i6o 



THE ANATOMY OF WOODY PLANTS 



morphologically equivalent to the normally single piliferous layer 

 of terrestrial monocotyledonous roots. The cells of the velum 

 have banded thickenings in their walls which in a general way 

 simulate those found in certain types of tracheids. The elements 

 in question are also sometimes porous, so that water enters readily 



into their cavities during the rainy 

 season, to be conserved for use in the 

 following period of drought. 



In vascular plants in general the 

 secondary roots are related in a definite 

 fashion to the primary structures of 

 the main root. In the greater number 

 of cases the young root appears as a 

 local development of cells on the cen- 

 tral cylinder of the main root. This 

 cellular proliferation is opposite one of 

 the clusters of protoxylem, so that the 

 arrangement of the rootlets is predeter- 

 mined by the organization of the pri- 

 mary structures of the main root. In 

 many monocotyledons the indication of 

 a developing root appears, not oppo- 

 site one of the protoxylem groups, but 

 in the interval between two of these, 

 thus constituting a departure from the 

 usual topography. It is evident that 



the secondary roots are formed endogenously and subsequently 

 bore their way outward. This internal mode of origin is 

 characteristic of roots and rootlets, the only exception being 

 found in the case of the primary root of the seedling. Fre- 

 quently roots are definitely related to other appendages, such 

 as branches and leaves, and in these instances originate at or 

 near the node. 



All roots, with the exception of the degenerate ones found in 

 certain types of parasitism, are provided with a protective cover 

 over their tender apex, and this is known as the root cap or pile- 

 orhiza. This structure is for the purpose of protecting the root 



FIG. 116. Transverse sec- 

 tion of an orchid root, showing 

 multiple piliferous layer devel- 

 oped as a velamen. 



