224 



PLANT MORPHOLOGY 



phyll, or at least potentially so, but in most species just the upper leaves 

 bear sporangia, the lower leaves being sterile and functioning merely as 

 foliage leaves (Figs. 179 and 180). An aggregation of sporophylls is 

 called a cone or strohilus. The sporophylls may be loosely arranged but, 

 more commonly, are compactly organized. They may be similar to the 



Fig. 180. Lycopodiiim obscurum, a conimou ipecacs occurring throughout the northeastern 

 United States, about one-half natural size. Upright branches bearing terminal cones arise 

 from long trailing stems. 



foliage leaves but generally are smaller, of a different form, and without 

 chlorophyll. 



Because of this variation among the species of Lycopodium, it is not 

 difficult to arrange a series representing progressive stages in the differen- 

 tiation of the sporophylls and organization of a strobilus. It seems rea- 

 sonable to suppose that such a series represents the course of evolution 

 that the more complex species have followed. This is confirmed by other 

 characters. Thus most of the species without a definite strobilus have 



