112 



MINUTE STRUCTURE OF THE HOOT. 



duccd to a single duct, as iu some Carices, or there maj- 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. ff., Pon- 

 tederia) ; but usually there are mauj-, which maj' be variously 

 disposed. 



340. Bast-fibres may be associated with the cribrose-oells in 

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

 occasionallj- with some sclerotic parenchjma) in the cortex. In 

 Philodeudrcm these scattered groups of bast-Iibres 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 b}' the alteration 

 of their existing tissue 

 elements or by the in- 

 troduction of new ele- 

 ments. Some roots, 

 however, do not suffer 

 much change fiom first 

 to last. Their cells may 

 become more strongly 

 cutinized or lignified 

 as the case maj' be, 

 but no new elements 

 are brought in. This 

 is true of the i-oots of 

 man3- monocotyledons, 

 but in dicotyledons the 

 secondarj- changes are 

 generall}' verj' marked. 

 The changes maj- af- 

 fect either the I'ortex or 

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

 latter. 



Fio.93. Soctioii Hirougli tlioceiUralcyliri'lerofabinary root of avascular cryptogam 

 (Cyatbea meilullaris): <?, iniernal layer of the proper cortex; p, eiuiodermis ; m, pe- 

 ripheral layer of the cyliiuler; I, llbor fasci^jles; r, woody fascicle; c, conjunctive paren- 

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



93 



