W. Carruthers — The Forests of the Coal Period. 291 



Excluding then the cellular cryptogams, we may shortly consider 

 the classification and structure of the vascular forms. They are 

 divided into four groups, all of which are represented in the indi- 

 genous Flora of Britain. 



I. Ferns (Filices). Polypody, Brake, Spleenwort, etc. 

 II. Horse-tails (Eqidsetacece) . Horse-tail. 



III. Club-mosses (Lj/copodiaceoe). Club-moss and Quill-wort. 



IV. Pill- worts (Marsileacece). Pill- wort. 



I. The Fejbns have a rhizome which creeps below or upon the 

 surface of the ground, or rises into the air like the trunk of a tree. 

 This trunk in some species attains a great height ; it is nearly uniform 

 in diameter throughout its whole length, and is covered with the 

 symmetrical and regularly-arranged markings of the stalks of the old 

 leaves. Internally it is composed of a central cellular pith sur- 

 rounded by a cylinder of scalariform tissue, and this is invested by 

 a cortical cellular layer or bark. 



The woody cylinder is composed of simultaneous vascular bundles, 

 which originate and are completely developed at the same time ; there 

 is consequently no addition to it from subsequent growth. It is 

 penetrated by large open meshes, each of which permits the passage 

 of the vascular bundles that supply a leaf, accompanied with a certain 

 amount of cellular tissue from the medulla which occupies the centre 

 of the mesh. 



The leaves, which are very variable in size and form, not only 

 perform the functions of ordinary leaves, but also bear the fruit, and 

 are hence called fronds. The fruit is produced in clusters on the 

 back or margin of the fronds ; each cluster contains many sporangia, 

 and each sporangium numerous uniform spores. 



Though there is a great diversity in the size of the plants of this 

 order — from the humble Wall Rue to the giant Alsophilas, — there is 

 a remarkable uniformity in the size of the spores. 



When the spore germinates it bursts through the outer membrane 

 and puts forth a tubular prolongation, which increases by cell-multi- 

 plication until a small green leaf is produced, called the prothallus, 

 on the under-surface of which two kinds of glandular-like bodies are 

 developed : the one, the antheridia, containing numerous cells with 

 spermatozoids, the other, the pistillidia, one of which when fertilized 

 develops into a true fern. 



II. The Horse-tails have slender, hollow, and jointed stems. 

 Each joint terminates in a toothed membranous sheath, composed of 

 leaves reduced to this elementary state. Whorls of branches and 

 branchlets are given off at the joints in some species. 



The fruit is produced in terminal cones composed of numerous 

 stalked peltate scales, each of which bears on its under-surface a 

 circle of sporangia filled with numerous uniform spores. The spores 

 have a spiral covering, which, when they are ripe, breaks up into four 

 clavate threads called elaters, which are remarkably hygrometric. 



The spores germinate like those of ferns. 



III. — The Club-mosses have solid stems composed of an axis of 

 spiral vessels, surrounded by a thickish cortical cellular layer. The 



