146 GENERAL BOTANY 



in them, and a higher osmotic pressure is thus developed. This 

 concentration of cell sap and the increased osmotic pressure pro- 

 duce a suction force on the less concentrated water content of 

 the adjacent cells of the veins. The result would be the with- 

 drawal of sufficient water from the vein cells at the top of the 

 water column to supply the mesophyll cells, which would cause 

 a corresponding deficiency of water at the base of the water 

 column in the roots. This deficiency in the roots would then be 

 supplied by absorption from the soil and by the forcible filtration 

 of water into the ducts by the cortex cells of the root. This is 

 in brief the conception of the -cohesion theory of water ascent, 

 which, while not entirely satisfactory, has the virtue of dealing 

 with known physical principles. Two of the physical principles 

 involved namely, leaf -cell suction and filtration, due to osmotic 

 pressure in the cortex cells of the root are known to be opera- 

 tive in the plant. The third principle of cohesion, while valid for 

 water columns in general, may not be applicable when applied 

 to water columns as they exist in the ducts of plants. 



The importance of water ascent in plants is also a question con- 

 cerning which there is considerable difference of opinion. Some 

 writers consider transpiration and water ascent as real functions 

 of the plant, designed to supply water and soil salts to the living 

 mesophyll cells of leaves as well as to other cells of the plant 

 body. Others regard it as a dangerous process which is necessi- 

 tated by the structural adaptations of the leaf for photosynthesis 

 and respiration. The great intercellular spaces of the leaf and the 

 stomata are certainly a menace to the plant when considered from 

 the viewpoint of conservation of water. We shall doubtless find 

 that both conceptions of transpiration are partly true. 



ECOLOGICAL RELATIONS OF PLANTS TO WATER 



Mesophytes. The plant structures thus far described are 

 those which pertain to plants living under medium conditions of 

 moisture and temperature. Such plants are termed mesophytes, 

 and they include the great plant populations which inhabit most 

 of the temperate regions of the earth. 



