ABSORPTION AND TRANSMISSION OF WATER 95 



In all organisms except the very lowest the power to 

 absorb moisture from outside the body is possessed by only 

 relatively few cells, whose external position fits them for this. 

 Thus, some submerged aquatics may perhaps absorb equally 

 throughout the whole extent of their comparatively thin epi- 

 dermis. Partially submerged forms can absorb only through 

 those surfaces which are under water. Ordinary land plants 

 absorb through the surface layers of the younger portions of 

 their roots, the surface layer being often greatly extended by 

 the development of root hairs. But in any event, no matter 

 where the water passes from the substratum into the plant 

 body, absorption always takes place in the same way. The 

 cellulose membranes are kept wet by imbibition, and water 

 diffuses into the protoplasm from them. The forces which 

 cause the entrance of water into the plant are, then, partly 

 those of adhesion and surface tension, and partly those of 

 simple diffusion. 



That the rate of root absorption varies with the tempera- 

 ture of the soil when the changes in temperature are grad- 

 ual, as was demonstrated by Vesque, 1 shows that this 

 absorption is an osmotic phenomenon. With a sudden rise 

 in temperature this author found that absorption is dimin- 

 ished temporarily, and with a sudden fall it is augmented. 

 This is probably due to the increased pressure of the inclosed 

 gas bubbles at higher temperatures. 



II. TRANSMISSION OF WATER 



a) Water loss. Within the single cell transmission of 

 water occurs mainly by simple diffusion, aided, no doubt, by 

 streaming movements of the protoplasm. In more complex 

 forms, like liverworts, and in simple tissue masses of the 

 higher plants, diffusion from cell to cell through the sat- 



1 J. VESQUE, " De 1'influence de la temperature du sol sur 1'absorption do 1'eau 

 par les racines," Ann. sci. nat. bot., Ser. VI, Vol. VI (1877), pp. 169-209. 



