Each sporophyll is supplied by a single vascular bundle which according to published statements never sends a branch to the sporangium base. The fertile tips of the foliage shoots of L. cernuum ([figs. 126], A–C) afford good examples of specialised cones. The surface of the cone is covered by the broadly triangular laminae of sporophylls ([fig. 126], C) which in their fimbriate margins resemble the Palaeozoic cone-scales described by Dr Kidston[109] as Lepidostrobus fimbriatus. The distal portions of the sporophylls are prolonged downwards ([fig. 126], A) to afford protection to the lower sporangia, their efficiency being increased by the lignified and thicker walls (A, a) of the cells in the lower portion of the laminar expansion. The cells of the sporangial wall are provided with strengthening bands which in surface-view ([fig. 126], B) present the appearance of prominent pegs. Since the appearance of Miss Sykes’s paper on the sporangium-bearing organs of the Lycopodiaceae, Dr Lang[110] has published a more complete account of the structure of the strobilus of Lycopodium cernuum in which he records certain features of special interest. The importance of these morphological characters is increased by their agreement, as shown by Lang, with those of the Palaeozoic cone Spencerites[111]. The sporophylls of a cone (12 mm. long by 3 mm. in diameter) of Lycopodium cernuum show an abrupt transition from the foliage leaves, but like these they occur in alternate whorls of five. A large sporangium is attached to the upper face of each sporophyll close to the base of the obliquely vertical distal lamina ([fig. 127]); each sporophyll, which is supplied with a single vascular bundle, has a large mucilage-cavity (m) in its lower region. “The mucilaginous change” in the sub-sporangial portion of a sporophyll “extends to the surface involving the epidermis, so that this portion of the sporophyll-base may be described as consisting of a mass of mucilage bounded below by a structureless membrane[112].” Dehiscence of the sporangia occurs at the middle of the distal face ([fig. 127], x). As seen in the radial section ([fig. 127], ma) the outer margin of the base of the sporophyll bears a short outgrowth. The leaf-bases of each whorl hang down between the sporangia of the alternating whorl below, and the base of each sporophyll is coherent with the margins of the two sporophylls of the next lower whorl between which it lies, the sporangia being thus closely packed and lying in a pocket “open only on the outer surface of the cone.” Fig. 128 represents a transverse section through a cone in the plane AA of [fig. 127]; this traverses the sporangia and their subtending bracts (b) of one whorl and the dependent bases of the sporophylls of the next higher whorl in the region of the mucilage-sacs (m), which are bounded at the periphery by the outer tissue of the sporophylls (a). A transverse section in the plane BB of [fig. 127] is shown in [fig. 129]: the pedicels and a part of each vascular strand are seen at b radiating from the axis of the cone; one sporophyll (sp, a) is cut through in the region of the pad of tracheal tissue that characterises the short sporangial stalks. The upper portions of the sporangia of the next lower whorl, which project upwards against the mucilaginous bases of the sporophylls above (cf. [fig. 127], BB) are shown at c and external to them, at a, the section has cut through the outer persistent portions of these sporophyll bases.
Fig. 127. Radial longitudinal section of the cone of Lycopodium cernuum. (After Lang.)
Fig. 128. Transverse section of the cone of Lycopodium cernuum, in its plane AA of [fig. 127]. (After Lang.)
Fig. 129. Transverse section of the cone of Lycopodium cernuum in the plane BB of [fig. 127]. (After Lang.)
As Lang points out, this highly complex structure is an expression of the complete protection afforded to the sporangia of a plant met with in exposed situations in the tropics; it is also of importance from a morphological standpoint as exhibiting an agreement with the extinct type of Lycopod cone represented by Spencerites.
Selaginellaceae.
Selaginella differs from Lycopodium in the production of two kinds of spores, megaspores and microspores, and, in the great majority of species, in the dimorphic character of the foliage leaves, which are usually arranged in four rows, the laminae of the upper rows being very much smaller than those of the lower ([fig. 130], 1–3). The smaller leaves are shown more clearly in [fig. 130], 1a. It is obvious from an examination of a Selaginella shoot, such as is shown in [fig. 130], that in fossil specimens it would often be almost impossible to recognise the existence of two kinds of leaves. Some species, e.g. Selaginella spinosa[113], the sole British representative of the genus, are homophyllous and agree in this respect with most species of Lycopodium. Another feature characteristic of Selaginella, as contrasted with Lycopodium, is the presence of a ligule in both foliage leaves and sporophylls. This is a colourless thin lamina attached by a comparatively stout foot to the base of a pit on the upper surface and close to the lower edge of the leaf ([fig. 130], 4, l; [fig. 131], E, F, l).