Physiologie. 617 



similar bodies, and they also include descriptions of two simpler 

 forms of Oedanometers, more suitable for class demonstrations. 



Agnes Arber (Cambridge). 



Dixon, H. H. and W. R. G. Atkins. Osmotic pressure in 

 Plant s. IV. On the constituents and concentration of 

 the case in the conducting tracts and on the circula- 

 tion of carbohydrates in plant s. (Sei, Proc. Roy. Dublin 

 Soc. XIV. N" 31. pp. 374-392. 1915.) 



The authors summarize their results as follows. Sugars (mono- 

 saccharides and disaccharides or both) are found at all times in the 

 sap of the tracheae of the trees examined {Acer^ Fagiis, Populus 

 and others) and usually in greater quantities than electrolytes. The 

 greatest concentration of sugars occurs during early spring; this is 

 followed by a rapid dilution in spring and early summer, so that a 

 minimum occurs during summer and autumn. A rise then takes 

 place in winter culminating with the vernal-maximum. The latter 

 coincides with the period of greatest root pressure and it is simul- 

 taneous with or just prior to the opening of the leaf buds. 



The rise in transpiration, initiated by the expanding leaves and 

 facilitated by the opening of the conducting Channels by root pres- 

 sure, is largely responsible for the dilution of carbohydrates. The 

 conveyance upwards of carbohydrates of which sucrose appears to 

 be the most important, is a continual and primary function of the 

 tracheae. The sheath of the wood parenchyma round the vessels 

 functions as a gland to secrete carbohydrates into the rising transpi- 

 ration stream. The relation of the medullary rays to these sheaths 

 Supports the view that they convey the carboh5^drates from the bark 

 to the glandulär sheaths. 



The presence of large quantities of soluble carbohydrates in the 

 wood sap of roots is probabl}^ responsible for root pressure and 

 bleeding, by producing an osmotic flow across the root cortex 

 which acts as semi-permeable membrane. The concentration of the 

 carbohydrates is generally greater in the tracheae of the stem than 

 in those of the root, except during summer. Ilex is an exception 

 to this rule. The electrolytes however are present in greater quantity 

 in the root. 



In general, the vessels function, in times when water is abun- 

 dant, to convey rapidh^ Solutions of organic and inorganic sub- 

 stances to the leaves. The tracheids may be supposed to afford a 

 permanent Channel for water and salts and to a less degree for 

 organic substances. F. Cavers. 



Dixon, H. H. and W. R. G. Atkins. Osmotic pressure in 

 Plant s. V. Seasonal Variations in the concentration of 

 the cell-sap of some deciduous and evergreen trees. 

 (Sei. Proc. Roy. Dublin Soc. XIV. N« 34. p. 445—461. 1915.) 



The authors conclude that the major part of the osmotic pres- 

 sure in tissues is due to dissolved carbohydrates. The variations in 

 the pressure are due largelj^ to fluctuation of the carbohydrate- 

 contents of the cells and to a smaller degree to changes in the 

 concentration of the electrolytes. 



A progressive average rise in the osmotic pressure has been 

 found during the development and life of each organ examined. 



