CHEMISTRY OF PLANT GROWTHS 241 



physical and chemical agents. This action can best be 

 understood by first considering a few of the properties 

 of plant tissues, as porosity, capillarity, and osmosis. 



340. Porosity of Tissues is a property common to all 

 forms of matter, and one possessed particularly by vege- 

 table substances. The living plant not only admits the 

 passage of water through its tissues, but absorbs it until 

 the pores are filled. Animal and vegetable tissues always 

 possess the power to take up and tenaciously hold 

 water within their fibers. This is, in part, due to capil- 

 lary action (see Section 20, Chemistry of Soils and Fer- 

 tilizers). Capillarity, assisted by evaporation, explains 

 only in part the movement of the plant juices. Com- 

 pounds formed within the leaf must be transported in an 

 opposite direction to that taken by the sap in moving 

 from the roots to the leaves. This movement is effected 

 by osmosis, and chemical reaction within the cells. 



341. Osmosis. When a bottle filled with a solution of 

 salt, colored with litmus, is placed in a large vessel of 

 water, the bottle will discharge its contents into the 

 water and the movement of the solutions can be followed 

 with the eye. If sugar and salt solutions are separated 

 by a membrane, there is a gradual interchange between 

 the two solutions. Some of the sugar finds its way into 

 the salt solution, and some of the salt finds its way into 

 the sugar solution. This action or interchange is still 

 further increased when the solutions are of different 

 densities and when chemical action is taking place on 

 both sides of the membrane. Such an action takes place 

 in plant tissues, which are composed of a large number 



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