680 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS 



plete in everything but the true generative organs, which evolve 

 themselves from the detached spores. Here we have, therefore, an 

 example of alternation of generations differing in one important 

 respect from that in mosses. In ferns the ' sexual generation ' or 

 ' oophyte ' which results from the germination of the spore consists 

 of the prothallium only with its archegones and antherids, the leafy 

 plant which bears the sporanges constituting the ' sporophyte ' or 

 ' non-sexual generation,' the product of the fertilisation of the arche- 

 gone by an antherozoid. In mosses, on the other hand, the leafy 

 plant belongs to the sexual generation. 



The singular discovery has recently been made by the researches 

 of De Bary, Farlow, and others, that the ordinary alternation of 

 generations in ferns may be interrupted by the suppression either of 

 the sporophyte, the non-sexual or spore-bearing generation, or of the 

 oophyte or sexual generation which bears the true reproductive 

 organs. These phenomena are called respectively apospory and 

 apogamy. The former has been observed especially in varieties of 

 Athyrium Filix-fcemina and Polystichum angular e, and is shown by 

 the production of prothalloid structures bearing antherids and 

 archegones on the fronds in the place of ordinary sori. The latter 

 occurs not unfrequently in Pteris serrulata, the sporophytic genera- 

 tion springing directly from the prothallium without the interven- 

 tion of archegones and antherids. 



The little group of Equisetaceae (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 considerable interest to the microscopist. The whole of 

 their structure is penetrated to such an extraordinary degree 

 by sileXy 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, 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 epiderm, 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 curvilinear quadrangle ; and these (which are, in 

 fact, the particles occupying the guard-cells of the stomates) are 

 arranged in pairs. Their form and arrangement are peculiarly well 

 seen under polarised light, for which the prepared epiderm 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. What 

 is usually designated as the fructification of the Equisetaceee forms a 

 cone or spike at the extremity of certain of the stem-like branches 

 (the real stem being a horizontal rhizome) and consists of a cluster 

 of shield-like discs, each of which carries a circle of sporanges or 

 spore-capsules, that open by longitudinal slits to set free the spores. 

 In addition to the spores each sporange contains a number of elastic 

 filaments (fig. 523), called elaters. These are at first coiled up around 



