380 



MA>:UAL OF BOTANY 



In Monocotyledons and Vascular Cryptogams no cambium 

 layer is developed in the roots, which consequently increase in 

 thickness only very slightly. 



Coincidently with the development of the secondary vascular 

 tissue in the stele, a phellogenlayer arises, usually in the pericycle, 

 though sometimes in the cortex, which forms cork externally 



Fie. 767. 



-p.ph. 



Fig. 767. Section of root showing secondary thiclcening. eii. Endodermis. 

 p.x. Primary xylem bundles. p.p}t. Primary pliloem bundle, s.37. Masses 

 of secondary xylem formed opposite to the latter, c. Cambium. Opposite 

 to the primary xylem the cambium has only formed parenchyma, consti- 

 tuting large secondary medullary rays. After Kny. 



and phelloderm on its inner face, as in the stem. The tissues 

 external to this periderm formation dry up and are thrown off, 

 so that the exterior of the root is covered by a layer of cork, 

 usually of stelar origin. The phellogen continues active for 

 some time, and then becomes either cork or permanent paren- 

 chyma. A new phellogen subsequently arises in the new-formed 



