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CHAPTER III 

 ADJUSTMENT TO WATER 



50. Responses to water stimuli. The primary responses of 

 the plant to the water of the habitat are four: namely, absorp- 

 tion, diffusion, transport, and transpiration. Absorption is the 

 response of the root to water content. Transpiration is the evap- 

 oration of water from the leaf, and in some measure from the 

 stem also, in response to the humidity of the air. Diffusion is 

 the process by which water is carried from cell to cell through 

 the various tissues. Transport is the movement of water along 

 certain prescribed pathways in its journey through the stem to 

 the leaves. None of these are simple processes; all involve several 

 factors that wall be considered in the proper place. Absorption 

 is the initial activity. It is followed by diffusion and this by 

 transport. The water transported to the leaves is carried to 

 the various cells by diffusion and finally passes off in consequence 

 of transpiration. In any living plant, all of these processes are 

 ordinarily taking place at the same moment, though it is equally 

 clear that transpiration must be preceded logically by transport, 

 transport by diffusion, and diffusion by absorption. 



Absorption 



51. General relations. Absorption is the function by which 

 water is taken into the body of the plant. It is an essential property 

 of every living cell in contact with water supply, and is practically 

 the same for the smallest one-celled water plant, and for the 

 largest tree. Primitive water plants, i.e., the algae, such as pond- 

 scums, seaweeds, etc., ordinarily use the entire surface of the 

 plant body for absorbing water. Terrestrial plants, on the other 

 hand, have reduced the absorbing surface as the plant emerged 



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