EQUISETACEJE : SILICEOUS CUTICLE J SPORES. 383 



Fern requires, the Prothallium, whose function as a ' nurse ' is now 

 discharged, decays away. 



281. The little group of Equisetacece (Horsetails), which seem 

 Dearly allied to the Ferns in the type of their Generative apparatus, 

 though that of their vegetative portion is very different, affords 

 certain objects of considerable 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 con- 

 sistent Skeleton still remains. This mineral, in fact, constitutes 

 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 Cuticle, which is used by Cabinet-makers for 

 smoothing the surface of wood. 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 cur- 

 vilinear quadrangle ; and these (which are, in fact, the particles 

 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 cuticle 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 and similar objects (such 

 as the Cuticles of Grasses) upon Polarized light 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.* What is usually designated as the 

 Fructification of the Equisetaceae forms a cone or spike at the 

 extremity 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 Thecce or Spore-cases, that open by 

 longitudinal slits to set free the spores. Each of the Spores has, 

 attached to it, two pairs of Elastic Filaments (Fig. 196), that are 

 originally formed as 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 closely 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 them- 

 selves in the manner shown at b, — the slightest application of 

 moisture, however, serving to make them close together (the assist- 

 ance which they afford in the dispersion of the spores being no 

 longer required) when the spores have alighted on a damp surface. 

 If a number of the Spores be spread out on a slip of glass under 

 the field of view, and, whilst the observer watches them, a by- 

 stander breathes gently upon the glass, all the filaments will be 

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



