EQUISETACE^ 107 



come ultimately to stand on a level. On the rhizomes the ridges and 

 furrows of the outer surface are generally less well marked, and the axial 

 cavity of the internode is sometimes wanting ; but the vallecular and 

 carinal canals are always present, and play an important part in the 

 diffusion of air through the tissue. The aerial stems, both barren and 

 fertile, are usually completely formed in miniature during the preceding 

 year within the underground bud, and their rapid growth after they 

 appear above the soil is mainly due to the great elongation of the 

 internodal cells. The ascending stem and all other aerial parts of the 

 plant are always entirely destitute of hairs ; while the rhizomes and the 

 underground leaf-sheaths are frequently covered with a felt of root- 

 hairs. 



The firmness and strength of the slender aerial stem are not due, as 

 in ferns, to the ' vascular ' bundles, but mainly to the siliceous epiderm 

 and the firm hypodermal tissue. The epiderm, consisting of a single 

 layer of elongated cells, is provided with stomates in the green, leafy, 

 aerial stem, but not usually in the colourless fertile stem, or in the 

 rhizomes. In most species of Equisetum the stomates lie in one or 

 more longitudinal rows in the furrows of the stem only ; but in 

 E. arvense, according to Miss E. A. Southworth ('American Naturalist,' 

 1884, p. 1 041), also on the ridges. Stomates also occur en the leaf- 

 sheaths. The stomates either have their orifice on a level with the 

 surface {Equiseta phafieropora) or considerably depressed below it 

 {Equiseta cryptopora) ; in the latter case they frequently do not open 

 directly into the surrounding air, but are situated in the hypodermal 

 tissue, beneath a ' false stomate,' or pore in the epiderm. The stomates 

 (fig. 82) differ from those of other classes of vascular plants in being 

 formed of two pairs instead of a single pair of guard-cells. Strasburger 

 (Beitr. zur Entwickelungsgesch. der Spaltoffnungen, in Pringsheim's 

 Jahrb., vol. v., p. 297) terms the lower pair ' subsidiary cells ' of the 

 true stomate. All the cells of the epiderm, even the guard-cells of the 

 stomates, have their outer walls or cuticle strongly silicified ; and these 

 deposits of silica frequently project above the surface in the form of 

 fine granules, bosses, rosettes, rings, transverse bands, teeth, or spines. 

 On the guard-cells they usually have the form of ridges radiating from 

 the orifice. Beneath the epiderm, except on the deciduous fertile stems, 

 bundles or layers of firm thick-walled cells generally constitute a 

 sclerenchymatous hypoder?nal tissue, which is especially developed in 

 the elevated ridges of the aerial internodes. On the underground stems 

 both epiderm and hypoderm frequently assume a beautiful brown-red 

 colour. In addition to silica, analysis of the ash of Equisetacese 

 (Dieulafaitj ' Compt. Rend.,' vol. c, 1885, p. 284) shows the presence 



