g;;33 PLANT STUDIES 



tinn^ meaning "horsetail "), but they are 1)ut the linger- 

 ing remnants of an abundant flora which lived in the time 

 of tlie Coal-measures, and helped to form the forest vegeta- 

 tion. The living forms are small and inconspicuous, but 

 very characteristic in ajipcarance. Tliey grow in moist or 

 dry places, sometimes in great abundance (Fig. o05). 



The stem is slender and conspicuouslv jointed, tlie joints 

 separating easily; it is also green, and fluted witli small 

 longitudinal ridges; and there is such an aliundant deposit 

 of silica in the epidermis that the jjlants 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 ci/rlic 

 arrangement, or sometimes the ir/iorlei/ arrangement, each 

 such set of leaves being called a fi/r/c or a n'liorl. These 

 leaves contain no chlorophyll and have evidently alsandoned 

 chlorophyll work, which is carried on liy tlie green stem. 

 Sucli leaves are known as .svYc/r.s-, to distinguisli tlicm from 

 foliage leaves. The aerial stem (really a branch) is either 

 simple or profusely liranched (Fig. 1)05). In tlie species 

 illustrated the early aerial Ijranclies are simple, usually not 

 green, and bear the stroliili ; wliile the later liranchcs are 

 sterile, profusely branched, and green. 



2'-*0. The strobilus. — One of the distinguisliing cliarac- 

 t(>rs of the group is that chlorophyll-work and sp(n'e-forma- 

 tion are completely differentiated. Although the foliage 

 leaves are reduced to scales, and the cldoroplivll-woi'k is 

 done liy the stem, there are well-organized spoi'ophvlls. 

 Tlie sporophylls are grouped close together at the cud of 

 tlie stem in a. compa,ct conical cluster \\'liich is called a. 

 s/ri)/ii//is, tin: Latin name for " jiine cone," which ihis clus- 

 ter of s})oropliylls resembles (Fig. :!(),">). 



Each sporopliyll consists of a, stalk-like ])oi-iioii and a 

 shield-like (pe/ia/c) top. Beneath the shield hang the 



