ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 531 



Movements of Water in Plants.* — E. Hartig has carried on a 

 series of experiments relating to the tissue through which the circula- 

 tion of the fluids takes place in plants, and the causes which set them 

 in motion. 



In the oak he finds the duramen, notwithstanding the large amount 

 of water it contains, quite incapable of conduction ; while in the birch, 

 on the other hand, the conduction takes place through the whole of 

 the wood ; and these may be taken as examples of two different types. 

 The organs for the conduction of the sap are chiefly those provided 

 with bordered pits, esj)ecially the tracheides ; and at times also the 

 true vessels. 



The absorption of water by the roots is nearly altogether inde- 

 pendent of the ascent of water through the wood. It is caused by the 

 osmotic forces of the cells of the root, especially of the root-hairs ; 

 often called the root-pressure. It is very largely dependent on the 

 temperature of the soil, less so on the amount of water which it 

 contains. Hence the beech, oak, larch, and pine contain the largest 

 quantity of water in the height of summer, the birch on the other hand 

 about April. 



The cause of the ascent of water in the wood is the difterence in 

 the density of the air in the conducting organs, by which the water is 

 pressed upwards from cell to cell. 



The proportion of water and air in the conducting organs varies 

 with the time of year ; when the cell-walls are saturated the cell- 

 sap may occupy from one-third to two-thirds of the cavity of the 

 cell. It rises in them by the action of capillarity. 



While therefore the movements of water in trees are brought 

 about mainly by the changes in the density of the air contained in 

 them, the pressure of the atmospheric air exercises no or very little 

 influence on the whole process. 



Movement of Water in the Vessels.! — J- Vesque thus summarizes 

 the results of his researches on this subject : — 



1. Water is conveyed through the vessels : — (a) when they are full 

 of air ; (h) when they inclose sufficiently long columns of water, inter- 

 rupted here and there by bubbles of air, local transference ; but (c) 

 no conveyance of water takes place when small portions of water are 

 everywhere separated by bubbles of air. 



2. When transpiration is active the vessels give up water to the 

 surrounding elements, and become full of air. 



3. When transpiration is sluggish, the air contained in the vessels 

 diminishes in volume, and finally disappears altogether. 



The vessels are therefore always reservoirs of water ; and, in 

 special circumstances, are the agents for its conveyance. 



Exudation of Water from Leaves.^ — By an examination of plants 

 in very early morning, Volkens has greatly extended the list of those 



* Unters. Forstbot. Inst. Miinchen, ii. (1882); iii. (1883) pp. 47, 94. See 

 Bot. Ztg., xli. (1883) p. 250. 



t Ann. Sci. Nat. (Bot.),xv. (1883) pp. 5-15. See also this Journal, ii. (1882) 

 p. 373. 



% Ber. K. Bot. Gartens Berlin, 1883. See Science, i. (1883) pp. 491-2. 



2 M 2 



