184 General Botany 



In fleshy roots like the beet, which form in a single growing 

 season, the thickening takes place through the continued forma- 

 tion of new cells by the cambium. As the cambium produces 

 conductive tissue, alternating rhythmically with parenchyma 

 tissue, the mature root appears to be composed of concentric 

 layers. 



Perennial roots. The perennial roots of shrubs and trees 

 increase in length from year to year, and the older roots increase 

 in thickness by the formation of annual rings. These roots soon 

 lose their epidermis, and later the cortex also dies, and a dead 

 bark similar to that of tree trunks is formed. In some instances 

 new cambiums arise in the cortex, which produce a layer of cork 

 enveloping the roots. 



Growth of roots. Roots develop from growing points near the 

 tip. The growing point, however, does not lie at the surface, as 

 in stems, but some distance below, being covered by a root cap 

 (Fig. 109). As it moves, or forces its way between the particles 

 of the soil, it is in this way protected from abrasion. The cells 

 of the growing point are alike, but at a very short distance 

 back from the growing point they become difTerentiated into the 

 vascular axis, with elongated cells, cortex, and epidermis. The 

 growing region of soil roots is usually very short. Lateral roots 

 arise from growing points that develop on the vascular axis and 

 push outward through the cortex, breaking it as they elongate. 



Root hairs. The young roots of land plants generally bear root 

 hairs. These are delicate elongations of the epidermal cells of 

 the root. They are especially concerned with absorption of 

 water and mineral salts, and their presence increases the absorb- 

 ing surface of the root from two to one hundred times. Since 

 the rate of absorption depends in part upon the surface area in 

 contact with the soil water, the advantage of root hairs is evident 

 (Fig. no). 



Root hairs are usually short-lived structures, their duration 

 being best measured in days. Their walls are not only thin, but 



