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 fecl 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 
fia. 190.—Larly tertile shoots of Hguise- — gerig] stem, which arises 
tum, which are not green, have con- t 
spicuous leaf-sheaths at the joints,and from an elongated root- 
bea coueiewous srobi beeimines stock, is either simple ot 
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,” which it resem- 
bles in general outline (Fig. 190). The strobilus is a com- 
