300 MORPHOLOGY OF ANGIOSPERMS 



of the type just mentioned have been designated by Seott 6 

 " exarch." Hence it may be stated that the bundles of the Fern- 

 like plants are characteristically mesarch ; that the Lyeopods 

 and their allies have exarch bundles ; and that the prevailing 

 type in the Spermatophytes is the endareli bundle, the primitive 

 wood here coming next the medulla. These anatomical distinc- 

 tions, however, are less trustworthy than those depending on 

 the presence and absence of foliar gaps, for many Ferns have 

 endarch bundles, while some (Lygodium, etc. ) have even exarch 

 tracheary strands; on the other hand, Phylloglossum, a recog- 

 nized Lycopod, has distinctly mesarch cauline bundles. There 

 are no known examples, however, of siphonostelic Lyeopods 

 (Jeffrey 10 ) with foliar gaps, or of siphonostelic Ferns without 

 them. 



CTCADOFILICES 



Recently Potonie n has established a group, the Cycado- 

 filices, to include a number of fossil forms which are neither 

 true Ferns nor typical Gymnosperms, but which possess to a 

 large degree anatomical features of both alliances. These forms 

 can now be more advantageously discussed after the general 

 anatomical account presented in the foregoing paragraphs. The 

 vegetative anatomy of the Cycadofilices is of special importance, 

 both because of our entire ignorance of their reproductive or- 

 gans at the present time and because their anatomical structure 

 presents such an interesting transition from the pteridophvtic 

 to the gymnospermous type. 



Heterangium. — Fig. 100, i, taken from Scott's admirable 

 Studies in Fossil Botany, shows the structural features of the 

 stem of Heterangium Grievii, a primitive representative of the 

 Cycadofilices. The central cylinder is obviously protostelic and 

 very similar to that of Gleichenia flabeUata of Fig. 108, .1. 

 A striking difference, however, is the presence, on the outside 

 of the pithless primary wood, of a narrow zone of secondary 

 wood which is clearly distinguishable by reason of the regular 

 radial arrangement of its elements. In the cortex mav be seen 

 leaf-traces and groups of sclerotic cells. The external cortex 

 is bounded by a very characteristic hypodermal zone, which in 

 transverse section appears to be made up of alternating stripes 

 of parenchymatous ami sclerencliymatous cells. Viewed longi- 



