RECENT ADVANCES IN SCIENCE 201 



sider the passage through the stem of the materials which 

 constitute the food reserve. Moreover, throughout the grow- 

 ing season the stem is also a channel for the continual upward 

 flow of the " transpiration stream," and the importance of 

 this in relation to storage and translocation, essentially pro- 

 cesses of solution and deposition from solution, is self-evident. 

 Nor is the work of investigation simplified by the recurrence 

 of annual, as well as diurnal, rhythmic changes in transpiration 

 rate, food manufacture and utilisation, superposed one upon 

 another. Consequently it is to be expected that progress 

 towards an understanding of these processes will be slow and 

 laborious. 



The significance of the role played by osmotic pressure in 

 the movement and storage of water and nutritive substances 

 compels attention to the osmotic properties of cell sap. N. A, 

 Maximov and I. A. Krasnoselskaia-Maximova {Piibl. Tiflis 

 Bot. Gard.y 191 6, 19, 10), in the course of extensive researches 

 on transpiration, made determinations also of fluctuations of 

 the osmotic pressure and of the sugar content of the cell sap 

 of evergreen leaves. They found that the osmotic concentra- 

 tion of the leaf cell-sap was greater in winter than in summer, 

 and this difference they were able to correlate with changes 

 in the quantities of sugars and glucosides present. The winter 

 accumulation of these substances, with the resulting concentra- 

 tion of the sap, is regarded as a protection against freezing and 

 consequent injury. Another aspect of the question of changes 

 of osmotic pressure of the sap at different stages of growth 

 has been discussed by B. F. Lutman {Amer. Journ. Bot., 1919, 

 6, 181-202) in the course of investigations of potatoes. He 

 arrived at some interesting conclusions with regard to trans- 

 location of stored food reserves, but in common with most 

 workers in this field, he is confronted with cases of movements 

 of nutritive materials which at present cannot be reconciled 

 with known osmotic phenomena. Young shoots growing from 

 a potato tuber yielded a sap which exhibited a shghtly higher 

 osmotic pressure than that obtained from the tissue of the 

 parent tuber, and further, the leaves of the young plant gave 

 a sap more concentrated than the stem sap. Later in the 

 growing season this relation was found to be reversed, the 

 stems then developing the higher pressure. As in the inves- 

 tigation first quoted, these changes were correlated with the 

 local accumulation of sugars, more especially sucrose. The 

 rather remarkable conclusion arrived at is that nutritive sub- 

 stances reach the growing parts of the plant from storage 

 regions by passing along a simple concentration gradient, but 

 the problem of the passage of assimilation products to storage 

 organs and fruits is not amenable to such a simple explanation ; 



