In roots of such plants as the carrot or parsnip we can distinguish an 

 easily broken outer layer and a tougher core, or "central cylinder", running 

 lengthwise. The two layers correspond respectively to the bark and the 

 wood seen in the stem of a tree. With a microscope we can see that there 

 are several different kinds of cells in the root (see illustration, p. 144). In 

 the central cylinder the cells are much longer in proportion to their width 

 than are those in the cortex^ or bark; and their long diameters run length- 

 wise of the root. 



Such fleshy roots illustrate a third function that many roots carry on, 

 namely, that of "storing", or accumulating, surpluses of food material. But 

 whether roots are fleshy or stringy or woody, they generally absorb and 

 transfer materials. 



Vessels and Fibers In the cortex of a root, movement of material re- 

 sults from simple diffusion or osmosis from cell to cell. In the central 

 cylinder, however, liquids move bodily through long tubes or vessels that 

 act as main channels in the plant. There are, in fact, two sets of conducting 

 tubes. Through the smaller vessels in the central cylinder food materials 

 produced in the leaves are carried down toward the growing parts of the 



Hugh Speueer 



AIR HOLES OF PLANTS 



Thin-walled "guard cells" enclose each stoma and carry on photosynthesis. When 

 they are turgid, the stomata are open; when they become flaccid, the stomata are 

 closed. Stomata occur in the epidermis of twigs, as well as on leaves. As the stem 

 grows tougher, the holes become larger and more irregular. The roughened spaces 

 on the bark are lenticels 



143 



