PHASEOLUS ROOT 151 



II. Cut sections successively at older points in the 

 same root, treat as before, and observe the mode of 

 origin of the lateral roots, noting more especially the 

 following facts : - 



a. The lateral roots arise opposite the groups of 

 primary xylem : this explains their arrangement in four 

 rows as above observed with the naked eye, since the 

 number of groups of primary xylem is usually four. 



1). The pericycle, endodermis, and a small portion 

 of the cortex, all take part in their formation. 



c. In the older lateral roots it may be seen that their 

 vascular system is continuous with that of the main root. 



d. The lateral roots, increasing in length, burst 

 through the outer layers of cortex and the piliferous 

 layer : since they originate from deeply seated tissues 

 and rupture the more superficial ones, they are said to 

 be of endogenous origin. 



This mode of origin of the lateral roots is the rule in the plants 

 with apical meristem, arranged according to Type II. (see below, 

 p. 159). In the plants whose root-apex follows Type I. the lateral 

 roots are mainly, or even entirely, derived from the pericyle : 

 this is the case with Helianthus, and sections should be made 

 from its roots to illustrate this point. 



III. Cut transverse sections of the root, six inches or 

 more from the apex, avoiding the lateral roots : take 

 care also to avoid the thick base of the hypocotyledonary 

 stem, which shows a structure characteristic neither of 

 the stem nor of the root : treat as before. 



The general arrangement of tissues is the same as 

 has been above described, though there has been 

 increase in bulk, and the xylem and phloem, being now 

 more fully developed, are more easily recognized. Ob- 



