« PredošláPokračovať »
and hyacinth. The flowering of plants takes place at different periods of the year, and thus a floral calendar has been formed in which each month is characterized by the flowering of certain plants. In addition to this, it is found that the opening of flowers takes place at different times of the day, and thus a sort of floral clock has been constructed in which the hours of the day are indicated by the opening of certain flowers. Some plants open their flowers late at night and are night-flowering. Of this kind is the Cereus grandiflorus, which expands its blossoms about ten P.m. in Britain. During flowering some plants develop heat. This depends on a chemical action going on between the oxygen of the air and the carbon of the flower, by which starch is converted into sugar, and carbonic acid is evolved. In the arums the production of heat is very marked.
The flower consists usually of four parts : the calyx on the outside, or below the other parts ; then the corolla, the stamens, and the pistil (Figs. 20, 21, p. 222). The calyx protects the whole flower;
the corolla guards the internal organs, and at the same time secretes a honey-like matter concerned in the nourishment of the plants ; while the stamens and pistil are connected essentially with the production of fruit and seed, the latter containing the embryo plant, with its nutriment, and preserving it until it is placed in circumstances suited to its growth. There is an adaptation in all these organs, however minute, which show the overruling providence of God in the construction and preservation of the meanest flower.
Each of the four parts of the flower consists of a set of leaves arranged in a circle or whorl. The leaves composing the different whorls are more or less modified, so as to serve the function which they have to perform. The number of parts in each whorl follows the series two, three, and five, or multiples of these. The number three prevails in the flowers of monocotyledonous plants, as seen in the tulip, hyacinth, and lily ; while the numbers two, four, and five, prevail in the flowers of dicotyledonous plants, as in speedwell, willow-herb, and buttercup. The parts in each whorl are arranged alternately with those of the next whorl (Fig. 22).
*(Fig. 22.) Diagram of a flower of four whorls, each of which contains five parts. These parts are arranged alternately, the petals al ernating with the sepals, the stamens with
The outer envelopes of the flower are the calyx and corolla. One or both of these may be wanting. When both are present, the flower is said to have two coverings ; when only one (the calyx), the flower is said to have one covering ; and when both are absent, the flower is said to be naked or to have no covering. The leaves composing these whorls may be separate or united. These characters are taken into account in forming the sub-classes of the natural system of classification.
The stamens and pistil are the essential organs of the flower. They are concerned in the production of seed, and they must be present in order to constitute a flower in a botanical sense. Occasionally the stamens and pistils are in separate flowers, either on the same plant, as in the hazel, or on distinct plants, as in the willow.
The calyx has often a green colour ; sometimes it assumes other colours, as in the globe flower and the aconite, in which it is respectively yellow and blue. The calyx is sometimes united to the ovary (Fig. 23). In the dandelion the calyx of each of the flowers becomes hairy, and remains attached to the fruit so as to scatter it.
The corolla is usually beautifully coloured, the colours being white, red, yellow, and blue. In the corolla reside commonly the odours of flowers.
A stamen consists generally of two parts (Fig. 21, p. 222), a stalk or filament, and an anther or bag containing powdery matter called pollen, which is often yellow. The number of the stamens was taken into account by Linnæus in the formation of some of the classes of his system. Stamens are sometimes united together
either by the stalks or by the anthers.
The pistil consists of modified leaves called carpels, either separate as in the buttercup, columbine, and stonecrop, or united as in the poppy, the chickweed, and the lily. Sometimes one carpel constitutes the pistil, as in the pea. In the double-flowering cherry, the pistil appears as one or two folded leaves, and it does not produce fruit. The parts of a pistil are the ovary 0, (Fig. 23), containing
the ovules or young seeds, and the stigma s, which the petals, and the carpels with the stamens. This is the normal or regular position of the parts.
1 (Fig. 23.) Pistil of a saxifrage with the calyx c partially attached to it. The ovary o is cut open vertically to show the ovules or young seeds. Above, is the style 1, terminated by the stigma 8.
is either placed on the top of the ovary directly, or separated from it by a stalk called the style t. The number of pistils was employed by Linnæus to characterize some of the orders in his system.
The essential organs of the flower are concerned in the production of fruit and seed. The anthers burst in various ways by slits or openings, in order to scatter the pollen which is thus applied to the stigma. There it is retained by means of a viscid secretion, and each of the grains sends out a small tube which descends to the ovule in the ovary. After this the little embryo plant begins to be developed in the ovule. Subsequent to this the fruit is formed, containing the mature seed with the embryo plant inside. Provision is made for the application of the pollen to the stigma. The anthers often burst in an elastic manner, so as to scatter the pollen. The stalks or filaments of the nettle and pellitory are curved inwards, and retained in their place by the covering of the flowers, and when the latter expands they start up in an elastic manner, and disperse the pollen. In the case of the hazel, the flowers containing stamens, and those containing pistils, are separate, and they are produced in spring before the leaves expand ; in this way the broad leaves are not allowed to interfere with the application of the pollen. Insects are often employed to scatter the pollen ; while insinuating themselves into flowers and collecting honey, they at the same time aid the essential organs in their functions.
