198 



A TEXT-BOOK OF BOTANY 



It is also green, and fluted with small longitudinal ridges; 

 and there is such an abundant deposit of silica in the epi- 

 dermis that the plants feel rough. This last property sug- 

 gested formerly its use in 

 scouring, and the name 

 " scouring rush." At each 

 joint there is a sheath of 

 minute leaves, more or less 

 coalesced, the individual 

 leaves sometimes being 

 indicated only by minute 

 teeth. This arrangement 

 of leaves in a circle about 

 the joint is the cyclic ar- 

 rangement, the leaves be- 

 ing said to be whorled (8). 

 These leaves contain no 

 chlorophyll and have evi- 

 dently abandoned food 

 manufacture, which is car- 

 ried on by the green stem; 

 hence they are scales rather 

 than foliage leaves. The 

 aerial stem, which arises 

 from an elongated root- 

 stock, is either simple or 

 profusely branched. In 

 some cases the aerial stems early in the season are simple, 

 usually not green, and bear the sporangia (Fig. 190); 

 while the later branches from the same rootstock are 

 sterile, profusely branched, and green (Fig. 191). 



113. Strobilus. At the apex of the aerial stem there 

 may be found a more or less conspicuous cone-like structure, 

 called the strobilus, meaning "pine cone," whrch it resem- 

 bles in general outline (Fig. 190). The strobilus is a com- 



FIG. 190. Early fertile shoots of Equise- 

 tum, which are not green, have con- 

 spicuous leaf -sheaths at the joints, and 

 bear conspicuous strobili; beginnings 

 of the later sterile shoots also seen. 



