Sporophyll

“ 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 organization of Lepidostrobus is essentially that of a Lycopodiaceous cone. The axis, which in anatomical structure resembles a vegetative twig, bears numerous spirally arranged sporophylls, each of which carries a single large sporangium on its upper surface (fig. 10) . The sporophyll, usually almost horizontal in position, has an upturned lamina beyond the sporangium, and a shorter dorsal lobe, so that the form of the whole is somewhat peltate. ”

“ The cone may be considered as a very rudimentary flower, and the annulus may be regarded as a very early stage in the formation of a flower (perianth) . See page 235.
The SPORANGIA are situated on the underside of the sporophylls, one at each angle; they are sac-like, and open inwardly by a longitudinal cleft (Fig. 224 d) . An annulus is wanting; but in the wall of the sporangium, as in the pollen-sacs of the Flowering-plants, a layer of cells, with annular or spiral thickenings, is developed, which assists in the dehiscence of the sporangium.
The SPORES are green ”

“ The mature sporangium had a wall of a single layer of cells, which were larger towards the base, where they continued into the epidermis of the sporangiophore. In Sphenophyllum fertile both the ventral lobes of the sporophyll (corresponding to the sporangiophores in other species) and the dorsal lobes, which in other species are sterile, were developed as peltate sporangiophores. In other species of Sphenophyllum, which are known only as impressions, single sporangia, or groups of four, appear to have been inserted directly on the upper surface of the bracts. ”

“ The fertile leaves or sporophylls are generally aggregated on special shoots to form flowers which may contain one or both kinds. The microspores are set free from the sporangium and carried generally by wind or insect agency to the vicinity of the macrospore, which never leaves the ovule. The male gametophyte is represented by one or few cells and, except in a few primitive forms where the male cell still retains the motile character as in the Pteridophyta, is carried passively to the macrospore in a development of the pollen grain, the pollen tube. ”

“ The mesophyll consists of palisade cells having the appearance of typical chlorophyll-tissue. The heterosporous strobili attributed to this species bore microsporangia on the upper and megasporangia on the lower sporophylls; the megaspores, of which a considerable number occur in each megasporangium, are identical in size with those of another Culm form, Lepidodendron rhodumnense. Some of these have retained traces of prothallus tissue, and in one spore Renault figures what he regards as an archegonium: the drawing is by no means convincing. ”

“ More recently Professor Thomas [29] of Auckland has carefully examined living plants, with the result that variations of different kinds are proved to be exceedingly common. He finds that sporophylls occur which exhibit repeated dichotomy of the axis (fig. 120, D, F) and thus each may bear four instead of two leaf-lobes and three synangia, one at the first fork and one at each of the forks of the second order [30] .
Other abnormalities occur in which the synangium is raised on a distinct stalk instead of being more or less sessile at the point from which the leaf-lobes diverge. ”

“ In the region of the cone the internodes remain shorter, and the whorls of appendages, known as sporophylls or sporangiophores, have the form of stalked structures terminating distally in a hexagonal peltate disc, which bears on its inner face a ring of five to ten oval sporangia (fig. 52, B) . Each sporangium contains numerous spores which249 eventually escape by the longitudinal dehiscence of the sporangial wall. The opening of the sporangia is probably assisted by the movements of the characteristic elaters formed from the outer wall of each spore. ”

“ The cone is of large size—3.5 cm. in diameter; the stout axis bears numerous whorls of compound sporophylls, the members of successive verticils being superposed. The sporophylls, of which there are eleven or twelve in a whorl, are each composed of six segments, three being inferior or dorsal, and three superior or ventral. The dorsal segments are sterile, corresponding to the bracts of Sphenophyllum Dawsoni, while the ventral segments constitute peltate sporangiophores, each bearing four sporangia, just as in a Calamarian fructification (fig. 7) . ”

“ A complete individual, which does not exceed 4 or 5 cm. in length, consists of a very small tubercle or protocorm bearing a rosette of slender subulate leaves and prolonged distally as a simple naked axis which overtops the foliage leaves and terminates in a compact cluster of 34small scale-like sporophylls, each subtending a single sporangium [91] . ”

“ The axis of the strobilus has a stele characterised by a pith of elongated elements, most of which have thin walls; the xylem cylinder possesses about twenty protoxylem strands forming more or less prominent exarch ridges. The cortex exhibits a differentiation comparable with that in the shoots of Lepidodendron. The sporophylls are arranged in alternating verticils, each whorl consisting of ten members: the narrow horizontal pedicel of a sporophyll, containing a single vascular bundle, as shown in fig. ”

“ Isoetes echinospora (After Motelay and Vendryès) .
Stem of I. lacustris. Base of sporophyll: l, ligule; spg, sporangium partially covered by velum.
There is as a rule little or no difference between the foliage leaves and sporophylls; in I. lacustris the latter are rather60 larger and in the terrestrial species I. hystrix [134] the sterile leaves are represented by the expanded basal portions only, which persist like the leaf-bases of Lepidodendron as dark brown scales to form a protective investment to the older part of the stem. The innermost leaves are usually sterile ”

“ The perigone has a twofold purpose, serving both to protect the sporophylls, and, at least in bright-colored flowers, to attract insects which, as we shall see, are important agents in transferring pollen from one flower to another.
When we compare the embryo sac (macrospore) of the angiosperms with that of the gymnosperms a great difference is noticed, there being much more difference than between the latter and the higher pteridophytes. Unfortunately there are very few plants where the structure of the embryo sac can be readily seen without very skilful manipulation. ”

