INTllODUCTION TO CRYPTOOAMIC BOTANY. 551 



after all, merely analogical The superior development of the 

 vascular tissue iudicates a higher type than that of ferns ; and 

 if the nobler forms of these are objected, we have but to point 

 to extinct Equisetacece. 



628. Almost the only especial use to which these plants are 

 put by man is that of polishing, which they effect in conse- 

 quence of the minute crystals of silex which are found in their 

 cuticle. Brewster has shown that the flinty particles are 

 arranged in lines parallel to the axis of the stem, and that 

 each possesses a regular axis of double refraction. Horsetails 

 have been supposed to possess medical properties. E. arvense, 

 for example, is said to be astringent and diuretic. 



629. Equiseta are found in most parts of the world, but 

 there are none in Australia or New Zealand. The tropics have 

 their species as well as more temperate climes. A few of the 

 species, as E. variegatum, sylvaticum, &c., have a very wide 

 distribution. The former occurs as far north as Iceland ; it is 

 found also in Quito, Bourbon, and Uitenhage, while E. sylva- 

 ticum is found from the Arctic regions of North America to 

 Simla. E. giganteum, a Brazilian species, attains several feet 

 in height, Avith a stout stem, three quarters of an inch in 

 diameter. One species, on the contrary, E. debile, is so weak 

 that it requires the support of low bushes, up w^hich it may be 

 said to climb, and Welwitsch describes E. elongatmn as 

 climbing up Agave Americana at Lisbon. 



630. The largest Equisetwni of the present day is not to be 

 compared with the noble representatives, as Calamites, which 

 occur in the Coal Measures and the New Red Sandstone. True 

 Equiseta also occur in a fossil state. The recent species vary 

 greatly in size, branching, and the length of their internodes- 

 E. arvense produces tubers on the creeping rhizoma. 



IV. MARSILEACE.E, R B. 



Ehizocarpe^, Batsch., Ag. Rhizospeemje, Roth., D. C. HxDKorTERiDES, 

 Willd. 



-3i^stivation straight or circinate, leaves various or reduced 

 to a petiole ; receptacles* more or less radicular, formed from 



* These receptacles are called iu the tabular view of the acrogenous 



