PAST HISTORIES OF PLANT FAMILIES 153 



CHAPTER XVI 



PAST HISTORIES OF PLANT FAMILIES 

 IX. Sphenophyllales 



The group to which Sphenophyllum belongs is of 

 considerable interest and importance, and is, further, one 

 of those extinct families whose very existence would 

 never have been suspected had it not been discovered 

 by fossil botanists. Not only is the family as a whole 

 extinct, it also shows features in its anatomy which are 

 not to be paralleled among living stems. Sphenophyllum 

 became extinct in the Palaeozoic period, but its interest 

 is very real and living to-day, and in the peculiar features 

 of its structure we see the first clue that suggests a 

 common ancestor for the still living groups of Lycopods 

 and Equisetaceae, which now stand so isolated and far 

 apart. 



Before, however, we can consider the affinities of 

 the group, we must describe the structure of a typical 

 plant belonging to it. The genus Sphenophyllum in- 

 cludes several species (for which there are no common 

 English names, as they are only known to science) whose 

 differences are of less importance than their points of 

 similarity, so that one species only, ,5*. plurifoliatum, 

 will be described. 



We have a general knowledge of the external ap- 

 pearance of Sphenophylhim from the numerous im- 

 pressions of leaves attached to twigs which are found 

 in the rocks of the Carboniferous period. These im- 

 pressions present a good deal of variety, but all have 

 rather delicate stems with whorls of leaves attached at 

 regular intervals. The specimens are generally easy to 

 recognize from the shape of the leaves, which are like 

 broad wedges attached at the point (see fig. 112). In 

 some cases the leaves are more finely divided and less 

 fanlike, and it may even happen that on the same branch 