“ The central cylinder of the root often shows a striking resemblance to that of the stem. The Lycopodiaceae are homosporous. The spores are formed in sporangia of considerable size, situated on the upper surface and near the base of the sporophylls. The latter may differ from the foliage leaves and be arranged in definite cones, or the two may be similar and occupy alternate zones of a shoot with continued growth ”

“ The fusion of the two gametes results in the production of the sexual spore, called the “o-o-spore;” it develops at once into the second generation, or second stage in the life-cycle of the moss plant, which is called the “spo-ro-phyte.” It consists of the seta and capsule; the lower end (“foot”) of the seta becomes early embedded and fixed in the tissue of the gametophyte, and from it is derived the nourishment necessary to complete the development of the sporophyte, or the plant that produces the numerous non-sexual spores. ”

“ When the ripe sporangium becomes dry, the ring of thickened cells (an.) contracts more strongly than the others, and acts like a spring pulling the sporangium open and shaking out the spores, which germinate readily under favorable conditions, and form after a time the sexual plants (prothallia) .
The roots of the sporophyte arise in large numbers, the youngest being always nearest the growing point of the stem or larger roots (Fig. 67, C) . The growing roots are pointed at the end which is also light-colored, the older parts becoming dark brown. ”

“ The spore on germination gives rise to a small thalloid structure, the prothallium, on which the sexual organs are developed; this is the gametophyte or sexual generation. The sexual organs have the form of typical archegonia and antheridia. From the fertilised egg-cell there is developed the Pteridophyte plant or sporophyte, which bears the spores. This asexual generation shows a well-marked external differentiation into stem and leaves, and bears true roots. Internally the tissues exhibit a high degree of differentiation into distinct tissue-systems. ”

“ The pollen-spores are borne in sporangia, which are united into what are called anthers. The embryo-sac, which contains the egg-cell, is borne in a sporangium known as an ovule. A gametophytic stage is present in both pollen and embryo sac: fertilization takes place, and a sporophyte arises. Soon this sporophyte becomes dormant, and is then known as an embryo. The embryo is packed away within tight-fitting coats, and the entire body is the seed. When the conditions are right the seed grows, and the sporophyte grows into herb, bush, or tree. ”

“ In mosses (fig. 12) a filamentous growth, the protonema, is first formed, and the leafy plants arise upon this. In liverworts this preliminary phase of the sexual generation is as a rule ill-marked or absent, and the plant may be said to develop directly from the spore.
It will be evident that the two generations exhibit a regular succession or alternation in the life-history of all Bryophytes. The gametophyte is developed from the spore and bears the sexual organs; the sporogonium is developed from the fertilized egg and produces spores. ”

“ When the young sporophyte first begins its independent life—when, that is, it exists in the form of the embryo in the seed—its living substance has no power of utilizing the simple inorganic compounds spoken of. Its nutritive pabulum is supplied to it in the shape of certain complex organic substances which have been stored in some part or other of the seed, sometimes even in its own tissues, by the parent plant from which it springs. ”

“ The spores grow slowly when they come on to moist earth, but as their development takes a long time, you had better get some from a gardener which have already grown. As the spore grows, the one cell composing it divides and divides again, until there is formed a little filmy heart-shaped green structure called a prothallium (see fig. 129) , which is not in the least like a fern plant, for it is not more than a quarter of an inch across. It has no stem or leaves, and is only a thin layer of green cells, with a few root-hairs on the under side. ”

“ In the life-history of a moss, the plant itself bears only sexual organs: it is the sexual form, and is distinguished as the gametophyte. The zygote (oospore) formed in the sexual act develops into an organism, the sporogonium, which is entirely asexual, producing only spores: it is distinguished as the sporophyte. When these spores germinate, they give rise to moss-plants. Thus the two forms, the sexual and the asexual, regularly alternate with each other—that is, the life-history presents that simple form of polymorphism which is known as alternation of generations. ”

“ I am afraid it has been rather a difficult history to follow step by step, and yet it is perfectly clear when once we master the succession of growths. Starting from a spore, the thread-mass or protonema gives rise to the moss-stems forming the dense green carpet, then the green flowers on some of the leaf-stems give rise to a plant-egg, which roots itself in the stem, and grows into a perfect plant without leaves, bearing merely the urn in which fresh spores are formed, and so the round goes on from year to year. ”

“ The sporangia agree in structure with those of other species of Sphenophyllum: the spores are of one size and elliptical, characterised by the presence of several sharp ridges or flanges encircling the spore-wall in the direction of the major-axis. Sphenophyllostachys fertilis differs from all previously recorded types in the absence of sterile bracts. The appendages of the cone-axis are all fertile, a striking contrast to the differentiation into protective and sporangia-bearing bracts which constitutes a constant feature in the cones of Sphenophyllum and Calamites. ”

“ The sporogenous tissue is separated from the stalk of the sporangium by a mass of parenchymatous tissue which may project as a prominent pad (fig. 126, D, F, p) into the interior of the sporogenous cavity. This basal tissue (the subarchesporial pad of Bower [107] ) has been observed in L. clavatum to send up irregular processes of sterile cells among the developing spores, suggesting a comparison with the trabeculae which form a characteristic feature of the sporangia of Isoetes and with similar sterile strands noticed by Bower [108] in Lepidostrobus (cone of Lepidodendron) . ”

“ But in the Bryophytes the sporogonium never becomes a sporophyte producing leaves and roots, and always remains dependent upon the gametophyte for its water and mineral food, and the facts give us no warrant for asserting homology (i.e. morphological identity) between the differentiated tissues of an Anthocerotean sporogonium and those of the sporophyte in the higher plants Opposed to the thalloid forms are the group of leafy Liverworts (Acrogynae) , whose plant-body consists of a thin supporting stem bearing leaves. ”