338 PLANT STUDIES 
turn, meaning " horsetail "), but they are but the linger- 
ing remnants of an abundant flora which lived in the time 
of the Coal-measures, and helped to form the forest vegeta- 
tion. The living forms are small and inconspicuous, but 
very characteristic in appearance. They grow in moist or 
dry places, sometimes in great abundance (Fig. 305). 
The stem is slender and conspicuously jointed, the joints 
separating easily ; it is also green, and fluted with small 
longitudinal ridges ; and there is such an abundant deposit 
of silica in the epidermis that the plants feel rough. This 
last property suggested its former use in scouring, and its 
name " scouring rush." At each joint is a sheath of minute 
leaves, more or less coalesced, the individual leaves some- 
times being indicated only by minute teeth. This arrange- 
ment of leaves in a circle about the joint is called the cyclic 
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 aerial stem (really a branch) is either 
simple or profusely branched (Fig. 305). In the species 
illustrated the early aerial branches are simple, usually not 
green, and bear the strobili ; while the later branches are 
sterile, profusely branched, and green. 
220. The strobilus. One of the distinguishing charac- 
ters of the group is that chlorophyll-work and spore-forma- 
tion are completely differentiated. Although the foliage 
leaves are reduced to scales, and the chlorophyll-work is 
done by the stem, there are well-organized sporophylls. 
The sporophylls 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 clus- 
ter of sporophylls resembles (Fig. 305). 
Each sporophyll consists of a stalk-like portion and a 
shield-like (peltate) top. Beneath the shield hang the 
