x OSMOTIC PRESSURE IN PLANTS 197 



In Experiments 21 and 22, the leaves of which are from 

 shaded positions, smaller pressures are found than in 

 Experiment 24, which was performed on sap from leaves 

 in a sunny position. Experiment 23, on leaves coming 

 from the outside of the crown facing a clear north sky, 

 and being consequently better illuminated than the other 

 two samples, used in 21 and 22, reveals a higher pressure 

 than they, though this pressure is considerably lower than 

 that found in 24. 



This effect of illumination in raising the osmotic pressure 

 in the cells of leaves appeared in a great number of 

 experiments. Conversely it was also found that the 

 pressure gradually falls when they are cut off from light. 

 The difference thus produced may amount to 141 1 atm. 



These experiments, taken in conjunction with observa- 

 tions on the mean molecular weight of the solutes, amply 

 show that conditions favouring the formation of soluble 

 carbohydrates are those which raise the osmotic pressure ; 

 and vice versa, the osmotic pressure falls when conditions 

 are unfavourable to the formation of these substances, while 

 not preventing their consumption. It was only to be 

 expected then that the cells in organs where these sub- 

 stances are normally stored should have high osmotic 

 pressures. This point is well illustrated by the high 

 pressures found in the fruit of Vitis vinifera (38*3 atm. 

 A=3T85), and of Pyrus malus (23T atm. A = 1-919), 

 and in the root of Beta vulgaris (2T2 atm. A = T761 ). 1 



1 Another point of interest with regard to the distribution of osmotic 

 pressure in plants may be mentioned. 



In almost every case it was found that the older leaves, ccnterls paribus, 

 had a higher osmotic pressure than the younger leaves on the same plant. 

 This was observed in Syringa vulgaris, Vitis veitchii, Eucalyptus globulus, 

 Hedera helix, and especially in Ilex aquifolium. The leaves of the last named 

 evergreen persist through four or five periods of growth, and it is generally 

 found at any time that the osmotic pressure of the sap of the leaves of each 

 successive growth is lower than that of those which precede it. Thus taking 

 the mean of many observations made throughout the year on the sap from 

 untreated leaves, the osmotic pressure of the sap of the ultimate three 



