426 PART III. THE CLASSIFICATION OF PLANTS. 



branches on the subterranean shoots : they have limited apical growth ; they 

 are branched, the mode of branching being probably the same as that of the 

 subterranean shoots ; and they bear small scattered leaves. 



The stem of Tmesipteris appears to agree in all essential morphological points 

 with that of Psilotum ; but with this conspicuous difference, that it is much less 

 branched. 



The leaves. In both genera the leaves of the subaerial shoots are of two 

 kinds. In Psilotum the vegetative leaves are minute scales, whereas in 

 Tmesipteris they are relatively well-developed as foliage-leaves : they are simple 

 and sessile. The sporophylls, on the contrary, are petiolate and bilobed in both 

 genera, a single sporangium being borne on the upper surface of each sporophyll 

 just at the junction of the bases of the two lobes : they are not borne in cones. 



The sporangia are synangia (p. 375) ; that is, they are not unilocular, but 

 multilocular capsules : in Psilotum the synangium is generally trilocular (some- 

 times 2-4 locular), in Tmesipteris bilocular. Each loculus opens by a 

 longitudinal slit. The origin of the archesporium has not been fully traced : 

 but it appears probable that it consists primarily of a layer of cells, some of 

 .vhich become the mother-cells of the spores, whilst the rest are sterile and form 

 the tissue of the walls separating the loculi. 



The spores are developed in tetrads from the mother-cells ; bilaterally, as in 

 Tmesipteris ; or either bilaterally or tetrahedrally, as in Psilotum. They have 

 the typical structure. 



General Histology. The shoots of both genera are monostelic. The arrange- 

 ment of the bundles is radial, much as in Lycopodium ; but the number of the 

 bundles is generally smaller, especially in the subterranean shoots where there 

 may be only two protoxylem-bundles. There is no secondary growth in 

 thickness. The growth in length of the shoots is effected by an apical growing- 

 point which, when the shoot is quite young, consists of small-celled meristem ; 

 but in which a three-sided pyramidal apical cell can in certain cases 

 (subterranean shoots of Psilotum) be detected at a later stage. 



The leaves have a midrib with a single bundle, whicii is, however, very 

 rudimentary in Psilotum triquetrum. 



Vegetative propagation takes place in Psilotum by means of gemmas, 

 developed on the rhizomes, consisting of a single layer of cells, ovoid and 

 flattened in form. 



THE GAMETOPHYTE. No observations have as yet been made on the gameto- 

 phyte of either genus, and consequently the embryogeny of the sporophyte is 

 also unknown. 



SUB-CLASS HETEKOSPORE^. 



OrderS. Selaginellaceae. This order consists of the single genus Selag- 

 inella, of which the numerous species are very widely distributed, but only one, 

 S. spinosa (selaginoides), is British. 



THE SPOROPHYTE. The primary stem is slender and elongated, erect, or more 

 commonly procumbent; its symmetry is bilateral, isobilateral when erect, 

 dorsiventral when procumbent ; the branches spring from the flanks of the 

 primary stem, and, as this is subsequently repeated, the resulting branch- system 

 lies in one plane ; the mode of branching is lateral, though it appears to be 



