The beautiful ringed sporangium of the fern ([Fig. 307]) when ruptured gives exit to the dust-like spores; these, examined under a moderate power, are seen to be sub-globose and pyramidal, the outer coat or exospore being a coloured hyaline cell with nuclei similar to the spores of mosses, but in which chlorophyll soon begins to form, and from this little green embryonic growth the organs of reproduction are formed.
In all ferns the pistillidia or archegonia are analogous to the ovules or nascent seeds of flowering plants, and contain, like them, a germinal vesicle, which becomes fertilized through the agency of the spiral filaments, and then gradually develops into an embryo plant possessing a terminal bud. This bud begins at once to unfold and push out leaves with a circinate vernation, of a very simple form at first, and growing up beneath the prothallium, coming out at the notch; single fibrous roots are at the same time sent down into the earth, the delicate expanded prothallium withers away, and the foundation of the perfect fern plant is laid. When a fern acquires a considerable stem, as in a tree fern, it consists of cellular tissue and an external cortical portion forming fibro-vascular bundles, scalariform ducts, and woody fibre. [Fig. 308], b, shows an oblique section of the footstalk of a fern leaf with its bundle of scalariform ducts.
These observations on ferns have acquired increased interest from subsequent investigations made on the allied Cryptogams, and on the processes occurring in the impregnation of the Conifers. Not only have later researches furnished a satisfactory interpretation of the archegonia and antheridia of the mosses and liverworts, but they have made known and co-ordinated the existence of analogous phenomena in the Equisetaceæ, Lycopodiaceæ, and Rhizocarpeæ, and prove, moreover, that the bodies described by Dr. Brown in the Conifers under the name of “corpuscles” are analogous to the archegonia of the Cryptogams; so that a link is hereby formed between these groups and the higher flowering plants.
Fig. 308.—a. Vertical section of Fern-root, showing spiral tissue and cells filled with granular bodies; b. Section of Footstalk.
Equisetaceæ.—The development of Horse-tails ([Fig. 309]), the name by which they are commonly known, corresponds in some respects with that of ferns. They comprise a little group, and the whole of their structure is composed in an extraordinary degree by silex, so that even when the organic portion has been destroyed by prolonged maceration in strong acid, a consistent skeleton still remains. It is this flinty material that constitutes their chief interest for microscopists. A portion of their silicious particles is distributed in two lines, arranged parallel to the axis of the plant, others are grouped into oval forms, and connected by a chain as in a necklace. The form and arrangement of the crystals are better seen under polarised light. [Plate VIII]., No. 170, a portion of the epidermis, forms an extremely beautiful object. Sir David Brewster pointed out that each silicious particle has a regular axis of double refraction. What is usually said to be the fructification of the Equisetaceæ forms a cone or spike-like extremity to the top of the stem ([Fig. 309]), the whole resembling a series of spike-like branches (the real stem being a horizontal rhizome), and a cluster of shield-like discs, each of which carries a circle of sporanges that open by longitudinal slits to set free the spores which are attached to it in two pairs of elastic filaments (shown in [Fig. 291], F, G), elaters; these are at first coiled up around the spore in the manner represented at G, but on their liberation they extend themselves as shown at F. The slightest moisture will close them up again, and their purpose having been served in the distribution of the spores, they are no longer required. If a number of spores be spread out on a glass-slip under the microscope and, while watching, a bystander breathes upon them, they immediately respond, are set in motion, presenting a curious appearance, but as soon as the hydroscopic effect has passed off they return to their previous condition. These spores can be mounted in a cell with a movable cover, and made to exhibit the same effect over and over again.
Fig. 309.—Equisetum giganticum.
a. Fragment of stem showing mode of branching out; b. Cone or spike of fructification; c. Scale detached from cone; d. Spore with elastic filaments; e. Vertical section of stem; f. Transverse section showing hexagonal cells.
The vascular tissue of the Equisetaceæ ([Fig. 309], e, f) shows them to be of a higher grade than the ferns. More recently discovered Horse-tails, of Brazil, grow to a gigantic size, but even these are comparatively small when compared with the Calamites, and other fossil Equisetaceæ of the coal measures and new red sandstone. They all require a calcareous flinty soil for growth. A spring water-course making its way to the sea, as in the Chines of the Isle of Wight, is very favourable, the author having gathered them more than once in Bramble Chine.