HORSETAILS AND CLUB-MOSSES 



199 



pact group of modified leaves bearing sporangia. Just 

 as in some ferns certain leaves are set apart to do chloro- 

 phyll work and others 

 to bear sporangia, so in 

 the Equisetum the same 

 division of work oc- 

 curs; but the notable 

 thing is that the spo- 

 rangium-bearing leaves 

 are massed together in 

 a cluster that is quite 

 distinct from the rest of 

 the plant. Leaves set 

 apart for bearing spo- 

 rangia are called spo- 

 rophylls, which means 

 "spore leaves.'' A 

 strobilus, therefore, is 

 a group of sporophylls 

 that form a more or 

 less distinct cluster, dis- 

 tinct from the rest of 

 the plant. 



In Equisetum each 

 sporophyll consists of 

 a stalk-like portion and 

 a shield-like top, be- 

 neath which the several 

 sporangia hang (Fig. 

 192, A). The spores 

 have a very peculiar 

 outer wall. It consists 

 of two spiral bands 



wound about the spore and fastened to it only at the point 

 where they intersect (Fig. 192, ). When dry, the bands 

 14 



FIG. 191. Later sterile shoots of the species 

 shown in Fig. 190, and photographed a 

 month later. 



