Fig. 23.—Spring Succession of Leaves of Mature Plant of Arrow-head (Sagittaria sagittifolia). 1/3.
problem of avoiding submergence; and the stomata are confined to the upper side, which alone is in contact with the atmosphere. Those water plants which raise their leaves into the air, on the other hand, have leaves of a variety of shapes, which in most respects approach those of land plants. An interesting progression of leaves illustrating all three stages may be watched in spring in the Arrow-head (Sagittaria sagittifolia). The first leaves produced are entirely submerged, and conform to the usual ribbon shape and delicate texture. Those which follow float on the surface. In them the lower part is contracted into a flaccid winged petiole, the upper part being expanded into an oblong floating blade with a waxy surface to keep the leaf dry on the upper side. These in turn give way to the characteristic aerial arrow-shaped leaves of summer, which approach in character the leaves of land plants, and are borne on stout, stiff petioles capable of resisting wind and wave.
Coming now to FLOWERS, it is possible here to refer only to a few macroscopic or “naked-eye” characters and modifications; the full study of the flower and its essential functions being a matter for the laboratory and the high-power microscope, as very minute structures are involved. As briefly described in [Chapter IV.], flowers are groups of modified leaves arranged mostly very close together at the ends of branches, the tip of the shoot being often expanded into a receptacle (very well seen in the Compositæ—e.g., Dandelion) for the accommodation of the crowded floral leaves. Just as the foliage leaves have become modified to carry on to the best advantage the process of assimilation, so the different series of floral leaves are specially adapted to their several functions. The sepals, which compose the calyx, having usually a protective rôle, in most cases enclose the young flower with a tough envelope; they usually retain their primitive green colour, and take part in the process of assimilation. They may drop off as the flower opens (e.g., Poppy), or wither as the petals wither, or remain fresh until the fruit is ripe. Sometimes, as in many Ranunculaceæ (compare Anemone, Caltha, Helleborus), they take on the advertising rôle usually assigned to the petals, being large and coloured, while the petals themselves are minute. In the Monocotyledons they usually join with the petals in adorning the flower. The next whorl, lying inside (that is, above) the sepals, is formed of petals, constituting the corolla. The connection of colour and form of petals with the visits of insects, and their relative insignificance in wind-pollinated flowers, has already been referred to ([p. 81]). The marvellous variety of colour and form observable in the corolla has for its main object the attracting of insects to the flower. The petals have departed much farther from the ordinary leaf-form than the sepals. They assume brilliant hues of every tint, the pigment being due either to colouring matter dissolved in the cell-sap (pinks and blues) or to small coloured solid bodies (chromoplasts) contained in the cells (reds and yellows). Chlorophyll being absent, the coloured petals do not assist assimilation: they are purely advertisements, though incidentally they often fulfil a useful protective rôle for the important organs which they surround. In this latter connection their sensitiveness to changes of light and temperature, which causes them to close in dark or cold weather, is a very familiar phenomenon; as is also the excellent protection which they provide in flowers such as those of the Labiatæ, where, fused together into a tube, they form a kind of cave in which the stamens and pistil nestle securely.
Fig. 24.—Fruit of Coriaria japonica. 1/1.
An exceptional use of petals, where indeed they are used for the purposes of advertisement, but to secure the dispersal not of the pollen, but of the seeds, is illustrated in [Fig. 24]. In the genus Coriaria the staminate and pistillate organs are borne on separate flowers. The flowers of both kinds are small and inconspicuous. But in the “female” flowers the petals persist after flowering, and, becoming fleshy and comparatively large, enclose the seed in a pulpy berry-like envelope, which no doubt serves the same purpose as a true berry in securing seed-dispersal by being devoured by birds. In C. terminalis, which comes from the Himalayas, the “ripe” corolla is bright orange; in C. japonica, from Japan, it is at first coral-red, and when mature velvet-black.
The stamens, which form the next ring (sometimes a double ring or a close spiral), are much less leaf-like than the sepals or petals, yet there can be no doubt that they are descended from leaf-shaped organs; this is especially clear from the study of certain primitive fossil types, in which the corresponding organs which bear the pollen are actually leaf-like. In most of the present-day Seed Plants the stamens conform to a uniform type—a slender stalk (filament) bearing a head (anther) containing four chambers, in which are produced pollen grains, which escape when the flower is mature by the splitting of the enclosing walls. The ways in which the pollen is then conveyed to the pistil of other flowers have been referred to briefly on a previous page ([p. 82]). The stamens in many flowers are few, and their number usually bears a relation to the number of the other floral parts; in other flowers, for instance Rose and St. John’s wort (Hypericum), they are of large and indefinite number. The peculiar arrangement of the pollen in Orchids has been already noted ([p. 94]).
The final ring of modified leaves in our typical flower constitutes the pistil, formed of one or many carpels, the essential structure of which has been touched on already ([p. 82]). In the present place it is desired only to point out some of the leading modifications which the pistil undergoes, so that its structure as seen by the naked eye may be understood. In the simpler forms of carpel, the affinity to leaves is still evident, though in forms of pistil made up of a number of carpels this may be very difficult to trace. With the Pea, for instance, we may begin, as presenting a very simple example. Take an oblong leaf like that of a Laurel, and fold it down the mid rib till the two edges are in contact. There is our pea-pod complete. The young seeds, or ovules, are borne in a row along the mid rib, a very usual arrangement. Examine next the young fruit of a Columbine (Aquilegia). Here there is a group of five separate erect carpels, but each is essentially like a pea-pod in structure. Compare the fruit of a Saxifrage. This clearly consists of two carpels which are grown together save at the tips, where the two styles stand out like little horns. From this we may go on to other pistils in which several carpels are completely fused together. Next, the compact body thus formed may be sunk down in the expanded top of the stem (the receptacle). Or the other parts of the flower—sepals, petals, stamens—may in their lower part be fused with the walls of the pistil, and may thus appear to spring from the top of it. In such cases the structure of the flower may easily be wrongly interpreted, and reference to a work on systematic botany is necessary if pitfalls are to be avoided. It is indeed to be noted that in flowers, as in other parts of plants, complicated structure or multiplication of parts is not necessarily an indication of advanced evolution; on the contrary, it is often indicative of a primitive condition. Just as in machinery or in organized human effort simplification often accompanies improvement, so it is with plant structures. Many of the more primitive types of flowers, such as Buttercups or Water Lilies, have a multitude of petals or stamens or carpels, while in many of the most specialized, such as Composites or Campanulas, the number of parts is much reduced. The primitive wind-pollinated flowers produce large quantities of pollen; in those which have adopted the improved method of utilizing insects, the amount of pollen is much less; in the highly specialized Orchids, a most successful group, the pollen is reduced to two small bundles.
Once the act of pollination is effected, the duty of the petals and stamens is finished, and they generally fade. The sepals often remain, as in the Rose. By the growth of the pollen tube from the stigma into the ovary, fertilization is effected, and mature seed is produced. The fruit—that is, the seed and its coverings or appendages—offers the most varied forms of any of the plant organs—compare Hazel, Strawberry, Pea, Apple, Cranesbill, Dandelion; the variety is endless. Many of these forms are connected with the means by which seed-dispersal is effected: this subject has been touched on in [Chapter III.] But in numerous instances we can no more assign a reason for their beautiful or fantastic forms than we can account for the infinite variety of shape assumed by leaves and flowers.