23 r. 



THE AMERICAN NATURALIST [Vol. XLIII 



or Araucarineae, and have not even the slightest affinity 

 with the living genus, which externally they so strikingly 

 simulate. The reference of the fossil genns just de- 

 scribed to its true affinities as well as similar results in 

 the case of a large number of other Mesozoic conifers, 

 likewise erroneously placed in the system, leads to im- 

 portant general conclusions in regard to the evolutionary 

 history of coniferous gymnosperms, which are too lengthy 

 and too technical even to be mentioned here. 



But it is not only in connection with extinct plants that 

 anatomy has shown itself the useful servant of phylogeny. 

 The enforced use of anatomical criteria in the case of 

 fossil forms, where such evidence is absolutely indispen- 

 sable, has resulted in a new and broader point of view in 

 general botanical morphology. Within the decade we 

 have begun to realize fully the great constancy of fibro- 

 vascular structures. This may perhaps be best exempli- 

 fied by a special case. Superficially there is no organ of 

 the plant more prone to vary extremely, within near 

 lines of affinity, than the leaf. If, however, we look 

 within, it presents anatomical features of great con- 

 stancy. In the case of the leaf, perhaps the most hope- 

 lessly variable feature is its size. Anatomically, how- 

 ever, there are just two sizes of leaves, large leaves 

 (megaphylls) and small leaves ( micro phylls), which are 

 absolutely characterized by their anatomical relations. 

 The foliar strands of the megaphyll, or large leaf, pass 

 off from the woody cylinder of the stem, leaving corre- 

 sponding gaps in its wall. Those of the microphyll, or 

 small leaf, equally constantly leave no such gaps in their 

 exit from the woody cylinder. It is even possible to 

 divide the whole vascular series into two clean-cut phyla 

 on the basis of the anatomical features of leaf size, viz., 

 the Pteropsida with, anatomically speaking, large leaves, 

 including the ferns, gymnosperms and angiosperms and 

 the Lycopsida, structurally speaking, small leaved forms 

 including among living plants only the club-mosses and 

 horsetails. 



