The chief characteristic of the Phanerogams does not lie in the formation of the flower (although they may quite properly be termed “Flowering-plants”), because Equisetums and Lycopods have reproductive shoots as highly differentiated as those of certain Gymnosperms and other Phanerogams. As regards the SEXUAL GENERATION the characteristics are found:—(1) in its great reduction; (2) in the transmission of the microspore (pollen-grain) to the macrosporangium, and its germination, with the formation of a pollen-tube (antheridium), the protoplasm of which is not differentiated into spermatozoids; (3) in the fact that the macrospore (embryo-sac) never leaves its sporangium (nucellus); and further in the Angiosperms, (4) in the peculiar development of the nutritive-tissue in two parts; and (5) in the great reduction of the archegonium.
As regards the ASEXUAL GENERATION the characteristic feature is that this generation is formed whilst the sporangium is still attached to the mother-plant, and for a long time is nourished by it; and that after the sporangium has become detached from the mother-plant, it spends a longer or shorter resting period as the embryo in the seed (enveloped by the testa), and does not make its appearance until the “germination” of the seed. In addition the shoot which bears sporangia undergoes greater modification than in the case of the Flowerless-plants.
The Phanerogams are separated into two Divisions as follows:—
Division 4. Gymnospermæ. The ovules, as well as the seeds, are borne naked on the surface of open carpels, or on the apex of a stem (ovary wanting). The pollen-grains are conveyed by the wind to the ovules, and caught by drops of mucilage, secreted by the micropyle. A “stigma” is wanting. The entire female prothallium (the endosperm), which serves for the nourishment of the embryo, is formed before fertilisation. The archegonia are embedded in the upper part of the prothallium. The pollen-grains are “multicellular,” i.e. there is always in their interior a distinct prothallium, formed by 1–3 cells, and a larger cell which gives rise to the pollen-tube.
Division 5. Angiospermæ. The carpels surround the ovules and form an entirely closed chamber (ovary), in which the ovules mature and ripen into seeds. The surface of a portion of the apex of the carpel is transformed into the “stigma,” which, by a sticky fluid and also by hair-structures, is capable of retaining the pollen-grains conveyed to it by the wind, or more frequently by insects. The pollen-tube grows from the stigma, through the “conducting cellular tissue” (style), to the ovules. The pollen-grains contain two cells, a vegetative and a free generative cell. The latter passes into the pollen-tube and there divides into two, one of which is the sperm-nucleus. The female prothallium, which is intended to serve as nutritive-tissue, is formed after fertilisation. Archegonia are wanting.
DIVISION IV.
GYMNOSPERMÆ.
The following characters should be added to those already given on page [2]:—
The Gymnosperms comprise only trees or shrubs. The flowers are always unisexual and destitute of perianth (except Gnetaceæ); the female plant of Cycas is the only one which has no flower. The MALE FLOWERS are constructed on the same type as the cones of the Horsetails and Club-Mosses, and are most frequently long shoots (Figs. [243], [258], [260] A, [267] J) bearing a number of spiral or verticillate stamens. The FEMALE FLOWERS are of a more varied structure (see the orders). The OVULE is orthotropous (except Podocarpus which is anatropous) and projects from the carpel uprightly, inverted, or horizontally; it has usually only one integument (compare however Taxaceæ) which proceeds from the upper part of the nucellus, so that the embryo-sac in part is placed below the integuments (Figs. [251], [264]). The drop of mucilage which catches the pollen-grains dries up and draws the pollen-grain through the micropyle to a space just above the nucellus—the pollen-chamber—in which the germination of the pollen-grain commences.
In each seed, only one of the many embryos which are formed proceeds to its full development. The seed is always endospermous, and the embryo has one, two, or a whorl of several cotyledons. A vigorous primary root is developed on germination. The vascular bundles in the stem are arranged in a ring, and increase in thickness takes place by a closed cambium-ring which forms bast (phlœem) externally, and wood (xylem) internally with distinct annual rings, as in the Dicotyledons. Only certain of the Cycadeæ deviate from this arrangement. The secondary wood is very uniform, as it is almost exclusively formed of tracheides with bordered pits, but true vessels are wanting; this also indicates a relationship with the Pteridophyta (see page [202]).
The Gymnosperms are biologically lower than the Angiosperms; they are wind-fertilised, and without extra floral-nectaries.