576 SCIENCE PROGRESS 



freezing-point, about 0'o6°, and a conductivity corresponding to 

 a freezing point of —0*02° or thereabouts. These figures, 

 however, were observed to undergo large seasonal variations, 

 especially the portion of the total depression due to non- 

 electrolytes. As in the leaves, these were shown to be almost 

 entirely sugars. In the wood of some deciduous trees osmotic 

 pressures of three or four atmospheres may be met with in the 

 wood-sap in early spring before the opening of the leaves. It is 

 then that the mobilisation of the colloidal reserve products 

 stored in the medullary rays and wood parenchyma occurs, 

 resulting in the stream, as it rises from the roots, being enriched 

 on the way by the diffusion or secretion into it of these sugars, 

 sucrose, hexoses, and maltose. Several per cent, of sugar 

 is shown to be present by calculation from the freezing-point 

 depression. Direct analysis showed that in some cases this was 

 a mixture of all four of the above sugars — there are two hexose 

 sugars — whereas more often maltose was absent ; at other times 

 cane sugar was the only one present, or, again, hexoses alone 

 might be found. Since the analysis of sugar mixtures is both 

 complex and tedious, it is easily seen what service cryoscopic 

 osmotic pressure determinations are in studying a problem of 

 this type. The chief factors influencing the pressure in wood 

 sap were found to be the nature of the tree, the season of the 

 year, and the height above ground from which the sample was 

 obtained. Examinations were made from base to summit 

 of similar trees of the same species at different seasons of the 

 year, and both deciduous and evergreen trees were examined. 

 As might be expected, the latter do not show such well-marked 

 seasonal changes. The transpiration stream then supplies all 

 parts of the tree with sugars and electrolytes in addition to 

 water. 



General Summary 



From the various examples given above it may be perceived 

 how different is the physiology of the animal and vegetable 

 kingdoms with respect to osmotic pressure. Were animal cells 

 to possess the power of assimilating carbon dioxide to form 

 sugars, these would have to be rapidly transformed into colloids 

 to prevent the development of too great a pressure, in water- 

 dwelling forms at least. Then since in the animal body all 

 neighbouring cells have to be in osmotic equilibrium, the local 



