I82 



SIZE A FACTOR IN STELAR MORPHOLOGY 



[CH. 



the trace of each successive leaf, and it is important to note that its entry is 

 effected without any break of continuity of the endodermal envelope, which 

 thus forms a gas-tight barrier surrounding the whole vascular system. It is 

 found in the adult stems of Gleichenia, Lygodiimi, and Cheiropleuria, and is 

 believed to have been present in Botryopteris. It is also characteristic of the 

 Hymenophyllaceae. All of these are relatively primitive types with stems 

 of moderate size. In Botryopteris cylindrica the stele is about "5 mm., in 

 Lygodmm i mm. to 2 mm., in Cliciropleiiria about i mm. and in Trichonianes 

 scandens, one of the larger tlymenophyllaceae, it is '5 mm. in diameter. In 



Fig. 174. Transverse section of a stem oi Botryopteris cylindrica, 

 showing a protostele with a solid central core of xylem, and 

 peripheral phloem. The endodermis is not clearly shown in 

 this fossil Fern. 



Gleichenia, and probably in all the others, its form is conical at first, but 

 after reaching a certain size the stele retains that size through life, as a 

 cylinder traversing the cylindrical rhizome. The limiting factors have come 

 into play, one of which is believed to be the proportion of surface of the 

 stele to its bulk. When the stele attains larger size, as it did in certain related 

 fossils while still retaining its protostelic state, it is seen to have undergone 

 a modification of form. For instance inAnkyropteris Grayi\which is 2 — 3 mm. 

 in diameter, it is corrugated, the insertion of the leaf-traces projecting, and 

 the surfaces between them being hollowed (Fig. 175, ii, iii): moreover the 



