NATURE AND SOURCES OF FOOD. 103 



may be obtained from it by evaporation, wbilst the descending 

 sap of the same tree does not possess any sweetness. 



Besides this general circulation of the sap through the entire 

 cellular and vascular system of the plant, an independent cir- 

 culation or movement of rotation has been observed in the cells 

 themselves, considered separately and individually. This is 

 well seen in those cells which form the hairs of plants which 

 are conveniently situated for observation. The string of bead- 

 like cells which compose the jointed hairs of the Tradescantia 

 Virginica, or the spiderwort, show this circulation distinctly 

 under a magnifying power of about 400 diameters. In the tubu- 

 lar cells of Chara, an aquatic plant growing in stagnant pools, 

 this circulation may be seen with an ordinary microscope. The 

 motion of the currents in the cells of these plants is rendered 

 visible by the minute grains of chlorophyl which they carry 

 along with them. The cause of these motions is at present 

 wholly unknown. 



