ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 593 



pressure. The variation is probably due chiefly to fluctuation in the 

 carbo-hydrate content of the cells, and is not defined by the height of the 

 leaves above the ground, nor by the resistance of the conducting tissue ; 

 the osmotic pressure is in any case much greater than the tension of the 

 water supply could have been. 



Photosynthesis leads to increase in osmotic pressure, and a similar but 

 smaller increase occurs in plucked leaves stored in darkness, because of 

 the hydrolysis of saccharose and starch. Shielding leaves from the light 

 leads to fall in osmotic pressure —e.g. l<s to 11^ atm. Mature leaves 

 showed higher pressure than developing ones, and the pressure in roots is 

 relatively low ( f to 6 atm.). The greatest osmotic pressure observed was 

 26*87 atm., in Lilac, but this is probably by no means a maximum for 

 this plant. The lowest record was 3*79 atm., in Chamseropx humilis. 



Ascent of Sap in Trees.* — Leclerc du Sablon points out the un- 

 settled state of this problem, and objects to previous explanations 

 based on the assumption of a difference of pressure between the two ends 

 of the stem on the score that no cause can be ascertained to account for 

 a difference of pressure sufficient to raise water from the roots to the 

 summit of a tall tree. Bonnier has shown that pressure-differences can 

 be transmitted for only a very short distance in the interior of the wood, 

 and Ewart has demonstrated the great resistance offered by the vessels 

 on account of their fine bore, and the presence of cross-walls and of 

 groups of air-bubbles (Jamin chains). 



The author suggests that the water-current is maintained by the 

 osmotic power of the living cells of the wood. The leaf-cells transpire, 

 their osmotic power increases and turgescence diminishes ; they there- 

 fore take water from the neighbouring vessels in which the pressure 

 will be less than atmospheric. This pressure-effect, we have seen, can- 

 not reach far, but it can reach at least the neighbouring living cell ; the 

 equilibrium between this and the vessel will then be broken, and sap 

 will pass into the vessel. This process is repeated from living cell to 

 living cell, and water circulates throughout the organism, quite inde- 

 pendently of its bulk. The necessary force is thus derived from 

 increase in osmotic pressure, and all causes which, like transpiration, 

 influence the osmotic strength, contribute to the ascent of the sap. 



Irritability. 



Influence of Light upon Fruit- and Seed-development.f — M. 

 Lubimenko publishes the results of various experiments in this con- 

 nexion, performed with Colutea arborescens, Pisum sativum, Lathyrus 

 latifolia, various Rosacea^, and other plants. The author concludes : — - 

 1. That a closed atmosphere is necessary for normal seed-development, 

 the oxygen necessary for respiration being derived from the photo- 

 synthetic activity of the pericarp wall ; and this necessity extends pro- 

 bably to the first stages of embryo development. 2. Mechanical pressure 

 of the surrounding tissues plays a significant part in limiting embryo de- 

 velopment. 3. A certain intensity of illumination is indispensable to the 

 initiation of fruit development. Darkness prevents the formation of 



* Cornptes Rendus, cli. (1910) pp. 154-7. 



t Rev. Gen. Bot., xxii. (1910) pp. 145-75 (figs.). 



