MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA. 349 



spore-capsules, but, as in Phanerogamia, into entire plants, complete in 

 everything but the true generative organs, which evolve themselves from 

 the detached spores. 



345. The little group of Equisetacea (Horse-tails) which seem nearly 

 allied to the Ferns in the type of their generative apparatus, though that 

 of their vegetative portion is very different, affords certain objects of con- 

 siderable interest to the Microscopist. The whole of their structure is 

 penetrated to such an extraordinary degree by silex, that even when its 

 organic portion has been destroyed by prolonged maceration in dilute nitric 

 acid, a consistent skeleton still remains. This mineral, in fact, consti- 

 tutes in some species not less than 13 per cent of the whole solid matter, 

 and 50 per cent of the inorganic ash; and it especially abounds in the 

 epidermis, which is used by cabinet-makers for smoothing the surface of 

 ^ood. Some of the siliceous particles are distributed in two lines, parallel 

 to the axis; others, however, are grouped into oval forms, connected with 

 each other, like the jewels of a necklace, by a chain of particles forming 

 a sort of curvilinear quadrangle; and these (which are, in fact, the parti- 

 cles occupying the cells of the stomata) are arranged in pairs. Their 

 form and arrangement are peculiarly well seen under Polarized light, for 

 which the prepared epidermis is an extremely beautiful object; and it is 

 asserted by Sir D. Brewster (whose authority upon this point has been 

 generally followed) that each siliceous particle has a regular axis of 

 double refraction. According to Prof. Bailey, however, the effect of this 



FIG. 23-L 



Spores of Equisetum with their Elastic Filaments. 



and similar objects (such as the epidermis of Grasses) upon Polarized 

 light, it is not produced by the siliceous particles, but by the organized 

 tissues; since, when the latter have been entirely got rid of, the residual 

 silex shows no doubly-refracting power. 1 What is usually designated as 

 the fructification of the Equisetacea3 forms a cone or spike at the extrem- 

 ity of certain of the stem-like branches (the real stem being a horizontal 

 rhizoma); and consists of a cluster of shield-like disks, each of which 

 carries a circle of theccB or spore-capsules, that open by longitudinal slits 

 to set free the spores. Each of these spores has, attached to it, two pairs 

 of elastic filament (Fig. 234), that are originally formed spiral fibres on 

 the interior of the wall of the primary cell within which it is generated, 

 and are set free by its rupture; these are at first coiled up around the 

 spore, in the manner represented at A, though more closely applied to the 

 surface; but, on the liberation of the spore, they extend themselves in the 

 manner shown at B, the slightest application of moisture, however, 

 serving to make them close together (the assistance which they afford in 

 the dispersion of the spores being no longer required) when the spores 



1 See " Silliman's American Journal of Science," May, 1856. 



