70 



FOOD FKOM THE SOIL 



attach a glass tube with a bore of approximately the diam- 

 eter of the stem. Pour in a little water. Observe the rise 

 of the water due to the pressure from below (Fig. 109). 



::. ; Some plants will force the column of water 



several feet. The water ascends chiefly in 

 the young wood, not between the bark and 

 wood, as commonly supposed. To illustrate 

 the path of water -ascent, insert a growing 

 shoot in water which is colored with eosin : 

 note the path which the color takes. (Eosin 

 dye may be had of dealers in microscope 

 supplies. Common aniline may answer very 

 well.) 



147. HOW THE SOIL HOLDS MOISTURE. The 



water which is valuable to the plant is not the 



free water, but the thin film of moisture 



which adheres to each little particle of soil. 



The finer the soil, the greater the number 



of particles, and therefore the greater is the 



quantity of film moisture which it can hold. 



This moisture surrounding the grains may 



not be perceptible, yet the plant can 



use it. Root absorption may continue in 



a soil which seems to be dust dry. 



148. THE ROOTS NEED AIR. Corn 

 on land which has been flooded by heavy 

 rains loses its green color and turns 

 yellow. Besides diluting plant -food, the 

 water drives the air from the soil, and 

 this suffocation of the roots is very soon 

 apparent in the general ill health of the 

 plant. Stirring or tilling the soil aerates 

 it. Water-plants and bog-plants have adapted themselves 

 to their particular conditions. They either get their air 

 by special surface roots, or from the water. 



109. 

 To show root-pressure 



