338 



MANUAL OF BOTANY 



stele. As we shall see later, the leaves are derived from the 

 periblem and dermatogen of the stem, the plerome taking no part 

 in their formation. We find that parts of the periblem give 

 rise in this way to vasciilar tissue, though the chief place of 

 origin of the latter is the plerome. The vascular strands con- 

 tinuous with those of the leaf are known as leaf -trace bundles. 



In rare cases certain strands of vascular tissue are found in 

 the cortex, developed longitudinally down the stem for some 

 distance parallel to the 'stele. These are generally leaf-trace 

 bundles, which ultimately curve and join the bundles of the stele. 



The innermost layer of the cortex is the endodermis. It 

 forms a band of peculiar character round the stele, or round 

 each stele if there is more than one. The cells composing this 

 layer are sometimes thickened uniformly, or on the side facing 

 the stele and on the lateral walls (fig. 721) : they are more 



Fig. 721. 



Fig. 722. 



end. end. 



Fij. 721. Endodermis and pericycle of root of Iris. end. Endodermie. 



p.c. Passage cell. p.e. Pericycle. Fig. 722. Endodermis of root with 



cogged thickening. 



commonly thin-walled with a peculiar ladder-like thickening 

 band extending round their radial and upper and lower surfaces. 

 This in section gives the appearance of fig. 722, end. In 

 other cases, particularly in stems, the endodermis is only recog- 

 nisable with difficulty. 



In rare cases, as in the roots of Eqiiisetuiii and the stems 

 of certain Ferns, the endodermis consists of two layers. 



In the cortex of all those stems which grow in thickness 

 a further modification of ground tissue is developed. This is 

 the tissue knovsm as corlc. It is not confined to the cortex, 

 though the latter is the chief seat of its formation. It may be 

 formed also in the cells of the epidermis or in some part of 

 the tissue of the stele. In most cases it arises from the outer- 

 most layer of the hypoderma becoming merismatic, and dividing 

 repeatedly by tangential walls, forming a layer, often several 



