166 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[ 32. 



FIG. 128. A cell 

 from the sub-endo- 

 dermal layer of tho 

 root of the Mustard, 

 seen obliquely from 

 the internal surface, 

 showing the su- 

 berised thickening 

 zone. (After van 

 Tieghem: x 350). 



which case the endodermis can, in many cases, be distinguished 



by the presence of starch-grains in its cells. 



When the endodermis is double, this marking is 



confined to the outer of the two layers. 



This marking is not confined to the endodermis ; 



it sometimes occurs also in the exodermis of 



roots (see p. 161), and in one or more layers of 



the internal cortex in some roots (one layer, next 



the endodermis, in Cupressus, Taxus, Prunus, 



Rosa, Lonicera, etc. ; several layers, Juniperus, 



Sequoia, many Crucifera3 such as Mustard and 



Wallflower). 



The walls of the endodermal cells frequently 



become sclerotic either over their whole surface, 



or more frequently on the internal and lateral 



surfaces (Fig. 129). When this is the case, some 



of the cells remain thin-walled, as passage-cells, 



opposite to the wood- bundles within. 



The Intra-stelar Fundamental Tissue, or Conjunctive Tissue of the 



Stele. MorpJtology. The morphology of the conjunctive tissue, 



of the stele varies somewhat in 

 accordance with the develop- 

 ment of the vascular tissue. 

 When a solid vascular cylinder 

 is produced, there may be no 

 conjunctive tissue at all, the 

 whole of the plerome having 

 developed into vascular tissue ; 

 or the conjunctive tissue may 

 be limited to one or more peri- 

 pheral layers, the pericycle, in- 

 vesting the vascular cylinder; 

 or, again, in addition to the 

 peri cycle, the conjunctive tissue 

 may extend inwards to some 

 extent between the bundles (in- 

 terfascicular) of the stele ; on the 

 other hand, when the vascular 

 cylinder is hollow (see Fig. 

 130), the central space is oc- 

 cupied by medullary conjunctive tissue, constituting the pith, and 



FIG. 129. Part of a transverse section of 

 a root of Iris florentt.na ; e sclerotic endo- 

 dermis, with / a thin-walled passage-cell ; 

 v bast; s wood-vessel; c cortical ground- 

 tissue; p pericycle. (After Strasburger : 

 x 2iO.) 



