278 
Fic. 217. The common scouring rush, or 
horsetail (Hquisetum arvense) 
INTRODUCTION TO BOTANY 
patches. In still others, as 
the maidenhair (Adiantum) 
and the bracken fern (Pteris 
aquilina) and other species of 
Prcris, 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 
AR plants. They are now 
\, represented by the sin- 
gle genus Aiyisetwi. 
Fossil remains tell in- 
A, aplant in early spring condition; r.s, rhizome; teresting stories of the 
b.8, spore-bearing branch; ce, collection of spo- 
rophylls (strobilus, or cone) ; /.b, foliage branch, 
ancestors of these plants 
which later expands as in B; (', one sporophyll thie l fey o 
fromthe cone, showing the stalk (st) “ad ever which lived “eee ae 
sporangia (sp). D and EF, spore with elaters. when coal was being 
A and B, one half natural size; (, magnified : 
about 20 times; 1 and £, greatly enlarged formed in abundance. 
