32 



PLANTS 



Structure and Function of the Roots 



68. The mechanism of water absorption by the roots may be 

 discovered by the study of cross sections of the smaller root- 

 lets. Such a section taken several centimeters from the tip, 

 i. e., through the region covered by root hairs, presents three 

 well marked kinds of tissues; viz., (i) a general ground tissue 

 made up of rounded or polygonal cells with thin walls, (2) 

 larger circular structures grouped around the axis of the root, 



which are longitudinal vessels 

 in cross section, and (3) root 

 hairs, which are tubular ex- 

 pansions of some of the thin 

 walled cells of the surface 

 layer. The vessels may usu- 

 ally be seen by the unaided 

 eye, especially in the larger 

 roots. The root hairs are 

 very conspicuous and, when 

 growing in a moist atmos- 

 phere, stand up rigidly from 

 Fig. 7.— Cross section of a young root, the surface of the root as 



slender cylindrical bodies 

 several millimeters in length. If they are exposed for a few 

 minutes to the dry air they soon become limp, topple over and 

 shrivel. This fact shows that the watery content of the hair 

 is rapidly extracted by evaporation from the surface^ and that, 

 therefore, the cell wall of the hair is highly pervious to water. 



69. By cutting off the stem of a growing plant near the 

 ground and connecting a glass tube with the stump it may be 

 shown that the roots have the power, not only of absorbing 

 moisture from the soil, but also of driving the sap up into the 

 stem under considerable pressure. In all probabihty the 

 force chiefly responsible for this root pressure is the osmotic 



