116 



PART II. ANATOMY AND HISTOLOGY. 



Most commonly the cells of the endodermis are thin-walled, with 

 a suberised thickening-zone extending round the lateral and upper 

 / and lower surfaces of the wall, and showing in transverse section 

 (Fig. 94) as a black dot on the radial wall. This peculiar mark- 

 ing is by no means always present : it is frequently wanting in 

 the endodermis of the stem, in which case the endodermisjsan, 

 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. 112), 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, Seqxioia, many Cruciferse such as 

 Mustard and Wallflower, Fig. 95). 



The walls of the endodermal cells frequently 

 become sclerotic either over their whole surface, 

 or more frequently on the internal lateral sur- 

 faces. W T hen this is the case, some of the cells 

 remain thin- walled, as passage-cells, opposite to 

 the wood-bundles within. 



32. The Stele. The plerome, constitut- 

 ing the young stele, always gives rise to vas- 

 cular tissue and usually to a certain amount of 

 other tissue which is termed conjunctive tissue. 

 The first indication of the development of 

 vascular tissue in the stele is afforded by the 

 differentiation of a varying amount of pro- 

 cambium, consisting of somewhat elongated 

 narrow cells formed by repeated longitudinal 

 division, which is the embryonic tissue from 

 which the vascular tissue is eventually formed. The procam- 

 bium frequently constitutes one solid central strand, surrounded 

 by more or less conjunctive tissue constituting the pericyclc; 

 this is sometimes the case in slender roots (see Fig. 94), in 

 slender monostelic stems (e.g. many aquatic Phanerogams, such 

 as Callitriche, Elodea, etc.; among Pteridophyta, Isoetes, Sal- 

 viniacese, Lycopodiaceae, Hymenophyllum, Schiztea), and gene- 

 rally in the steles of polystelic stems. More commonly, however, 

 the procambium of stout roots and monostelic stems is developed 

 as a number of strands variously arranged in the stele, generally 



Pio. 96.-A cell 



cortex of tbe root 

 of the Mustard, seen 

 obliquely from tho 

 internal surface, 

 showing the su- 

 berised thickening 

 zone. (After van 

 Ticghcm: x 350.) 



