i8 



BOTANY OF CROP PLANTS 



dies are called the xylem, the softer, food- 

 conducting bundles, phloem. The central 

 portion of the stele is composed of large, 

 loosely fitting cells, making up the pith or 

 medulla. 



Side roots arise from the outer edge of 

 ^j, the stele (central cylinder), and push their 

 way through the cortex and epidermis 

 (Fig. 7). This method of origin of side 

 branches is characteristic of roots. In 

 stems the, side branches arise from the outer 

 part of the cortex (Fig. 15). Branch roots 

 are said to have an endogenous origin, 

 while branch shoots (except those in mono- 

 cots) have an exogenous origin. 



As the root grows older, new xylem and 

 phloem are formed, and by and by, it be- 

 comes very tough and woody, serving as an 

 efficient anchorage organ. 



Root Hairsn^the Absorbing Organs of a 

 Plant. — The great problem of all our com- 

 mon plants is to take in as much water from 

 the soil as they lose to the air, i.e., to main- 

 tain a balance between water intake and 

 water outgo. We speak of the roots as the 

 absorbing organs of the plant. In a sense 

 this is true, but it must be understood that 

 water and soil solutions are not taken in at 

 all points on the surface of the root system. 

 Practically all absorbed substances enter 



Fig. 7. — Young root of white lupine showing origin 

 of lateral roots from the stele. {After Gager.) 



