60 AGRICULTURE. 



attraction, or the ov^ercoming of the inOuence 

 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 



