54 ° Bancroft.—A Contribution to our 
In some cases, sieve-tubes are not present, or the ring may be incomplete ; 
in these cases the phloem consists of a uniform zone of smaller, narrower 
cells. 1 On the whole, the ring of sieve-tubes is less marked in /3 type than 
in a type stems. 
In some well-preserved examples a clearly-defined continuous layer of 
cells external to the phloem may be considered as representing the peri- 
cycle 2 (PL XXVII, Fig. 4); it is, however, frequently impossible to refer 
the outer tissues of the stele to a definite series (Text-fig. 5). 
Text-fig. 5. A portion of the stele of an a stem, showing the xylem ( x ) } xylem parenchyma 
(x.flar.), sieve-tubes (st.), phloem parenchyma ( ph.par '.). The outermost layer of cells (u.) belongs 
to the inner cortex, having the typical radially-compressed form. Endodermis and pericycle do not 
constitute distinct layers in this specimen, x 400. (From slide K 20 k, University College, London.) 
There is some doubt as to the existence of a true endodermis in stems 
of R. cylindrical The stele is usually limited by a somewhat irregular 
layer of dark-coloured cells, fairly thin-walled, and elongated both tan¬ 
gentially and vertically. This layer, particularly in a stems, sometimes has 
1 Hick (’ 96 ), p. 7, notes the apparent lack of differentiation of the phloem cells, but mentions 
that elements resembling sieve-tubes occur in specimen Q 104. 
Tansley (’ 08 ), p. 14, Fig. 4, shows sieve-tubes in specimen K 14, University College collection. 
In Williamson’s figure (’ 78 , PI. 24, Fig. 80) there are no sieve-tubes; see p. 350. 
2 Hick (’ 96 ), p. 7. 3 lb., p. 6. 
