358 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA. 



to form the root ; and when these organs have been sufficiently 

 developed to absorb and prepare the nutriment which the young 

 plant requires, the prothallium, whose functions as a " nurse" is 

 now discharged, decays away. 



220. The little group of Uquisetacece (Horsetails), 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 considerable interest to the Microscopist. The 

 whole of their structure is penetrated to so extraordinary a degree 

 by silex, that, even when the organic portion has been destroyed 

 by prolonged maceration in dilute nitric acid, a consistent skele- 

 ton 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 their inorganic ash. The cuticle, which is used 

 by cabinet-makers for smoothing the surface of wood, becomes, 

 through the peculiar arrangement of its siliceous particles, an 

 extremely beautiful object under polarized light. Of these par- 

 ticles (each of which has a regular axis of double refraction), 

 some 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 

 particles occupying the cells of the stomata) are arranged in pairs. 

 "What is usually designated as the fructification of the Equise- 

 tacese, forms a cone or spike at the extremity of certain of the 

 stem-like branches (the real stem being a horizontal rhizoma) ; 



Fio. 148. 



Spores of £quisetum, 



and consists of a cluster of shield-like disks, each of which car- 

 ries a circle of thecce or spore-cases, that open by longtitudinal 

 slits to set free the spores. Each of these bodies has, attached 

 to it, a pair of elastic filaments (Fig. 148), that are originally 

 formed as spiral fibres on the interior of the wall of the primary 

 cell within which the spore 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 slightest application of moisture, they 

 suddenly extend themselves in the manner shown at B ; and this 

 motion, like the extension of the spiral elaters of Marchantia, 

 probably serves to assist in the dispersion of the spores. 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 bystander 



