ABSORPTION AND MOVEMENT OF WATER 117 



Evaporation and excretion, taking place on the exposed 

 surfaces and also from those cells bordering on air-passages, 

 increase the density of the cell-sap of the cells directly con- 

 cerned. These draw osmotically upon their neighbors for 

 water to make good their loss. The neighboring cells in 

 their turn draw upon cells still more remote from the losing 

 surface. In this way the demand for water is developed in 

 cell after cell away from the surface. To meet the demand, 

 water is transferred from cell to cell toward the surface. 

 Ordinarily, cells are small and short, and though their 

 bounding cellulose membranes and their component proto- 

 plasm may be freely permeable, water can move more 

 i-apidly in response to other than osmotic forces if only the 

 way is clear. Through living cells water can make its way 

 best by osmosis, but as water will pass more rapidly through 

 a tube in which no filtering membrane is interposed, so water 

 in the plant will pass more rapidly through elongated cells 

 than through a series of short ones, through dead and 

 empty tracheids than through living cells of the same di- 

 mensions (other things being equal), and through con- 

 tinuous ducts than through a succession of tracheids. These 

 stages in the development of conducting tissues one finds in 

 the larger erect mosses, in the Coniferse, and in the Angio- 

 sperms. The most perfect development of a vascular sys- 

 tem is found perhaps in twining plants, especially those of 

 tropical countries, * in the slender stems of which the ducts 

 are large, long, and no thicker walled than is consistent 

 with the necessary mechanical strength. 



Upon the vascular tissues, throughout their whole length, 

 parenchyma cells abut directly. In the root these paren- 

 chyma cells receive more and more water so long as t'he 

 root-hairs continue to absorb any from the soil, and pres- 

 ently, not being able to expand beyond a certain point by 

 reason of the pressure of their neighbors, they are obliged to 

 get rid of the excess in some way or other. Into the vascular 

 tissues of the root, therefore, the parenchyma cells discharge 

 the water and dissolved matters, the discharge taking place 



* Schenck, H. Beitrage zur Biologie der Lianen. Bot. Mittheilungen 

 aus den Tropen, Bd. II., 1893. 



