126 AGRICULTURAL BOTANY 



through membranes easily, while mucilaginous, or jelly- 

 like, substances pass through membranes slowly or do 

 not pass through at all. 



The passage of water into root hairs, and the retention 

 of the protoplasm inside of the root hair, well illustrate 

 both of these laws. The root hair is surrounded by 

 soil water that is not so dense as the cell sap, which 

 is somewhat sugary. Hence the soil water passes 

 through the wall of the hair into the cell, here mixing 

 with cell sap, thus making it less dense than before. 

 This root hair, or cell, touches another cell. The cell 

 sap in the second cell, being as yet undiluted, is more 

 dense than that in the first, therefore some of the cell 

 sap of this first will pass through the wall of the second 

 and dilute the second. Thus the diluted cell sap is 

 passed on from cell to cell till it reaches the small 

 tubes, or ducts, in the root that carry it into the stem. 

 By the operation of the second law, the protoplasm, 

 or life element of the cell, remains in the cell. 



Root Pressure. --This absorptive power of the 

 root hairs causes what is called root pressure. This 

 is the force which sends the upward current of water 

 through the plant. Hales, the English physicist, 

 found the root pressure of a grape vine to be equal to 

 the weight of a column of mercury thirty-two and one 

 half inches high. It is probably this force which 

 causes the sap to flow freely from broken stems and 

 from tapped maple trees in spring. Within certain 

 limits it is affected by the temperature of the soil, 

 lessening as the temperature falls. During cool nights, 

 when evaporation from the leaves of plants is not great, 

 the root pressure may be great enough to force water 

 from the tips of the leaves. The drops of water that 



