FACTORS AFFECTING STRUCTURE OF ENDODERMIS 373 



While it is clear that such strengthening devices must tend to 

 increase the resistant properties of the endodermis, it it nevertheless 

 by no means easy to form a clear conception of the mechanical signi- 

 ficance of that layer. We may, however, regard it as certain, that this 

 mechanical function is in some way connected with the tissue-tensions 

 that are set up between the parenchymatous ground- tissue and the 

 central cylinder. Injury to the leptome and other vulnerable vascular 

 tissues by the resulting tensions and compressions, is prevented by the 

 interpolation of an appropriately constructed protective sheath in the 

 shape of the endodermis. " Young Iris-roots, for instance, in which 

 the tissue-tensions are never very great, possess an unthickened 

 endodermis : in older roots, on the contrary, in which the endodermis 

 is strongly thickened, very considerable tissue-tensions prevail." 



Schwendener further remarks that " climatic and edaphic conditions 

 also react upon the structure of the endodermis. Thus the endo- 

 dermis is always specially strengthened in the roots of lithophytes and 

 steppe-plants. In certain megathermic members of these ecological 

 classes {Dasylirion, Restio), and also in some of our native Ferns, which 

 grow on limestone rocks or on dry walls, the endodermis becomes 

 thickened to quite an extraordinary extent, evidently in adaptation 

 to the alternation of periods of abundant water supply with severe 

 droughts. Greatly thickened endodermal layers are also found in 

 many marsh plants which inhabit places that are liable to dry up 

 periodically. Obviously, rapid variations of turgor are just as likely 

 to occur under these conditions, as in the case of the above-mentioned 

 xerophytes. Where, on the contrary, the substratum is constantly 

 moist and soft, as in deep bogs, backwaters, lakes, etc., the endodermal 

 cells of the root do not as a rule develop any cellulose thickening 

 layers (e.g. Najas, Potamogeton, Sparganium natans, Sagittaria sagittac- 

 folia, Alisma natans, Calla palustris, etc.). In the case of plants that 

 grow in running water, however, the stem may be furnished with a 

 thick-walled endodermis even if the corresponding layer in the root is 

 unthickened." 



Very little is known concerning the factors which determine the 

 presence or absence of a typical endodermis ; why, further, in some 

 cases each vascular bundle is enveloped in a distinct protective sheath, 

 while in other instances the entire central cylinder is surrounded by a 

 common endodermis, is also quite obscure. Schwendener has shown 

 that the presence of a foliar endodermis in some tribes of the Gramineae, 

 and its absence in others, cannot be ascribed to the influence of environ- 

 ment. Thus each bundle of the leaf is furnished with a typical endo- 

 dermis in all the Festuceae, in the hygrophilous genus Glyceria, as well 

 as in the xerophilous Festucas. The presence or absence of endodermal 



