THE ROOT 



143 



piliferous layer, wither and shrivel. The older portions of a 

 root after becoming covered by a protective periderm lose 

 their absorptive function and henceforward act chiefly as con- 

 ductors of the watery solutions absorbed by the younger parts 

 still possessing root-hairs. For an account of the characteristic 

 root-cap which covers the growing point of practically all roots 

 see pp. 149 and 150. 



1 2 



Fig. 73. — Diagram illustrating secondary growth in thickness of 

 the root of a dicotyledon, i. Transverse section of a very young 

 root. 2. The same after the cambium {c) has formed a continuous 

 band. 3. The same after secondary thickening has been in progress 

 some time. / Piliferous layer ; j primary cortex ; e endodermis ; 

 n pericycle ; b ' primary bast ; iv ' primary wood ; c cambium ; 

 b " secondary bast ; w " secondary wood ; r secondary cortex 

 m primary medullary ray. 



Ex. 69. — Soak some peas and barley grains in water for six or seven 

 hours, and afterwards allow them to germinate on damp blotting paper or 

 flannel as in Ex. 3. When the root-hairs are visible on the young roots 

 examine them with a lens and make sketches noting especially their origin 

 away from the extreme tip. 



Strip off with forceps a piece of the outer portion of a root, so as to 



