158 



HARDWICKE'S SCIENCE-GOSSIP. 



have belonged to allied plants, such as the Calamo- 

 dendron and others. 



Besides leaves, fruits have sometimes been found 

 which have been shown by Mr. Carruthers to be 

 closely allied to the fruit of the Equisetacex-. The fossil 

 fruit M-as called, before its affinities were discovered, 

 Volkmannia. It is a small cone, differing from that 

 of the recent horsetail in no respect save that the 

 fossil has a protecting leaflet shielding each spor- 

 angium-bearing scale. The remains of the elaters 

 may even sometimes be observed attached to the 

 fossil spores. 



From the Carboniferous period to the present day 

 the Equiseta have been gradually losing both their 

 size and importance, although perhaps multiplying in 

 the number of species ; but the large tropical forms of 

 the early ages disappeared about the middle of the 

 tertiary age. 



A plant closely allied to the Calamites was the 

 Calamodendron, which was, however, far more woody 

 in its structure ; but intermediate forms between 

 Calamites and Calamodendron have been described, 

 so both may belong to one family. The tissues of the 

 higher Calamodendra, according to Dawson, are 

 similar to those of gymnospermous plants ; he has 

 also considered them to form a connecting link 

 between the Calamites and the ribbed Sigillarice. 



LycopodiacecE. — Amongst the commonest of the 

 coal-measure fossils are portions of various species of 

 a genus of plants to which the name Lepidodendron 

 has been given ; the appearance of these fossils is 

 familiar to every one who has seen any collection of 

 organic remains from the Carboniferous rocks. They 

 occur in the shales and in the sandstones, and also in the 

 substance of the coal itself, and consist of fragments 

 of stems, sometimes cylindrical, at others, flattened 

 and varying in thickness from an inch or two up to 

 several feet in diameter. The surface of these stems 

 is gracefully marked by more or less diamond-shaped 

 leaf-scars, running spirally round the dichotomously 

 branched stems. But not only have the stems of 

 Lepidodendra been found, but certain cones called 

 Lepidostrobus have proved to be the fruit-bearing 

 organs of these plants, and fortunately for science, 

 the internal structure of some of these cones is fre- 

 quently so well preserved that its details are now 

 perfectly recognisable ; and these show very plainly 

 that the Lepidodendra were closely allied to the 

 existing Lycopodiacere, or club-mosses, but allies of a 

 gigantic growth. The largest of the living club- 

 mosses is a mere slender herbaceous thread of a 

 plant, seldom exceeding six feet in length, and, with 

 the single exception of the New Zealand species, 

 Lycopodium denswn, which is erect and attains a 

 height of three feet, the entire family creeps along 

 the ground. But the Lepidodendra and the allied 

 forms of the coal formation grew into tall trees, 

 occasionally attaining an altitude of as much as one 

 hundred feet and upwards, with a diameter of ten or 



twelve feet. Let us now proceed to the examination 

 of the internal structure of these plants ; and as with 

 the preceding ones we will first describe that of their 

 allies. The existing Lycopodiacece have a vascular 

 axis of spiral vessels, surrounded by a thick cortical 

 cellular layer, forming a solid stem, upon this, simple 

 leaves grow with a spiral an'angement, being supplied 

 by vascular bundles ; the stem branches dichotomously 

 as does that of Lepidodendron. 



A section of a Lepidodendron stem exhibits "a 

 central pith, surrounded by a thin cylinder of 

 scalariform woody tissue and by a large cortical layer 

 divided into two portions, the inner consisting of 

 large spherical thin-walled cells, the outer built up of 

 regularly arranged elongated cells of small diameter. 

 The vascular cylinder is penetrated by radiating 

 meshes thrtjugh which the vascular bundles passed to 

 the leaves, which were, at any rate on the younger 

 branches, small and lanceolate with a single mid- 

 vein."* Let us next compare the fruits : the fruit 

 of a Lycopodium " is produced in terminal cones 

 composed of imbricating scales, each scale bearing on 

 its pedicel a small sporangium full of spores. In the 

 genus Selaginella two kinds of spores exist, the one 

 called microspores, or male spores, producing sper- 

 matozoids, the other macrospores, or female spore?, 

 which germinate, and produce a prothallus on which 

 pistillidia appear ; these when fertilized by the 

 spermatozoids grow into perfect plants." In Lyco- 

 podium microspores only have been seen, and the 

 process of its germination is still unknown. There 

 is an allied plant, thelsoetes or quill-wort, which has 

 two kinds of spores like Selaginella, but it differs from 

 the true club-moss in the possession of a more woody 

 cylinder which has an exogenous mode of growth. 

 It has already been observed that the fruit of the 

 Lei^idodendron has been found, and is called 

 Lepidostrobus ; other cones have also been met with, 

 evidently belonging to the same order of plants, and 

 the examination of these cones strikingly confirms 

 their relationship to the club-mosses. They are seen 

 to be composed . of imbricating scales, spirally 

 arranged, and bearing sporangia on their horizontal 

 pedicels. 



Three different kinds of cones belonging to this 

 group have been described. Robert Brown many 

 years ago described one of the cones, which he 

 named Triplosporites. In this the large sporangia, 

 a single one of which is borne on each scale, are 

 seen to have a double wall, the outer composed of a 

 compact layer of oblong cells, placed perpendicularly 

 to the surface, whilst the inner is a delicate cellular 

 membrane. The spores, according to this author, are 

 each composed of three roundish sporules, but 

 Schimper says this is an error of observation, that 

 the spores are always united in fours, an arrange- 

 ment which is proved by the tetrahedral form of the 



* Carruthers. 



