290 



ESSENTIALS OF BOTANY 



371. Discussion. — The horsetails of the present day 

 number only about forty species, but they are the insig- 

 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 _£/. 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 

 a, mouth of a fertilized secd-plants. The specics of Eqvisetum 

 archegonium. 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. 



Pig. 212. A Lobe of 

 the Mature Female 

 Prothallium of Equi- 

 setum. (x about 50.) 



