THE ROOT 



159 



certain that the presence of a pith in roots is not a primitive feature, 

 but one which has been secondarily acquired. 



In the mass of mono- 

 cotyledonous roots there 

 is very little departure 

 from the state of affairs 

 found in the illustrations 

 shown above. At most, 

 the modification in the 

 central cylinder consists 

 of variations in the num- 

 ber of groups of xylem 

 and phloem. An interest- 

 ing complication of the 

 piliferous or root-hair- 

 forming layer is fre- 

 quently present in certain 

 orchids and aroids. Here 

 the normally uniseriate 

 piliferous layer becomes 

 multiplied, giving rise to 

 a white spongy substance 

 covering the surface of 

 the root and known as 

 the velamen or veil. This 

 structure is peculiarly 

 characteristic of the aerial 

 roots of tropical epiphytes 

 belonging to the two 

 orders mentioned above, 

 although sometimes found 



in a condition of imper- 



, j , . FIG. 115. Part of transverse section of root 



feet development in oi Smilax herbacea , 



terrestrial orchids of tem- 

 perate climates. The general relations presented by the velamen 

 are shown in Fig. 116. Here it is clear that the structure under 

 consideration lies external to the exodermis and is consequently 



