Dixon and Atkins — Variations in Osmotic Pressure, 6fc. 245 



An experiment made in December, 1911, indicates the amount of influence 

 we may attribute to photosynthesis during dark weather about that date. In 

 this experiment comparison was made of the sap of mature leaves freely 

 exposed to the light in the two aspects with that of leaves close by covered from 

 the light for a period of eight days. The results are shown in Table IV. 



Table IV. 

 Hedem Helix : Leaves. 



Here the cry oseopic values of the exposed leaves from the north aspect closely 

 correspond with those of the covered leaves from the south aspect. This may 

 be taken as indicating that the conditions of the north-aspect leaves did not at 

 the time allow of very active photosynthesis. The slightly higher value for 

 the sap of the exposed leaves in the south aspect, together with the higher 

 mean molecular weight of its dissolved substances, indicates that in this case 

 photosynthesis was active in raising the concentration. During these observa- 

 tions very wet weather prevailed, so that transpiration was reduced to a 

 minimum, and concentration from that cause may be neglected. 



The rise in the leaves from the north aspect which were covered for eight 

 days may possibly be explained bj' the activity of enzymes bringing insoluble 

 bodies into solution. This process in the covered leaves in the south aspect 

 may have been masked by more vigorous respiration in the warmer situation. 



A similar rise in the eryoscopic value of the sap is often observed in leaves 

 which have been taken from the plant and kept in the dark ; but sometimes 

 this rise is more than counterbalanced by some other process, probably respira- 

 tion, which reduces the eryoscopic value. This latter process is largely or 

 entirely removed when the sap is pressed from the leaves, so that the eryo- 

 scopic value of the sap in the uninjured leaves may fall or rise but slightly, 

 while that of the sap pressed from the same sample of leaves will rise con- 

 siderably, and continue to do so for some days. The figures in Table V show 

 that the change taking place in sap in vitro is large compared with that 

 proceeding in the picked leaves, both being kept under the same conditions, 

 and, of course, protected from evaporation. 



