ROOTS OF PLANTS 



43 



66. The tissues of a root. If we examine the root of a plant 

 freshly removed from the ground, we shall find that there is a 

 soft, easily broken outer layer covering a tougher central portion. 

 This central portion, running lengthwise in the root, is called 

 the central cylinder, and corresponds in position to the wood of 

 a stem. In a cross or longitudi- 

 nal section of a fleshy root, like a 

 carrot or parsnip, we may distin- 

 guish the central cylinder from 

 the cortex. In very thin slices cut 

 lengthwise through young root- 

 lets we are able, with the help of 

 a microscope, to see the character 

 of the cells (Fig. lo). The cortical 

 la3'er can be distinguished from 

 the central cylinder by the fact 

 that the cells of the former have 

 about the same diameter in one 

 direction as in another, whereas 

 the cells of the central cylinder 

 are considerably longer than they 

 are wide, and their long diameter 

 is parallel with the long diameter of 

 the root. 



67. Vessels and fibers. In the 

 cortex, transportation of material 

 probably takes place by diffusion 

 from cell to cell. In the central 

 cylinder, however, we can find that 



liquids are moved bodily through the long tubes or vessels that 

 act as the main channels in the transportation of materials taken 

 in by the root hairs. Other vessels in the central cylinder carry 

 food materials from the leaves and stems down into the growing 

 parts of the root. In addition to the cells which form ducts 

 there are others with thickened walls. These fibers add to 



Fig. lo. Diagramof root structure 



c. epidermis, or skin layer ; <r, cortex ; 

 cyl, central cylinder, consisting of con- 

 ducting vessels and, supporting fibers 

 and the pith, / 



