338 PLANT STUDIES 



tum^ 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 wJiorled arrangement, each 

 such set of leaves being called a cycle or a icliorl. 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 



