Dixon and Atkins — Variations in Osmotic Pressure, 6cc. 243 



From this table it will be seen that, in spite of a few exceptions, the 

 cryoscopic value of the sap of the leaves from the south aspect is very 

 consistently greater than that of the leaves from the north aspect. The 

 average depression of freezing-point over a period of nearly two years was 

 0-799° for the sap of the south-aspect leaves, and 0-748° for that of the leaves 

 from the north aspect, indicating osmotic pressures of 9-61 atm. and 9-00 

 atm. respectively. 



This result is quite parallel to Trinchieri's observations on the cryoscopic 

 values of the sap of Salpichroa rhomboidea. He found that, on the whole, the 

 depression of freezing-point of the sap of the aerial portions was greater in 

 plants grown in a sunny position than in those grown in a more shaded 

 one.' The higher average cryoscopic value of the insolated leaves finds an 

 obvious explanation in the increased photosynthesis and evaporation. 



Fig. 1 is a graphic representation of the results set out in the table, 

 omitting the first few observations. This shows there is a rough general 

 correspondence in the variations of the osmotic pressures of the leaves of 

 Hedera with those already described in Ilex. The maximum is attained in 

 the early spring ; the pressure then falls rapidly to a minimum in the summer, 

 rising later in the autumn to a second cusp. There is a second depression 

 indicated, as in Ilex, in the winter months. In spite of the greater 

 irregularity of the curve for the south-aspect leaves, it is surprising how 

 closely parallel it keeps to that traced by the cryoscopic values of the sap of 

 the leaves from the north aspect. 



As in the case of Ilex, the depressions in the curve seem to correspond to 

 the periods of elongation of the shoots and the formation of new leaves ; but 

 in the case of Hedera so great is the summer depression that it seems reason- 

 able to assume that not only is the average age of the mature leaves less at 

 that period, but that also in all probability the sap of the mature leaves is 

 made less concentrated by the transport of dissolved materials into the growing 

 organs, and by other causes. 



As in the case of Ilex, it seems impossible to correlate the form of the 

 curves closely with external conditions. In the first place, the plant seems to 

 maintain its cryoscopic values quite independently of the rain-fall. The large 

 rain-fall in November and December, 1910, did not succeed in depressing the 

 curves, nor did the drought lasting from January-July, 1911, succeed in 

 raising them. 



The General Form of the Curves. — The conditions recorded in the 

 second and third columns of Table III show that the amount of sunshine is 

 not an all-important factor ; this is also borne out by the fact that some- 

 times in sunny weather, e.g., June 7, 1911, the cryoscopic value of the north- 



1 G. Trinchieri. Bull, dell'orto botanico. Napoli, 1910. 



