290 ESSENTIALS OF BOTANY 
371. Discussion. — The horsetails of the present day 
number only about forty species, but they are the insig- 
Fie, 212. A Lobe of 
the Mature Female 
Prothallium of Hqui- 
setum. (xX about 50.) 
a, Mouth of a fertilized 
archegonium. 
nificant descendants of what was in 
earlier times a highly important class 
of plants, often tree-like in size. 
The work of photosynthesis in Equi- 
setum is wholly performed by the green 
tissue of the stems and branches, not 
at all by the scale-like leaves. The 
stomata occur in the channels which 
run lengthwise of the stems. The thick 
flinty epidermis and well-developed 
rigid tissue under the epidermis, to- 
gether with the moderate amount of 
plant-surface exposed to the air, make 
the horsetails decidedly xerophytic in 
their structure. 
Some horsetails, particularly Z. hie- 
male, have so much silica in the epi- 
dermis that they were formerly much 
used and are still somewhat employed 
to polish tinware and other metal sur- 
faces. Hence were derived the common 
names scouring-rush and gunbright. 
Equisetum differs from ferns in having 
the spore-bearing leaves grouped into 
a cone. This is a distinct advance 
toward the flower-bearing condition of 
seed-plants. The species of Equisetum 
studied has pale fertile stems, living 
almost like parasites at the expense of material drawn from 
the rootstock and wholly set apart for spore production. 
