8io 



A TEXTBOOK OF THEORETICAL BOTANY 



It should be remembered that, whereas the primary xylems and phloems 

 of the root are alternate, the secondary xylems and phloems lie on the same 

 radii, with the cambium between them, which corresponds to the arrangement 

 in the stem (Fig. 805). 



Lenticel 



Fig. 805. — Vitis quadrangularis. Transverse section of 

 a hexarch aerial root showing secondar}' thickening. 

 Lenticels are present at the surface. Note the 

 general resemblance to a stem. 



Periderm. 



When secondary thickening has begun, the expansion in diameter is rapid 

 and the need for a new outer covering which is self-regenerating makes itself 

 felt. Only a tissue capable of equally rapid expansion can keep pace with 

 the growth of the cambium. This is provided by a second, independent 

 cambium, which produces the new surface covering. The new cambium is 

 called the phellogen or cork cambium, and the tissue it forms is the 

 phellem or cork. Together they make up the periderm. 



The pericycle, immediately outside the phloem, becomes the phellogen, 

 by the rejuvenation of its cells. They divide repeatedly in the tangential 

 direction, and at each division the outer cell becomes part of the cork, while 

 the inner cell remains part of the phellogen, which is thus always one cell 

 thick. As there are far more tangential than radial divisions, the cork cells 

 are built up in regular rows, each row corresponding to one phellogen cell, 

 and they soon form a very impervious covering. Naturally all the tissues 

 external to the cork are cut off from their source of food supplies. They 

 wither and die and are eventually shed. We shall discuss the nature and 

 functions of cork more fully in the next chapter. . 



Modification of Roots. 



Many roots of peculiar form or function are known among Angiosperms, 

 though very few occur in lower groups. These are all derivable from the 



