Plants in Relation to Man 467 



When the end of a strip of cloth is dipped into water, one may 

 observe the water steadily creeping above the surface level 

 on the cloth. A dry lamp wick inserted into the oil reservoir 

 of a lamp carries the oil to the top of the wick, where it may 

 evaporate or be consumed by burning. If one corner of a 

 cube of sugar is dipped into tea or coffee, the moisture gradually 

 extends throughout the cube. Blotting paper, wood, and many 

 other substances illustrate the same fact. This may also be 

 the way by which water absorbed by the roots of plants is forced 

 to the tips of twigs and leaves, where part of it is lost through 

 evaporation and the rest is used as a medium of circulation 

 in the plant. 



Exercise : Capillarity. Holding closely together two pieces of 

 glass, dip one end into water. Note the gradual rise of water between 

 the glasses. If practicable, vary the space between the glasses, and 

 note difference in rate at which water rises as the space becomes 

 smaller. Arrange three or four pieces of glass tubing of different 

 diameters, ranging from one eighth of an inch to the smallest size 

 obtainable, in a pan containing a small amount of water. Observe 

 the behavior of the water in the tubes of different bore. Does the 

 result indicate that the smaller the bore, the higher and more rapidly 

 water rises? 



This behavior of water in very small spaces is usually de- 

 scribed as capillarity. The term capillary means strictly 

 " hair " from the Latin word capillus; capillary tubes are 

 tubes with hair-like openings. 



Capillarity in the soil. The soil is literally shot through 

 and through with such hair-like openings or pores. Water 

 retained after natural or artificial drainage rises gradually 

 toward the surface, where it is evaporated or absorbed by plants. 

 The water is said to be " attracted " or lifted by capillarity 

 to the surface. It is exactly the same as in the case of the 

 lamp wick, which carries oil from the reservoir to the burning 

 top. The rate at which capillary action of water occurs may be 

 shown experimentally to depend upon the character or texture 

 of the soil. 



