106 BRITISH FEKNS. 



CHAPTER XXV. 



THE BRITISH HORSETAILS. 



THIS race of plants bears an aspect altogether different from 

 that of the foregoing groups ; and indeed they have no very 

 obvious affinity with any existing order of plants. In their 

 mode of growth they have a certain resemblance to the 

 Ephedras and Casuarinas, but this resemblance is confined 

 to their general aspect. With Ferns and Club-mosses they 

 have little in common. Their most direct relationship is 

 with the aquatic group Chara. 



The Horsetails are distinguished from other plants by the 

 following characteristics. They are leafless, branching, with 

 hollow jointed stems, separable at certain joints, which occur 

 at intervals where they are solid, and surrounded by mem- 

 branous toothed sheaths : each length, in fact, terminates 

 above in one of these sheaths, into which the base of the 

 next length fits. The sheaths seem to represent abortive 

 leaves. The fructification consists of terminal cone-like 

 heads. 



The stems consist chiefly of cellular matter, coated exter- 

 nally by a layer of hard woody tubes, from which plates of 

 a similar nature project towards the central cavity. Between 

 the outer and inner surface of this cylinder-like stem, occur 

 one or more circles of tubes, or air-cavities, differing in size 

 and position; these afford, by their comparative size, number, 

 and arrangement, excellent auxiliary marks for the recogni- 

 tion of the species. The cuticle or skin abounds in silicious 

 particles secreted in the form of little warts, which impart 

 to the surface a greater or less degree of roughness in pro- 

 portion to their prominence. In some species this deposit 

 of silicious matter is so great, that the whole of the vegetable 

 Bubstance may be destroyed by maceration, the form of the 

 plant being preserved entire in the flinty coating. It has 

 been found that the ashes contain half their weight of 

 silica. 



On subjecting a portion of the cuticle to the analysis of 

 polarized light under a high magnifying power, Dr. Brevster 

 detected a beautiful arrangement of the silicious par>/cles, 

 which are found to be distributed in two lines parallel to the 

 axis of the stem, and extending over the whole surface. 

 The greater number of the particles were seen to form 

 simple straight lines, but the rest were grouped into oval 



