170 



TEXTBOOK OF BOTANY 



are not always sharply separated, and sometimes we find 

 an organ that will fit into one class about as well as into 

 another. 



There are three general forms that a root system may take : 

 (i) The primary root may grow into a long tap root that 

 bears many branches ; this form is rather 

 common among gymnosperms and di- 

 cotyledons, particularly in those that 

 become trees ; (2) sometimes, as in the 

 cucumber, the bean, and many other 

 dicotyledons, the primary root remains 

 short and gives rise to a cluster of long 

 branches ; (3) in many monocotyledons 

 the root system consists largely or 

 entirely of a cluster of adventive roots 

 that grow from the lower part of the 

 stem ; the primary root may remain 

 short (as in the corn) or it may die (as 

 .in the lily). But while most root 

 systems fall into one or the other of 

 these general classes, roots of very many 

 FIG. 105. A barley different kinds and forms have been 

 root, showing the re- developed to meet different needs. The 

 gion just back of the g^^h of a root in length goes on in a 



tip in which root hairs , , , . , 



are formed, and the short zone J ust back of the root tl P 



region a little farther (c, Fig. 104). Back of the region of 



back in which they die growth , and to some extent perhaps in 



and drop off. After . '. . ,. 



Stevens, this region itself, root hairs are borne. 



A little farther back still, the root hairs 

 and the other cells of the epidermal layer have died and 

 disappeared ; these older parts of the root are commonly 

 protected against the loss of water by the formation of cork 

 in the outer walls of what are now the surface cells. Roots 

 of gymnosperms and dicotyledons, especially of those which 

 live for more than one year, grow in thickness and form 



