PROTODERMAL STEREOME 203 



organs the advantage gained by the conversion of any layer into 

 mechanical tissue increases with the distance of that layer from the 

 centre, provided of course that the transformation is not rendered inad- 

 visable by opposing physiological considerations of even greater moment. 

 The collenchymatous thickening of the epidermal walls which is seen 

 in the leaves of many Liliaceae (especially in the genus Allium), 

 the thick-walled and lignified condition of the epidermis in the 

 colourless bracts of Papyrus antiquoruru and in the glumes of some 

 species of Cyperus, and many similar phenomena, can be most readily 

 explained from this point of view. 



Eeference has been made on a previous occasion to the conversion 

 of epidermal cells into typical stereides ; this so-called transfor- 

 mation of course merely consists in the fact that the meristematic 

 layer, which in some ancestor of the plant under consideration was 

 entirely engaged in the production of epidermal cells, at the present 

 time gives rise in the same region to stereides instead of typical 

 dermal elements. The meristematic layer in question is, of course., 

 the protoderm. In the simplest type of transformation the proto- 

 dermal cells are converted straightway i.e. without any preliminary 

 tangential division into thick-walled prosenchymatous stereides. This 

 case is illustrated by the leaves of many Ferns, very excellently, for 

 example, by those of Pteris serrata, where, in the smaller veins, each 

 flange of the I-girder is seen in cross-section to consist of a single 

 " bast-fibre " of protodermal origin (Fig. 74). More often, however, 

 such protodermal stereides are merely the outermost members of a 

 typical strand or cylinder of bast ; thus, for example, in the haulms of 

 many Grasses {Bromus, Mclica, etc.). In other cases, again, the 

 protodermal cells first undergo tangential divisions ; of the resulting 

 daughter-cells only the internal ones give rise to fibrous tissue, whereas 

 the outermost become secondarily incorporated in the epidermis. 

 The small but complete protodermal procambial strands that give rise 

 to perfectly normal fibrous bundles in the leaves of certain Cyperaceae 

 (e.g. Cyperus vegetus, C. longus, C. glaber, C. glomeratus and other 

 species of Cyperus) originate in this manner. In cross-section such a 

 procambial strand may consist either of a single protodermal cell or of 

 a group of protodermal elements. [For further details the reader may 

 consult Fig. 75 A-E, and the authors dissertation on "The Ontogeny 

 of the Mechanical System."^ It is worth noting that in many cases 

 e.g. in the haulms of Papyrus antiquorum the peripheral fibrous 

 strands originate at the boundary between protoderm and fundamental 

 meristem, so that both these meristematic layers may take part in the 

 formation of the same procambial strand (Fig. 75 v,g). 



Collenchyma, like bast, may originate from protodermal cells 



