ENDODERMIS 360 



primary layer being overlain by a suberin lamella, which in its turn is 

 covered by a layer of cellulose ; it is the last-mentioned layer that 

 may become thickened for mechanical purposes, in which event it 

 frequently exhibits the reactions of lignified membranes. This form of 

 endodermal cell is especially prevalent among Monocotyledons. Another 

 type of endodermis is characterised by the fact that its cells always 

 remain thin-walled, the tangential walls being altogether unsuberised, 

 while the radial septa are furnished with a so-called Casparian strip. 

 This Casparian strip is a peculiarly modified narrow zone of the radial 

 [and transverse] endodermal walls ; more rarely a wider strip, or even the 

 entire radial wall, is specialised in the manner about to be described. 

 Microscopic examination of an endodermal cell of this second type 

 shows that the region of the wall corresponding to the Casparian strip 

 is thrown into folds ; in a transverse section this corrugated area 

 appears as a dark patch or shadow. In the strip the primary layer of 

 the wall is chemically modified ; according to a widely accepted view, 

 it is suberised, like the suberin lamella in an endodermal cell of the 

 first-mentioned type. The recent investigations of Kroemer, however, 

 have rendered it very doubtful, whether the Casparian strip can be 

 properly described as suberised. Nevertheless, as the effect of the 

 chemical modification of the strip is such as to diminish its permeability, 

 the current designation will be retained in the present treatise. This 

 conservative attitude is further justified by the fact that the term 

 " suberisation " is still used as a collective name for a number of distinct, 

 though doubtless closely related, chemical modifications involving certain 

 characteristic alterations of the physical properties of the cell-wall. 



According to Schwendener, the Casparian strip, as seen in a section, 

 is not a natural feature of the endodermal cell, but is directly due to 

 the mode of preparation. As he points out, the living endodermal cells 

 are in a state of tension, either as a result of their own turgescence, or 

 owing to the pressure exerted by adjoining cells. On cutting a section 

 the turgor falls, and the tension in the walls is relieved. But those parts 

 of the membrane which consist of unaltered cellulose, contract much more 

 strongly than the less extensible suberised strips ; the latter consequently 

 become thrown into folds. Van Wisselingh accepts this explanation for a 

 number of cases, but maintains that in other instances the corrugation 

 of the Casparian strip has nothing to do with the mode of preparation 

 and pre-exists in the intact organ. In the latter event the folding 

 must arise in the course of development, owing to the fact that 

 the suberised strips undergo more active growth in surface than the 

 rest of the wall. 



The suberised condition of the cell membrane renders the endo- 

 dermis comparatively impervious. This physiologically important fact 



2A 



