PHYSIOLOGY. 



to know how plants can obtain water from soil which is not wet. If we at- 

 tempt to wash off the soil from the roots, being careful not to break away the 



Fig. 44- 

 Root hairs of corn seedling with soil particles adhering closely. 



root hairs, we find that small particles cling so tenaciously to 

 the root hairs that they are not removed. Placing a few such 



root hairs under the microscope it appears as if here and there the root hairs 



were glued to the minute soil particles. 



63. If now we take some of the soil which is only moist, weigh it, and 

 then permit it to become quite dry on exposure to dry air, and weigh again, 

 we find that it loses weight in drying. Moisture has been given oft. 

 This moisture, it has been found, forms an exceedingly thin film on the sur- 

 face of the minute soil particles. Where these soil particles lie closely to- 

 gether, as they usually do when massed together in the pot or elsewhere, this 

 thin film of moisture is continuous from the surface of one particle to that of an- 

 ther. Thus the soil particles which are so closely attached to the root hairs 

 connect the surface of the root hairs with this film of moisture. As the cell- 

 sap of the root hairs draws on the moisture film with which they are in con- 

 tact, the tension of this film is sufficient to draw moisture from distant parti- 

 cles. In this way the roots are supplied with water in soil which is only 

 moist. 



64. Plants cannot remove all the moisture from the soil. If we now take 

 a potted plant, or a pot containing a number of seedlings, place it in a moder- 

 ately dry room, and do not add water to the soil we find in a few days that 

 the plant is wilting. The soil if examined will appear quite dry fo the 

 sense of touch. Let us weigh some of this soil, then dry it by artificial 



