THE STELA R THEORY. 147 



The conception is governed throughout by the idea of 

 the endodermis as a definite morphological layer, always 

 separating" stelar from extra-stelar tissue. And the en- 

 dodermis is to be recognised by the suberised thicken- 

 ings on its radial walls. It is simply by the disposition of 

 layers of cells so thickened that we are supposed to be 

 able to distinguish the various arrangements described. 

 It is easy to show that this criterion is quite illegitimate. 

 The term endodermis is defined by Van Tieghem as the 

 innermost layer of the cortex which " offre frequemment " 

 the special character in question (10, pp. 738-9). Not 

 only, however, do cell layers with the same character 

 occur in quite other situations {e.g., in the middle of many 

 periderms), but the innermost layer of the cortex certainly 

 does not always possess it. So that these thickenings 

 cannot be used to mark a layer of invariable morpho- 

 logical value. And even in Equisetum, Van Tieghem does 

 not keep to his own criterion. For when the " astelic 

 gamodesmic " passes to the apparently monostelic con- 

 dition we are told that the inner endodermis is still 

 present though its special characters have disappeared. 

 But, we may well ask, if such great importance is to be 

 attached to these special characters as to justify us in 

 founding new types of structure simply upon the disposition 

 of the layers exhibiting them, why should we be suddenly 

 asked to recognise as equivalent a layer which does not 

 exhibit them ? The criterion becomes completely chimeri- 

 cal. 



Strasburger (15) has pointed out that an endodermoid 

 layer is an air-tight barrier which does not prevent the 

 passage of water through its cells. Such a layer is found 

 in a position to shut off the water-conducting system of a 

 plant from its air-containing lacunar system, but this posi- 

 tion may vary within the same genus [Ranunculus, 

 Equisetum), and has no necessary connection with any 

 morphological region. As a matter of fact it is most often 

 formed from the inner layer of the cortex, but may be 

 developed from conjunctive tissue, or even (leaf of Isoetes) 

 from intra-fascicular parenchyma. Since the innermost 



