PHYLOGENY 



Figure 41. Carboniferous Flora. (Courtesy of the Chicago Natural History Museum.) 



the tips of branches rather than as emergences from the stem. The leaves 

 are not adequate for the photosynthetic needs of the plant, and the entire 

 stem has retained this function. 



The origin of the horsetails from psilophytes is strongly indicated not 

 only by the similarity of shoots and leaves but by the fact that Hijcnia 

 is an excellent intermediate between the two groups and might with equal 

 justification be assigned to either. From the Hyenia stock there were de- 

 veloped herbaceous species, comparable to the horsetails of today, and 

 gigantic, tree-like species as much as forty feet tall. These flourished dur- 

 ing the Carboniferous, and they form an important part of the coal beds 

 (Figure 41). They became extinct during tlie Triassic, the first period of 

 the Mesozoic Era. The modern horsetails comprise a single genus, and it 

 is uncertain whether they are descended from the herbaceous species of 

 Paleozoic times, or whether they have been derived from the horsetail 

 trees bv reduction. 



SUBPHYLUM PTEROPSIDA 



Ferns. Th(> last of the subphvla of the phvlum Tracheophyta is the 

 Pteropsida, comprising ferns, conifers, and flowering plants. Like club 

 mosses and horsetails, the ferns— class Filicineae— appear to have devel- 

 oped from psilophvte ancestors of Devonian age. Some of the psilophytes, 

 for example V HCU(\oH])ovochnnH and Proloplcridiiiin, show a tendencv to- 

 ward leaf formation by llattening of whole branches, and it is probable 



120 



