60 AGRICULTURE. 



attraction, or the overcoming of the influence 

 of gravity by the adhesion between the water 

 and the solid particles, and is of direct use to 

 plants. 



3. Hygroscopic Moisture. Each particle of 

 soil is surrounded by a film of moisture, or hy- 

 groscopic water. It is held so firmly that even 

 roadside dust contains this film. 



EXPERIMENT 8. Fill a test-tube one-third full of 

 dry roadside dust; heat it gradually to a high temper- 

 ature. Allow it to cool, and see if any moisture con- 

 denses upon the tube. 



II. Relation to Plants. 



1. Dissolves Plant-food. This surface film of 

 water, through the carbonic and humic acids 

 which it contains (Chapter I.), acts directly 

 upon the plant-foods locked up in the soil, dis- 

 solving the mineral substances and giving them 

 up to the surrounding capillary water. 



2. Conveys Plant-food. As has been seen, 

 solids have an attraction for liquids. It is also 

 true that denser or thicker liquids have an at- 

 traction for thinner ones; so it is, as the mois- 

 ture is evaporated from the leaves and green 

 bark of plants, leaving behind the solid sub- 

 stances, the fluid in the plant becomes denser 

 than the soil water, and there is thus established, 

 through the cell wall of the plant, a flow of the 

 thinner liquid, or soil water, toward the denser 

 protoplasm of the cells. This process is called 



