178 



General Botany 



will show the essential features of roots. The primary root 

 extends downward from the base of the stem. On its sides are 

 numerous secondary roots which extend 

 at right angles, or grow obliquely 

 downward (Fig. 102). Unlike stems, 

 roots possess no definite nodes from 

 which branches arise. A secondary 

 root may originate at any point on the 

 primary root. Some common plants, 

 like parsnip and carrot, have primary 

 roots that thicken above ,^,,^_^.^^,,^,__ 

 and taper gradually, ex- 1§i|ft|* 

 tending deeply in the soil. 

 These are called tap roots 

 (Fig. 104). 

 Adventitious roots. In 



many seedlings there are Fig. 103. stages in the growth of an onion 



, i . 1 . 1 1 r seedUng, showing the lifting and shedding of 



also roots that develop from ^^e seed coats and the development of the 



the first node of the stem. primary and secondary roots. 



All roots arising from stems and 

 leaves are called adventitious roots. 

 The '' prop roots " that develop 

 from the lower nodes of corn stems 

 and the roots that grow from 

 ^' cuttings " are familiar examples. 

 The adventitious roots of corn, 

 sorghum, wheat, and many other 

 grasses are far more important than 

 the primary and secondary roots. 

 In many instances only the adven- 

 titious roots remain at maturity 

 (Fig. 108). 



Fig. X04. Dandelion plant showing the AdveutitioUS rOOtS dcvelop alsO 



primary tap root and its branches, the ^ 



secondary roots. from the stems of many plants like 



