112 



MINUTE STKUUTUKE OF THE HOOT. 



duced to a single duct, as in some Carices, or there may be a 

 large duct surrounded by smaller ones with or without inter- 

 vening cells, or many large and small ducts variously conjoined. 

 Moreover, there are all degrees of compactness in the union of 

 the different bundles of woody tissue with each other. 



339. The cribrose part of the bundle may be reduced to a 

 single cribrose tube (e.g., Anacharis), or two or three (e. g., Pon- 

 tederia) ; but usually there are man}', which may be variously 

 disposed. 



340. Bast-fibres ma}- be associated with the cribrose-cells in 

 the primary structure of the root, and they may be scattered (and 

 occasionally with some sclerotic parenctyma) in the cortex. In 

 Philodendron these scattered groups of bast-fibres frequently 

 contain oleo-resin canals. 



\ 



SECONDARY STRUCTURE. 



341 . The older parts of roots, even the recently formed por- 

 tions lying just back of the root-hairs, may undergo changes 



either by the alteration 

 of their existing tissue 

 elements or by the in- 

 troduction of new ele- 

 ments. Some roots, 

 however, do not suffer 

 much change from first 

 to last. Their cells may 

 become more strongly 

 cutinized or lignified 

 as the case may be, 

 but no new elements 

 are brought in. This 

 is true of the roots of 

 man}' monocotyledons, 

 but in dicotyledons the 

 secondary changes are 

 generall}' very marked. 

 The changes may af- 

 fect either the cortex or 



the central cylinder ; in some cases the former more than the 

 latter. 



FIG. 93. Section through the central cylinder of ablnary root of a vascular cryptogam 

 (Cyatheamedullaris): e, internal layer of the proper cortex; p, endcHlermis; m, 1*- 

 ripheral layer of the cylinder; /, liber fascicles; r, woody fascicle; c, conjunctive paren- 

 chyma (pith and medullary rays). (Van Tieghem ) 



