372 CONDUCTING SYSTEM 



pressure into the central cylinder of a severed root, it does not escape 

 from incisions made in the cortical parenchyma, near the root-tip, so 

 long as the endodermis remains intact. The living protoplasts of the 

 endodermal cells allow no water to pass, because their turgor-pressure is 

 much greater than the exudation-pressure in the vessels and tracheides, 

 while the strips of suberised membrane form a continuous barrier 

 which effectually prevents any movement of water through the substance 

 of the radial walls of the endodermis. The same argument naturally 

 applies in the case of the dissolved substances that travel in the 

 vascular bundles. These are likewise confined to their proper channels, 

 partly by the plasmatic membranes of the endodermal protoplasts, and 

 partly by the suberised strips of the radial walls. An endodermal 

 layer can therefore prevent the escape of all the substances that are 

 transported in the vascular bundles, even though its walls are only 

 partially suberised. 



It should be remarked that mere suberisation of the walls does not 

 increase the mechanical strength of the endodermis to any great extent. 

 This result can only be achieved by pronounced thickening of the 

 walls, such as takes place more particularly among Monocotyledons. 

 The endodermal walls may be equally thickened all round (Eussow's 

 " O-type "), or the thickening may be confined to the radial and the 

 inner tangential walls (Iiussow's " C-type "). As regards such points 

 of detail, however, which are in any case not of great mechanical 

 importance, differences may occur even within a single genus (cf. 

 Carex, Smilax, Ruscus, Potamogcton). 



Very frecpuently the mechanical efficiency of the endodermis is 

 enhanced by the development of thickened walls in neighbouring 

 layers. Among Ferns, indeed, the endodermal cells themselves remain 

 permanently thin-walled, and the thickening is entirely confined to 

 the adjoining layer of the cortex. In certain roots {e.g. those of 

 faxns, the Cupressineae, Viburnum, some Pomaceae, etc.), the cortical 

 layers immediately outside the endodermis are furnished with thicken- 

 ing ridges forming a close and continuous mesh work, the mechanical 

 effect of which is similar to that of the above-described suberised mesh- 

 work of the Casparian strips. These accessory mechanical sheaths 

 constitute the "^-endodermis" of Eussow, so-called like his other types 

 on account of the appearance which the thickened walls present in 

 transverse section. In the roots of Laukaceae, the endodermis is locally 

 strengthened by semilunar fibrous sheaths associated with the leptome- 

 strands. The endodermis itself is not infrequently more strongly 

 developed opposite the delicate leptome-groups, its cells tending to 

 become radially elongated at these points (e.g. in the Iridaceae and 

 in the aerial roots of Orchids). 



