460 Scientific Proceedings, Royal Dublin Society. 



Summary. 



1. The major part of the osmotic pressure of tissues is due to dissolved 

 carbohydrates. 



2. The variations in the osmotic pressure are due to a large extent to 

 fluctuations in the carbohydrate content of the cells, and to a smaller degree 

 to clianges in tlie concentratiou of the electrolytes. 



3. A progressive average rise in the osmotic pressure has been found 

 during the development and life of each organ examined. 



4. This progressive rise is due in the ease of leaves to the accumulation 

 of electrolytes with age. In the case of the only root examined, viz., that of 

 Ilex aquifoHum, it vfas due to the concentration of carbohydrates. 



5. The osmotic pressure of the deciduous tree, St/ringa vulgaris, reached 

 its maximum (about 18 atm.) in August, rising irregularly from about 13 atm. 

 at the opening of the buds. No very pronounced diminution was observed 

 before the fall of the leaf. 



6. The concentration of carbohydrates in falling leaves is considerable. 



7. Two specimens grown in the country possessed higher osmotic pressures 

 than the specimen grown nearer town. The maxima for the town and 

 country specimens were 183 atm. and 25'5 atm. respectively. 



8. The leaves of the evergreens examined, viz., 7/m aquifolium and Hedera 

 helix, possess higher osmotic pressures during the winter months than during 

 the summer months. 



9. The curve representing tlie seasonal variations of osmotic pressure in 

 the leaves of H. helix is similar for specimens growing in a sunny and in a 

 shaded position. But on the whole the osmotic pressure in the insolated is 

 higher than that in the shaded leaves, the mean pressure being 15'0 atin. 

 and 15'7atm. respectively. 



10. The osmotic pressure of the sap of the roots of /. aquifolium rose 

 from a minimum of about 6 atm. in October to a maximum of 14 atm. in 

 September. 



11. No concentration of electrolytes with age was observed in these roots, 

 the higher osmotic pressure in the older roots being due evidently to 

 increased concentration of the carbohj'drates. 



12. In each case the concentratiou of the total solutes of the sap pressed 

 after freezing was greater than that of sap pressed from the same tissues 

 untreated. The seasonal variations in concentration of the sap obtained by 

 the two methods showed a remarkable similarity. 



