310 Scientific Proceedings^ Royal Dublin Society. 



These determinations agree well with the highest determinations obtained 

 by the gas-pressure method. It is also possible that, being observed late in 

 the summer, when the conditions for assimilation and transpiration were not 

 optimal, these pressures are not maxima, but might be higher under more 

 favourable conditions ; and in several cases it has been found possible to 

 considerably raise the pressure by evaporation before wilting ensues. In 

 any ease the observations show that the osmotic pressures in the cells of tlie 

 leaves are more than ample to resist the tensions developed in the transpira- 

 tion-current. 



Conclusions. 



1. Osmotic pressures are variable with the species and individual. 



2. Leaves of the same individual under similar conditions have the same 

 osmotic pressure. 



3. In the same individual considerable variations are found under varying 

 conditions ; for example, in 8yringa vulgaris, the pressure in the leaves of one 

 plant was found to vary from 24*58 atmospheres to 11'58 atmospheres. 



4. Variation in pressure is not defined by the heiglit of the leaves above 

 the ground, nor by the resistance of the conducting tracts supplying the 

 leaves. In each case the osmotic pressure was much greater tlian the tension 

 of the water-supply could have been. 



5. The variations in the osmotic pressure observed are probably due 

 principally to fluctuations in tlie carbohydrate - contents of tlie cells. 

 Assimilation leads to a rise in the osmotic pressure, and in the mean mole- 

 cular weight of the solutes. 



6. A similar, but smaller, rise in osmotic pressure, and a similar rise in 

 mean molecular weight, may be observed in plucked leaves stored in the 

 dark. These changes are probably due largely to the hydrolysis of saccha- 

 rose and of starch. 



7. No such rise was observed in starved leaves or in roots kept in the 

 dark. 



8. The osmotic pressure of leaves still attached to a plant may be 

 reduced greatly (e.g. from 18'10 to 11*58 atmospheres] by shielding from 

 light. 



9. Other things being equal, mature leaves showed a higher osmotic 

 pressure than developing leaves. 



10. The roots examined had comparatively low osmotic pressures, viz., 

 4 to 6 atmospheres. 



