44 ELEMENTARY BIOLOGY 



68. 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. Through some of these tubes materials 

 are also brought down from the stem to the growing layer of 

 cells. In the central cylinder we can find that many of the cells, 

 instead of forming ducts, become thick-walled and stiff. These 

 " fibers " give the cylinder its toughness and rigidity. Bundles 

 of fibers and vessels are sometimes called fibro-vascular bundles, 

 the \sxmfibro meaning " of fibers," and vascular meaning " of 

 vessels," or tubes (Fig. 10, w). 



69. Forms of roots. The structure of roots is fairly uniform 

 for different kinds of plants. But roots nevertheless appear 

 in very many different forms, from the thin, stringy roots of 

 grains to the massive fleshy or woody roots of beets or trees. 

 These differences are found to be closely related, in many 

 cases, to the conditions under which the plants live. Thus, 

 fleshy roots are often associated with the biennial habit. In 

 such plants as beets, carrots, and parsnips the first season of 

 the plant's growth is spent in manufacturing food and deposit- 

 ing it in the root. The next year comparatively little foliage 

 is produced, but a stalk bearing flowers (which in turn develop 

 into fruit, bearing seeds) uses up practically all the food that 

 has been left over from the previous season. In contrast with 

 this habit of life we find the plants that sprout, grow ifp into 

 maturity, and die, all within one season. These annual plants 

 have, as a rule, rather delicate, or fibrous, roots. 



Trees and woody shrubs, which continue to live year after 

 year, develop massive shoots. Corresponding to this fact we 

 may note that such plants also develop elaborate, strong roots. 

 From this we may see that the structure of the root and its 

 functions are closely related to each other and to the character 

 of the plant. There is a connection, on the one hand, between 



