164 



XII. STRUCTURE OF ROOTS. 



central, and perhaps also the adjoining, vessels are thin- walled, 

 and not fully developed. Adjoining the one or more central 

 vessels are almost always six narrower scalariform vessels (so*) ; 

 to each of the last succeed a group of quite narrow spiral and 

 annular vessels, or vascular trachei'des, which constitute the 

 protoxylem (sp, sp + a). The size of these elements, therefore, 

 diminishes from within outwards, and it is the spiral and annular 

 protoxylem which lies outermost ; an exactly opposite condition to 

 that in the stem, and which has arisen from the twisting of the 



FIG. 65. Cross-section of the base, of a strong adventitious root of Alliuni Cepa* 

 c , cortex ; e, elidoderrnis ; p, pericycle ; , annular vessels ; sp, sp*, spiral vessels ; 

 g c and sc*, scalariform vessels ; v, bast (x 240). 



xylem bundle through 180 degrees. The xylem strings are in 

 this case united into a six-rayed star, and the central cylinder is 

 therefore described as " hexarch ". The bast strings (v) alternate 

 with the xylem, a relation which is universal for the central cy- 

 linders of roots. Wood and bast are separated from one another 

 by a layer of parenchymatous ground-tissue cells the conjunc- 

 tive tissue. The bast portions can be recognised by the white 

 shining walls of their cells ; they consist of some sieve-tubes 

 and companion-cells, which latter are, however, not to be dis- 



