THE ORGANIZATION OF THE PLANT 



61 



substances. The new ducts are provided by a ring of growing 

 cells, called cambium cells, that originate just inside the cor- 

 tex and encircle the central cylinder. The cambium may also 

 add other fibrous and corky cells to the cortex, and in time 

 these form the bark seen in the roots of old trees. The activi- 

 ties of the cambium, therefore, result in increasing the diam- 

 eter of the root from year to year, but growth in length takes 

 place at the tip of the 

 root only, and never at 

 the base, as is generally 

 supposed. If growth in 

 length occurred at the 

 base of the root instead 

 of at the tip, the whole 

 root would have to be 

 pushed through the soil, 

 a task which the plant 

 would soon find impossi- 

 ble of accomplishment. 



In the wild state the 

 seeds of plants are scat- 

 tered on the surface of 

 the soil and germinate 

 without getting very far 

 below it, but at maturity 

 we commonly find the 

 base of the stem, or even the whole stem, some distance 

 underground. In many cases this burial of the stem is due 

 to the contraction of the roots. These penetrate the soil, 

 and, after becoming established, contract and pull the plant 

 downward. The contraction is mainly in the central cylinder 

 and results in wrinkling the cortex. Such contraction wrin- 

 kles are well shown in the roots of the skunk cabbage or 

 the his. 



FIG. 34. Enlarged cross section of a young 



root showing epidermis, cortex, and central 



cylinder 



The large openings in the central cylinder are 



the ducts. Note the root hairs growing from 



the epidermis 



