6o OUTLINES OF PLANT LIFE. 



however, the primary root persists throughout the life of the 

 plant. 



Secondary roots are later formed. They are roots which 

 arise upon stem or leaf, or even upon the primary root itself. 

 In the last case they are distinguished from branches of the 

 primary root, which arise in regular succession toward the 

 apex, by originating out of this regular order. Secondary 

 roots are also called adventitious roots. They may take their 

 origin at any point upon any of the members. Their point 

 of origin will depend largely upon external conditions. A 

 wound may cause them to appear. They are especially likely 

 to be formed upon those parts which are in contact with the 

 substratum, or from those parts which are kept moist. Upon 

 stems they are most apt to appear near the nodes. (See 

 ^ 104.) If the plant as a whole is surrounded by very moist 

 air, roots may appear at any point of the surface. Secondary 

 roots arising thus upon a part of the plant exposed to the 

 air, and growing for all or part of their existence in the air, 

 are also called aerial roots. Familiar examples are to be 

 seen about the lower part of the stem of Indian corn, the 

 English ivy, the poison-oak, the trunks of palms and tree- 

 ferns. Secondary roots often arise in regular succession 

 toward the growing apex of the stem, particularly in plants 

 which have creeping or subterranean stems. 



71. Growing point. — Primary and secondary roots do not 

 differ materially in their structure. Near the tip they consist 

 of a mass of actively dividing cells, the growing point of the 

 root (compare ^ 87). The real tip of the root is covered by 

 a mass of cells called the root-cap (ep, fig. 50), which is at- 

 tached only to the growing point. Since the cells of the 

 free surface of the root -cap are older and firmer than the 

 inner ones and the growing point, and lie in front of them, 

 they serve to protect these more delicate parts as the growth 

 constantly pushes the apex forward through the soil. 