The Fruit and Seed.
The fruit is the mature pistil containing the seeds. To the fruit the calyx sometimes remains attached (Fig. 23, c), as in the gooseberry and apple. Some fruits are formed by the pistil of one flower, and are called simple, as the strawberry and raspberry; others are formed by the pistils of several flowers, and are called multiple, as the mulberry and pine-apple, cone, fig, and bread-fruit.
When mature, the fruit either bursts to scatter the seed, as in the pea, broom, campion, and iris, or it falls with the seeds in its interior, as the gooseberry, apple, cherry, and peach.
The varieties of cultivated fruit are produced by the art of horticulture, and they are kept up by the process of grafting. When a particular kind of fruit, as the apple or pear, has been improved
Ly high cultivation, slips are taken from the plant, and grafted on well-grown stocks, and thus prized varieties are produced and kept пр. All the fine kinds of apples have arisen from one parent kind, viz., the crab-apple. They can only be produced by cultivation in good soil and by grafting. The seeds of the finest varieties of these fruits, if sown in ordinary poor soil, would revert to the original crab.
A perfect seed contains the embryo plant. Seeds are some times provided with hairs, as in the cotton plant, or wing-like appendages, as in the fir, by means of which they are wafted to a distance. When they reach the soil, and are exposed to moisture and heat, they begin to sprout. We have already given an account of the mode in which the young plant goes through its different phases of germination. In the case of cultivated plants, such as corn, it
of importance that moisture, heat, and air, should be supplied in proper quantity to the plants. Hence the necessity for draining, so as to allow superfluous moisture to be carried off, and the soil to be heated and aërated; well-drained soil is much warmer than water-logged soil.
When a seed germinates or sprouts, chemical changes take place in it, by means of which the starch is converted into sugar, carbonic acid is evolved, and heat is produced. These changes are well seen in the malting of barley; the embryo contained in the grains begins to send out its little roots, heat is developed, and the starchy matter of the grain is changed into sugar.
When this change takes place, the process is arrested by drying the grain, and thus the sugar can be used by man for fermentation.
In ferns, and the lower classes of plants, in place of seeds, minute cells called spores (Fig. 5, p. 212) are formed which serve the purpose of seeds. Ferns have no flowers. They produce leafy fronds which bear cases or bags, in the interior of which the spores are formed (Fig. 24). The brown dust of ferns consists of these spores. The cases often burst by means of an elastic ring surrounding them (Fig. 24 a).
*(Fig. 24.) Spore-case of a fern, supported on a stalk p, and surrounded by a ring a, which by its elasticity causes the case to open, and scatter the spores or germs.
ILLUSTRATIONS TAKEN FROM COMMON WEEDS.
Dicotyledonous Plants.-(Exogenous and Phanerogamous.)
1. THE BUTTER-CUP.— As an illustration of an exogenous dicotyledonous plant, take the common buttercup of the fields. Dig up the plant, and you will find the roots consisting of a number of tapering divisions which end in little fibrils bearing the absorbing points called spongioles. The leaves are of a green colour, and much divided ; some of them are near the root, others are further up the stem, the latter having their divisions narrower than the former. Small leaflets are placed on the flower stalk below the flower; these are bracts. We next come to the flower with its four whorls, calyx, corolla, stamens, and pistil. These whorls are usually arranged in fives or multiples of five. The outer row consists of the greenish-yellow sepals, which alternate with the second row of yellow petals, bearing at their base little depressions covered with scales and secreting a honey-like fluid. Proceeding inwards, we have next the yellow stamens in several rows, with their stalks or filaments supporting the anthers in which the pollen is contained. In the centre of the flower is the green pistil, consisting of numerous small folded leaves called carpels. Each carpel consists of an ovary, style, and stigma, and in the interior of the ovary there is a single ovule. These ovules ultimately become seeds, having the little embryo plant in their interior along with a quantity of nourishing matter ; what are commonly called butter-cup seeds are in reality single-seeded fruits.
2. HEART'S-EASE. The common heart's-ease will form a useful subject of study. Its leaves are of a somewhat oval or egg-like shape, the broadest part of the leaf being next the stem. At the base of the leaves there are remarkably divided leaflets called stipules; these give a peculiar character to the plant. The calyx consists of five green leaves attached to the receptacle a little above their base, so that the base is extended. The corolla is irregular, its five parts being of different sizes ; one of them has a long spur. The stamens are five in number, united by their yellow anthers, which have orange-coloured appendages at their summits. Two of the anthers send small processes or spurs into the hollow spur