46 



ROOT-HAIRS. 



[CH. Ill 



in length it pierces the tissues and burrowing through the 

 cortex breaks out at the surface, leaving in the cleft 

 surrounding its base, evidence of its internal manner of 

 origination. 



FIG. 18. 

 TRANSVERSE SECTION OF THE PRIMARY ROOT OF THE BEAN, 



showing a secondary root developing. 



p. I, piliferous layer. c, cortex. &. s, endodermis. 



x, xylem. phi, phloem. p, pith. 



The surface of root in contact with the soil is still 

 further increased by the growth of what are known as 

 root-hairs. These can be especially well seen in seedlings 

 of the mustard, cabbage, or one of the cereals. A seedling 

 mustard which has germinated in damp air gives the 

 appearance shown in fig. 19. The base of the root, where 

 it joins the young stem, bears a dense frill of delicate 

 colourless hairs ; nearer to the tip of the root they are 

 younger and therefore shorter, and at the tip of the root 

 they are not found. A transverse section (fig. 13) would 



