A handbook of systematic botany

by Eugenius Warming

Fig. 245.—A carpel of Cycas revoluta with 5 ovules (s), at half to one-third nat. size.

Fig. 246.—Carpel with 2 ovules of Ceratozamia robusta (1/1).

Carpels. The simplest forms of carpels are found in Cycas. In this genus both the foliage and fertile leaves are pinnate, and hence present great similarity; the ovules (macrosporangia) are situated on the margin of the central portion, just as the sporangia are placed on the edge of the fertile leaf of Ophioglossum (Fig. [245], compare with Fig. [209]). The carpels of the other Cycadeæ present greater divergence from the foliage-leaves, being peltate, for instance, in Zamia and Ceratozamia (Fig. [246]). The ovules in the Coniferæ are situated on the upper side and near the base of the ovuliferous scales, almost in the same position as the sporangia in the Lycopodinæ (Figs. [269], [272], [273] H, compare Figs. [230], [239]). In Taxus the uninclosed ovule is placed on the apex of a shoot (Fig. [264]). In all these plants the ovules are not enclosed by the carpels, that is, they are not enclosed in chambers formed by the turning in of the walls of the carpel, and hence the name Gymnospermæ is given to them. In the higher Flowering-plants, the Angiospermæ, the ovules are distinctly situated on the edge, the upper surface, or base of the carpel; but the carpel closes round the ovules which are therefore enclosed in a chamber—the ovary. In a few cases, for example in the Polygonaceæ, an ovule is situated apparently on the apex of the stem itself, as in the Yew; in other cases, as in the Primulaceæ, many ovules are apparently developed on the apex of the stem, which seems to have been specially adapted as a placenta, but it is also possible and correct in these cases to suppose that the ovules are in reality developed on the carpels.[22] A single fully-developed carpel or a collection of carpels joined together is termed the pistil. The extremity of the carpel, which is specially developed to catch the pollen-grains and form a suitable nidus on which they may germinate, is called the stigma. The united edges of a carpel which bear the ovules are termed the ventral suture. The back of the carpel forms the dorsal suture. The Marsiliaceæ take a position among the Hydropterideæ analogous to that occupied by the Angiosperms; the sporangia are in a corresponding manner enveloped in a closed leaf.

The collection of stamens in a flower is termed the andrœcium, and all the carpels, whether individually free or united into one pistil, the gynœceum.

The Sporangia. The asexual generation of the Mosses is the sporogonium, in which the spores arise in tetrads from the mother-cells. The sporangia in the Filicinæ take their origin either from a single cell (Leptosporangiatæ) or, what probably may be regarded as an older stand-point, from a group of cells (Eusporangiatæ). In both cases there may be distinguished in a mature sporangium three tissues, which have different significance (Fig. [204]): (1) an external layer, the sporangium-wall, most frequently composed of one layer of cells made up of cells of dissimilar structure, so that on desiccation the wall is ruptured and the sporangium opens in a definite manner; (2) an internal group of cells, consisting of the spore-mother-cells, developed from an archesporium, and which by division into four gives rise to the spores; (3) a layer of cells lying between the two already mentioned, which is dissolved before maturity. The intermediate cellular layer, which directly surrounds the spore-forming cells, is in form and contents more worthy of note than the others, and is termed the tapetum. The construction of the sporangium in the Equisetinæ and Lycopodinæ is in the main the same.

Fig. 247.—Development of an anther. A Transverse section of a young anther of Doronicum macrophyllum. The formation of the 4 pollen-sacs commences by divisions of the hypodermal cells (at m, for instance). These cells divide by periclinal walls into external cells which only take part in forming the anther-wall; and internal cells, which correspond to the Archesporium, and from which the spores are derived. These spore-forming cells are drawn with thicker walls in B-E. The commencement of the vascular bundle is seen in the centre. B An older stage; the pollen-sacs already project considerably. It is the cells in the hypodermal layer which are active and in which tangential divisions particularly occur; fv vascular bundle. C A corresponding longitudinal section. D Transverse section through an older anther, the thickness of the wall outside the mother-cells of the pollen-grains is already increased, and it becomes still thicker by the division of the hypodermal cells: its most external layer of cells but one, becomes transformed into the “fibrous cells.” E Transverse section of a still older pollen-sac of Menyanthes; sm are the mother-cells of the pollen-grains surrounded by the tapetum (t), external to the tapetum is the anther-wall, which is still far from being fully developed. The sub-epidermal layer becomes “fibrous,” and the cells lying inside it become dissolved, together with the tapetum.

In the Phanerogams the Microsporangia are termed Pollen-sacs. They take their origin from a large group of cells, which, in the Angiosperms, lie immediately beneath the epidermal cells of the anther. In the developed, but not yet mature, sporangium (pollen-sac) there are to be found: (as in the Vascular Cryptogams) (1) an internal group of mother-cells which give rise to the pollen-grains (microspores), in this case also formed in tetrads; (2) a group of cells surrounding these, of which the internal ones form a tapetal layer, similar to that in the Vascular Cryptogams; the tapetum and some of the cells surrounding it in this group, become dissolved before maturity; the more external ones, on the other hand, are provided with peculiar thickenings, and form the “fibrous” layer by the aid of which the dehiscence of the anther takes place; (3) an external layer, the epidermis, enclosing all the other layers (Fig. [247]).

In some Coniferæ (Cupressus, Thuja, and several species of Juniperus) the microsporangia (pollen-sacs), which are situated on the under side of the stamen, are covered by a thin structure which seems to be a continuation of the lamina and which is supposed to be homologous with the indusium of the Ferns.