THE PRIMARY PERMANENT TISSUES 35 



them is intended to reduce permeability between primary cortex 

 and stele; and this conception is strengthened by the occurrence, in 

 such an endodermis, of thin-walled cells just in front of the xylem 

 portion of roots where water absorbed from the soil has need of 

 access to the water tubes (Fig. i6). In old portions of roots it 

 often happens that the outer parts of the primary cortex slough 

 away, leaving the endodermis to protect the stele. 



The Pericycle. — The pericycle lies between the starch sheath 

 or endodermis and the outer rim of the phloem part of the vas- 

 cular bundles (Fig. 14). In stems we commonly find it composed 

 of two kinds of tissues, thin-walled parenchyma and bast fibers, 

 the origin of which from the ground meristem is as stated for the 

 corresponding tissues in the primary cortex. The bast fibers 



A B C 



Fic. 17. — Diagram to show different plans in the distribution of bast fibers. A, bast a 

 •continuous cylind-;r in the pericycle; B, isolated strands of bast in the cortex and in the 

 pericycle in fron of each vascular bundle; C, a combination of A and B. (After Green.) 



may form a continuous zone all around the stem, or they may 

 occur as isolated groups, either associated with, and seemingly a 

 part of 'he phloem of the bundles, or dissociated from the phloem 

 (Fig. ^7). In the stems of most dicotyledonous plants the bast 

 fibers are restricted to the pericycle. They serve, of course, for 

 givin*., strength; and unless, or until, the cambium later adds a 

 substantial amount of wood fibers or fiber tracheids they remain 

 the chief reliance in this respect. 



The thin-walled parenchyma cells of the pericycle, like those 

 of the primary cortex, often contain chloroplasts, and they serve 

 for the slow conduction and storage of reserve foods, particularly 

 of the non-nitrogenous class. 



In roots the pericycle occurs usually as a single layer of 



