7 8 



DICOTYLEDONOUS STEM 



vascular bundle, and so presents a sinuous outline. Such a 

 starch-sheath is not uncommon in herbaceous Dicotyledonous 

 stems (cf. also Fig. 57, S.s.), and its readily movable starch-grains 

 have been regarded as fulfilling a similar function in perceiving 

 geotropic stimuli as that attributed to the starch-grains in the 

 cells of the root-tip (cf. p. 75). The central cylinder, which is 

 separated off from the cortex by the starch-sheath, is termed 



5- 



c.c. 



Pt: 



FIG. 35. A single vascular bundle from the stem of the Sunflower (Heli- 

 anlhus) in transverse section, c.c., companion cells (shaded) ;/., fibres 

 of the pericycle ; M.xy., metaxylem ; p., parenchyma of the rays; 

 ph.p., phloem-parenchyma ; Pt.xy., protoxylem ; s.p., sieve-plate. 



the stele, and comprises the vascular bundles and the accom- 

 .panying ground tissue. 



The zone of tissue between the starch-sheath and the phloem 

 constitutes the pericycle, which, in the Sunflower, is composed of 

 groups of fibres (as yet not fully thickened) opposite the bundles 

 (Fig. 35, /. ; Fig. 57, F), and elsewhere of parenchyma (Fig. 

 34). In other plants the pericycle is often parenchymatous 

 throughout, consisting of one or more layers of cells. The term, 



