EQUISETACE^— FILICES. 637 



an Equisetum fifteen feet high, and three inches in circumference at 

 the lower part of the stem. There is only 1 known genus, compre- 

 liending about 25 species. Example — Equisetum. 



From the quantity of silicic acid contained in them, some of the 

 species of Equisetum are used in polishing mahogany. An analysis of 

 them is given at page 131. The spiral filaments which surround 

 their spores are interesting objects under the microscope, exhibiting 

 marked movements according to the moisture or dryness of the atmo- 

 .sphere around them. The stomata are arranged in lines on the 

 cuticle. In Equisetum hyemale, often called Dutch Eushes, the sili- 

 ceous stomatic apparatus is well seen after the action of nitric acid on 

 the stem. There are regular rows of tubercles of a siliceous nature, 

 in each of which is a transverse fissure, and at the bottom of the 

 fissure a stoma is placed, with its opening at right angles to that of 

 the tubercle. Each portion of the stoma has a pectinated (comb-like) 

 appearance. The distinctions between the species of Equisetum are 

 founded on the nature of the fertile and barren stems, the number of 

 strise or furrows, and the number of teeth at the articulations. 



Order 216. — Filices, the Fern Family. Stem, a rhizome (fig. 

 881), which creeps along or under the surface of the ground, emitting 

 descending roots and ascending fronds (leaves), or which rises into 

 the air so as to form an acrogenous trunk (fig. 135, p. 71). This 

 trunk (stipe) is of nearly uniform diameter, often hollow in the interior, 

 marked on the hard outer rind by the scars (cicatrices) of the leaves, 

 and contains vascular bundles of woody, dotted, and scalariform 

 vessels, which are enclosed in hard tissue, and are arranged in an 

 irregular manner (fig. 136, p. 71). Ferns have a continuous woody 

 cylinder in their stem. The stem of many tree-ferns is composed of 

 a. mass of parenchyma traversed by vascular bundles of scalariform 

 tissue, which form a closed circle separating the medulla in the in- 

 terior from the cortex of the exterior. The tissue of this vascular 

 cylinder is entirely destitute of medullary rays, but it is penetrated by 

 large meshes, through which pass the vascular bundles that supply the 

 fronds, and which invariably rise from the inner surface of the cylinder. 

 Sometimes the trunk is dichotomous (fig. 137, p. 72). The outer 

 fibrous covering is formed by the bases of the leaves, and is thicker 

 at the lower than at the upper part of the stem. The leaves (fronds) 

 have a circinate (gyrate) vernation (fig. 881 /' /") ; their veins are 

 generally of 'equal thickness, and either simple or dividing in a forked 

 manner (fig. 882), or somewhat reticulated, and occasionally stomata 

 occur. Eeproductive organs consist of spore-cases (thecse, spo- 

 rangia), which arise from the veins on the under surface of the fronds 

 (figs. 881 /"", 882 s, 883), or from their margin. Spore-cases, either 

 stalked, with the pedicel passing round them in the form of an elastic 

 ring (fig. 884), or sessile and destitute of a ring. The thecse some- 



