150 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



tended, the slackening of the tension consequent upon 

 rupture results in their engorgement with water from the 

 adjoining tracheae. 



3. The rise in osmotic pressure which occurs as leaves 

 increase in age. 



4. The marked influence of exposure to sunlight, espe- 

 cially in deciduous leaves. For the sugars formed during 

 insolation increase the pressure greatly. 



5. The possibility of the osmotic pressure of a leaf being 

 more than doubled by the combined effects of exposure to 

 light and evaporation to the stage of incipient wilting. 



6. The maintenance of a slightly higher pressure, in 

 evergreens such as Hedera helix which grow in a southerly 

 sunny aspect, as compared with the same species when 

 facing north and shaded from direct sunlight. 



7. The want of correlation between osmotic pressure 

 and height above ground-level in the leaves of tall trees, 

 any such apparent connection being due rather to the 

 increase in sugar formation occurring in well-illuminated 

 positions. 



Dixon has also compiled a table showing the maximum 

 pressures found in the plants examined by him and the 

 writer. In unwilted leaves of Syringa vulgar is a pressure 

 of 25-68 atmospheres was found as a maximum. In this 

 determination liquid air was not used in the extraction, so 

 the actual pressure may have been higher. 



Subjoined are some of the most interesting of the values 

 obtained by Cavara (1905), Nicolosi-Roncati (1907), Trin- 

 chieri (1909), Marie and Gatin (1912), and Ohlweiler (1912). 

 These are all under- estimates, as previously explained, 

 owing to the fact that no attempt was made to render the 

 protoplasm permeable. In view of this fact, the numerous 

 determinations carried out to compare the osmotic pres- 

 sures met with in the leaves of various natural orders of 



