotic Pressures in Plants : iv. 



On the Constituents and Concentration of the Sap in^the Conducting 

 Tracts, and on the Circulation of Carbohydrates in Plants. Sci. Proc. 

 Eoy. Dub. Soc, 1915, vol. xiv (N.S.), pp. 374-392. 



2. Osteehout, W. J. V. — Some Quantitative Eesearches on the Permeability 



of Plant Cells. Plant World, 1913, vol. xvi, pp. 129-144. 



