340 



MANUAL OF BOTANY 



interior of a plant which is exposed to the air in consequence 

 of injury. The injury stimulates the exposed uninjured cells to 

 merismatic activity, and a pheUogen is formed over the v^ound. 

 Cork is formed in all stems and roots that increase in thick- 

 ness ; it occurs in most Dicotyledons and Gymnosperms ; some 



Fig. 725. 



Fiij. 726. Section of stele of root. After Kny. en. Endodermis. pp.. Peri- 

 cycle. X. Xylem bundles, the protoxylem, Xi, abutting on the perleycle. 

 ph. Phloem bundles alternating with the bundles of xylem. p. Pith, or 

 conjunctive tissue of the stele. 



Monocotyledons ; and in Marattiacea and Botryclvkiin among 

 the Cryptogams. It is not found in herbaceous plants. 



The development of cork and its effect upon the structure ot 



the stem and root in woody plants will be further discussed in 

 connection with the anatomy of these members. 



From the character of its cell-walls and the arrangement of 



its cells, the cork forms a layer impervious to the passage of 



