26 EXPERIMENT STATION EECOED. [Vol.35 



in plants, H. H. Dixon and W. R. G. Atkins (ScL Proc. Roy. Dublin Soc, n. 

 scr., llf (1915), No. 31, pp. 374-392, figs. 6).— The authors, having followed up 

 their previous work (E. S. R., 30, p. 523), give the results of observations ou 

 the sap drawn from the conducting tracts of several trees by centrifuging sec- 

 tions 10 cm. long by 2 cm. in diameter. The sap obtained by this method was 

 neutral to litmus and clearer and much less concentrated than that obtained 

 by pressure with consequent bursting of the cells. 



Sugars were found at all times in the trees examined, being usually more 

 plentiful than electrolytes. Sugars showed the greatest concentration in early 

 spring, a dilution in spring and summer progressing to a minimum concentra- 

 tion in summer or autumn, then a rise in concentration, slow at first, culmi- 

 nating in the vernal maximum, which also coincided Avith the period of greatest 

 root pressure and was simultaneous with or just preceded the opening of the 

 leaf buds. These changes in concentration were due largely to changes in the 

 transpiration rate. 



Tlie conveyance upward of carbohydrates, notably sucrose, is apparently a 

 primary and continual function of the tracheae. The sheath of wood paren- 

 chyma round the vessels functions as a gland to secrete carbohydrates into 

 the rising transpiration stream. The relation of the medullary rays to these 

 sheaths supports the view that they convey the carbohydrates from the bark 

 to the glandular sheaths. The abundant presence of soluble carbohydrates in 

 the wood sap of roots probably causes root pressure and bleeding by producing 

 an osmotic pressure across the root cortex, which acts as a semipermeable mem- 

 brane. The concentration of the carbohydrates is generally greater in the 

 tracheae of the stem than in those of the root, except during the summer. The 

 electrolytes, however, are generally present in greater quantity in the root. 



In general the vessels function, when water is abundant, to convey rapidly 

 solutions of organic and inorganic substances to the leaves. The columns of 

 tracheids may be supposed to afford a permanent channel for water and salts, 

 and to a less degree, for the organic substances. Even in times of greatest 

 drought, this is never put out of action. 



A bibliography is given. 



Osmotic pressures in plants. — V, Seasonal variations in the concentration 

 of the cell sap of some deciduous and evergreen trees, H. H. Dixon and 

 W. R. G. Atkins (Sci. Proc. Roy. Dnhlin Soc, n. scr., 14 (1915), No. 34, pp. 

 445-4GI, figs. 5). — The authors continue this series (see above) by reporting, 

 with certain additions, the results of a revision, the necessity for which has 

 been previously indicated (E. S. R., 29, p. 828). The sap for freezing-point 

 determinations and conductivity measurements was pressed from tissues previ- 

 ously frozen in liquid air. 



The authors state that the osmotic pressures in tissue and their variations 

 are largely due respectively to dissolved carbohydrates and to fluctuations 

 therein, but that electrolytes also play a part. A progressive average rise in the 

 osmotic pressure has been found during the development and life of each organ 

 examined. This is due in case of leaves to the accumulation of electrolytes 

 with age, but in case of the only root examined, to carbohydrates. 



The leaves of the two evergreens examined possessed higher osmotic pressure 

 during the winter than during the summer months. The curve of seasonal 

 variations in loaves of Iledcra helix was alike for specimens growing either in 

 a sunny or a shaded position, but on the whole the osmotic pressure was some- 

 what higher for the insolated leaves. The osmotic pressure of the root sap of 

 Ilex nfinifolium rose from a mininnnn of 6 atmospheres in October to a maxi- 

 mum of 14 atmospheres in September. 



