INTRODUCTION 



many cases they consist of two unequal lobes, 

 folded together face to face, with the marc(in 



2. 



either entire or toothed. The arrangement of 

 leaves on the stem may be succubous, or disposed 

 in a spiral which turns from left to right, so 

 that the anterior border of each inferior leaf 

 is covered by the posterior border of that im- 

 mediately above Or the arrangement may be 

 incubous, in which case the spiral turns from 

 right to left, and the anterior border of each 

 inferior leaf covers the posterior border of the leaf 

 placed immediately above it. If one of these stems 



is examined 

 carefully on 

 both sides with 

 a lens, it will be 

 observed that 

 theydiffermuch 

 in appearance, and especially that the under, or ven- 

 tral, surface exhibits a series of smaller leaves, some- 



