OSMOTIC PRESSURES AND ELECTRICAL CONDUCTIVITIES 169 



maxima are shown. Ilex and Hedera put forth their 

 leaves more or less continuously, but the majority of leaves 

 appear to arise at two periods the beginning of winter 

 and the early summer. At these periods new leaves 

 open out in quantity at intervals during one or two months ; 

 consequently the proportion of young leaves is a maximum 

 at such times. For this reason, and on account of the 

 heavy drain upon the sugars of the mature leaves occasioned 

 by the rapid growth of the younger ones, the osmotic 

 pressure is found to be at its lowest during the summer 

 and at the onset of winter. 



Fig. 21 also shows that, as previously mentioned, the 

 osmotic pressure of leaves of Hedera is slightly greater in 

 plants from a southern aspect than in those facing north- 

 wards. 



SUMMARY. 



From the foregoing tables an idea may be gained of the 

 magnitudes of the osmotic pressures commonly met with 

 in plant tissues. The determinations of Dixon and Atkins 

 were primarily undertaken with the object of ascertaining 

 whether the pressures were sufficient for the requirements 

 of the cohesion theory of the ascent of sap. In passing it 

 may be remarked that they were found to be ample. 



More generally osmotic pressure measurements serve as 

 a ready means of studying the variations of the molecular 

 concentration of the sap solutes. By combining them with 

 determinations of electrical conductivity it is possible to 

 distinguish between the portion of the pressure caused by 

 electrolytes and that due to non-electrolytes, which are 

 mainly sugars. The former, as a rule, remain remarkably 

 constant, but rise slowly with age in tissues from which 

 evaporation *is continually taking place. The latter, on 

 the other hand, are subject to large fluctuations, accord- 



