CHAPTER II. 



MOVEMENTS OF WATER IN THE . PLANT. 



15. Root-pressure. — The roots, by reason of the osmotic 

 activity of the substances which they contain, are constantly 

 absorbing water. The amount taken up during the winter 

 season when the soil is either frozen or at a very low tempera- 

 ture is very small. At the beginning of spring the storage 

 products which were accumulated in the roots during the latter 

 part of the previous season are changed into substances, such as 

 sugar, dextrine, asparagin, etc., which are soluble and possess 

 great osmotic activity. At this season the leaves are not yet 

 formed, and only a limited amount of water is carried up and 

 transpired by the plant ; consequently the water taken in by 

 the roots is slowly forced upward in a stream, almost filling 

 the wood-cells in the lower part of the plant. The action of 

 the roots is well illustrated by the osmometer described in 

 Experiment 14. The pressure with which the water is forced 

 upward by a Nettle will sustain a column of water 3 or 4 meters 

 high. In the Grape the root-pressure is sufficient to sustain a 

 column of water 10 meters in height. A yearly periodicity 

 of root-pressure is noticed in trees and other perennial plants. 

 In addition it can be demonstrated that daily variations due 

 to temperature of soil and air and the humidity of the air occur. 

 In the Grape the pressure is greatest in the forenoon, and 

 decreases from 12 to 6 P.M. The root-pressure of the Sunflower 

 reaches its maximum and begins to decrease at lo A.M. 



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