MONOCOTYLEDONOUS ROOT. 



167 



or other Flag Iris, shows in its axial vascular cylinder the closest 

 resemblance to Acorns, but the endodermis is somewhat differently 

 constructed (Fig. 67). The cells of this (e) are unilaterally thick- 

 ened on their side towards the 

 interior, into the form of the letter 

 U, and the thickening is beauti- 

 fully stratified. At isolated points 

 a single unthickened cell occurs, 

 and it can be determined that, * 

 wherever present, such a cell (/) 

 always lies in front of a ray of 

 the wood. These cells we will 

 call "passage-cells"; they are 

 permeable, and maintain the con- 

 nection with the surrounding 

 cortex (c). In concentrated sul- 

 phuric acid the thickening layers 



of the endodermis swell and are FIG. 67. Part of a cross-section 

 dissolved; only the cuticularised thr ff h the root of /'/*. , 



J eudodermis ; p, pencycle ; /, passage- 

 middle lamellae, forming a deli- oells; v, bast; s, vessel in 'the wood; 



i j *u 5 j f, cortex (x240). 



cate layer around the endodermic 



cells and also around the passage-cells, remain. Similarly, the 

 middle lamellae between the vessels and in the pith are not dis- 

 solved, and form a delicate brownish-yellow network. A tangen- 

 tial longitudinal section, which skirts the endodermis, shows us 

 that the longitudinal strip of this endodermis which lies outside 

 the xylem strands consists of an alternation of long thickened cells 

 and of short unthickened passage-cells, the latter with abundant 

 cell-contents. Here and there two such passage-cells follow one 

 another. 



Where the median strip of the radial walls of the endodermis 

 is cutinised, the layer would probably entirely exclude the air 

 which is present in the cortical intercellular spaces from pene- 

 trating to the central cylinder, where, by obtaining access to 

 the cavities of the vessels, it might act as an obstacle to the 

 passage of the water ; while, on the other hand, water itself can 

 freely pass through the non-cuticularised inner and outer walls. 

 This would be of especial importance for endosmotic purposes in 

 the root-hair, or absorbing region, of the root. In older parts of 

 the root, where passage of water through the endodermis is no 

 longer of importance, the cells may be cuticularised, or thickened, 



