HYPODERM: ENDODERMTS. 



H3 



walls exhibit pad-like projecting thickenings deposited in the angles of the cells, 

 which swell up strongly in water, and still more in dilute potash solution, and 

 give to the transverse section an uncommonly characteristic appearance. In the 

 leaves of Conifers, Cycads, and in many other cases, the collenchyma is replaced by 

 very thick-walled, prismatic, and even fibrous 

 cells, in part lignified; these, as in Figs. 149 

 and 151, run close beneath the epidermis, and 

 are grouped together in strands or layers. 

 These, and similar so-called Hypodermal 

 structures, serve to render the organs con- 

 cerned elastic and solid ; since, by means of 

 their own stiffness, they increase, or, in 

 the case of collenchyma, aid the elastic re- 

 sistance which the epidermis opposes to the 

 expansion of the succulent, turgid paren- 

 chyma. 



Where the fundamental tissue bounds 

 the vascular bundles, there are always formed 

 layers, commonly sharply marked, which may 

 be distinguished generally as vascular bundle- 

 sheaths; these however, like the hypodermal 

 layers, are, according to circumstances, of very 

 different constitution. The commonest form 



of these vascular bundle-sheaths is usefully distinguished by De Bary as Endo- 

 dermis. It is particularly well marked in the roots of all vascular plants, at the 

 circumference of the axial strand, in the form of a simple layer of small cells, 

 the walls of which are more or less suberized, cf. Fig. 8 s. In the shoot-axis 



Fig. 150.— Epidermis (f) and Collenchyma \cl) from the 

 petiole of Hegoiiia. The outer walls of the epidermis cells 

 are equally thickened ; where they adjoin the collenchyma 

 the thickening only occurs at the angles where three cells 

 meet. These thickenings have a great capacity for swell- 

 ing, clil chlorophyll grains; / parenchyma.cell. 



Fig. 151.— Transverse section of the leaf of 

 Pinus Pinaster ( X 3o)- ' epidermis ; es hypo- 

 dermal prosenchyma ; sp stomata ; h resin-pas- 

 sages ; / parenchyma containing chlorophyll; g, * 

 colourless internal tissue e"'''"''""" * — *^' — ' 

 vascular bundles. 



two fibro- 



FIG. 152.— The lefl-hand comer of the previous 

 figure (X 500). c cuticularlsed layer of the epidermal 

 cells ; i inner layer of the same, not cuticularlsed ; 

 f' very strongly-thickened outer wall of the epidermis 

 cell lying in the comer ; g, i' hypoderm cells : g- 

 middle lamella; i' stratified thickening mass; p 

 parenchyma containing chlorophyll ; fr contracted 



also a very similar endodermis is frequently found, enveloping all the leaf-traces 

 as a hollow cylinder, and therefore appearing on the transverse section as a 

 continuous ring, by which the whole tissue is separated into a cortex, lying outside 

 the endodermis, and an internal core of tissue (pleromc), c*^. Fig. 138. Such 



