290 THE ANATOMY OF WOODY PLANTS 



the paleontological facts, but is likewise at variance with the 

 fundamental principles of comparative anatomy. It presents 

 the further serious difficulty of supposing that the simpler type 

 of stele characteristic of the more modern representatives of the 

 Osmundaceae, although primitive, has nevertheless come from a 

 more complicated condition in the past. Finally, the hypothesis 

 of the greater primitiveness of tubular steles without internal 

 phloem supplies no valid explanation of the frequent occurrence 

 of internal phloem, internal endodermis, and cortical sclerenchyma 

 in the medullary region of various existing species of the Osmun- 

 daceae. The view that such structures where they occur are 

 "physiological" has little to commend it and is, moreover, a 

 hypothesis which cuts both ways; for if we are to interpret cortical 

 structures occurring in the medulla as merely physiological, we 

 must likewise consider the possibility of a similar explanation 

 of the rare and quite exceptional occurrence of a so-called "mixed 

 pith," consisting partially of tracheary tissue and partially of 

 parenchyma. 



The Osmundaceae on the whole present the evidence in regard 

 to the evolution of the tubular stele in later geological times more 

 clearly than does any other group of ferns. It should, however, 

 be emphasized that in practically all cases where the siphonostelic 

 central cylinder without internal phloem or endodermis is present 

 there are found elements in the region of the medulla which 

 present the characteristics of cortical tissues. It seems illogical 

 to interpret these structures as possessing in every case merely a 

 physiological significance, particularly since, in view of their 

 imperfect and sporadic development, it is difficult to attribute 

 to them any functional importance whatever. The structures 

 in question possess, in fact, all the criteria of vestigial features 

 persisting from an earlier more complicated condition of organiza- 

 tion. Our knowledge of groups which have their climax of devel- 

 opment in the past justifies the view that evolution in decadent 

 series proceeds by simplification. A final general objection to 

 the tracheary hypothesis of the origin of the pith is the outstanding 

 fact that the more modern groups of plants, and especially the 

 seed plants, are the ones which particularly and universally represent 



