256 



THE ANATOMY OF WOODY PLANTS 



among the tracheids, particularly toward the central region of 

 the stele. This peculiar organization of the median area of the 

 stele in protostelic lepidodendrids is responsible for a hypothesis 

 as to the origin of the medulla or pith. Quite generally it is con- 

 sidered that by continuing the process of transformation of 

 tracheids, first into short tracheary elements and then into paren- 

 chymatous cells, there is formed in the center of the stele a pith 



of stelar origin. In 

 accordance with this 

 view the central re- 

 gion of the stele in 

 many protostelic 

 lepidodendrids is 

 called a "partial 

 pith." There is no 

 conclusive evidence, 

 however, that the so- 

 called " partial pith " 

 in reality gives rise 

 to the true medulla 

 in those lepidoden- 

 droid types which 

 possess it. More- 

 over, the evidence in 

 the case of the 

 Pteropsida, which are very much better displayed in the period 

 of time which we are able to investigate, is distinctly against the 

 validity of the stelar origin of the pith, since the medulla in the 

 large-leaved vascular cryptogams shows very marked indications 

 of derivation from the fundamental system. It seems on the 

 whole more likely that the medulla in the lepidodendrids, where 

 such a structure is found, is an inclusion of fundamental tissues 

 on the part of the stele. This conclusion is particularly favored 

 by conditions found in lepidodendroid stems in which there is no 

 indication of secondary growth, as, for example, in Lepidodendron 

 Spenceri, shown in Figs. 183 and 184. Here the medulla is 

 largely occupied by dark-brown sclerenchymatous tissues similar 



FIG. 183. Transverse section of the stem of Lepido- 

 dendron Spenceri. 



