2T8 



INTKODUCTION TO BOTANY 



FIG. 217. The common scouring rush, 

 horsetail (Equisetum arvense) 



A, a plant in early spring condition ; r.s, rhizi 

 b.s, spore-bearing branch; c, collection of 



rophylls (strobilus, or cone) ; f.b, foliage branch, 

 which later expands as in B ; C, one sporophyll 

 from the cone, showing the stalk (st) and several 

 sporangia (sp). D and E, spore with elaters. 

 A and B, one half natural size ; C, magnified 

 about 20 times; D and E, greatly enlarged 



patches. In still others, as 

 the maidenhair (Adiantum) 

 and the bracken fern (Pteris 

 aquilina) and other species of 

 PteriSj the sporangia are cov- 

 ered by the folded leaf margins 

 the so-called false indusium. 

 In some, as the sensitive fern, 

 or oak fern (fig. 216), the leaf 

 is differentiated into a spore- 

 bearing branch and a chloro- 

 phyll branch. In such cases 

 the former is called the sporo- 

 phyll (spore leaf), and the 

 latter the foliage leaf, the 

 chlorophyll bearer. Setting 

 apart special structures for 

 special pieces of work (divi- 

 sion of labor), as here shown, 

 ordinarily increases the quan- 

 tity and quality of work done. 

 262. Horsetails, or scouring 

 rushes. The class of pterido- 

 phytes to which these plants 

 belong once constituted a 

 prominent part of the 

 earth's flora as tree-like 

 plants. They are now 

 represented by the sin- 

 gle genus Eguisetum. 

 Fossil remains tell in- 

 teresting stories of the 

 ancestors of these plants 

 which lived ages ago 

 when coal was being 

 formed in abundance. 



