THE GREAT GROUPS OF PTERlDOPHYTES 



161 



arrangement, or sometimes the whorled arrangement, each 

 such set of leaves being called a cycle or a whorl. These 

 leaves contain no chlorophyll and have evidently abandoned 

 chlorophyll work, which is carried on by the green stem. 

 Such leaves are known as scales, to distinguish them from 

 foliage leaves. The stem is either simple or profusely 

 branched (Fig. 135). 



80. The strobilus.— One of the distinguishing characters 

 of the group is that chlorophyll-work and spore-formation 

 are completely differentiated. Although the foliage leaves 



Fig. 136. Dioecious gametophytes of Equisetum: A, the female gametophyte, show- 

 ing branching, rhizoids. and an archegonium {ar)\ B, the male gametophyte, 

 showing several antheridia ( <5 }.— After Campbell. 



are reduced to scales, and the chlorophyll-work is done by 

 the stem, there are well-organized sporophylls. The sporo- 

 phylls are grouped close together at the end of the stem in 



a compact conical cluster which is called a strobilus, the 

 Latin name for " pine cone," which this cluster of sporo- 

 phylls resembles (Fig. 135). 



Each sporophyll consists of a stalk-like portion and a 

 shield-like {peltate) top. Beneath the shield hang the 





