810. Fertilization.—One of the sperms unites with the egg in the embryo sac. This is fertilization, and from the fertilized egg the young embryo is formed still within the ovule. Double fertilization,—the other sperm cell sometimes unites with one or both of the “polar” nuclei which have united to form the “definitive” or “endosperm” nucleus. As a result of fertilization, the embryo plant is formed within the ovule, the coats of which enlarge by growth forming the seed coats, and altogether forming the seed. (See [Chapters XXXIV], [XXXV], [XXXVI].)
[II. Kinds of Flowers.]
811. Absence of certain flower parts.—The complete flower contains all the four series of parts. When any one of the series of parts is lacking, the flower is said to be incomplete. Where only one series of the floral envelopes is present the flowers are said to be apetalous (the petals are absent), examples: elm, buckwheat, etc. Flowers which lack both floral envelopes are naked. When pistils are absent but stamens are present the flowers are staminate, whether floral envelopes are present or not; and so when stamens are absent and pistils present the flower is pistillate. If both stamens and pistils are absent the flower is said to be sterile or neutral (snowball, marginal or showy flowers in hydrangea). Flowers with both stamens and pistils, whether or not they have floral envelopes, are perfect (or hermaphrodite), so if only one of these sets of essential organs of the flower is present the flower is imperfect, or diclinous. Sometimes the imperfect, or diclinous, flowers are on the same plant, and the plant is said to be monœcious (of one household). When staminate flowers are on certain individual plants, and the pistillate flowers of the same species are on other individuals, the plant is diœcious (or of two households). When some of the flowers of a plant are diclinous and others are perfect, they are said to be polygamous.
Many of these variations relating to the presence or absence of flower parts in one way or another contribute to the well-being of the plant. Some indicate a division of labor; thus in the neutral flowers of certain species of hydrangea or viburnum, the showy petals serve to attract insects which aid in the pollination of the fertile flowers. It must not be understood, however, that all variations in plants which results in new or different forms of flowers is for the good of the species. For example, under cultivation the flowers of viburnum and hydrangea sometimes are all neutral and showy. While such variations sometimes contribute to the happiness of man, the plant has lost the power of developing seed. In diclinous flowers cross pollination is necessitated.
812. Form of the flower.—The flower as a whole has form. This is so characteristic that in general all flowers of the different individuals of a species are of the same shape, though they may vary in size. In general, flowers of closely related plants of different species are of the same type as to form, so that often in the shape of the flower alone we can see the relationship of kind, though the form of the flower is not the most important nor always the sure index of kinship. Since many flowers resemble certain familiar objects, names are often used which relate to these objects.
Flowers are said to be regular, or irregular. In a regular flower all of the parts of a set or series are of the same shape and size, while in irregular flowers the parts are of a different shape or size in some of the sets. The flowers of the pea family (Papilionaceæ), of the mint family (Labiatæ), of the morning glory, larkspur, monkshood, etc., are irregular ([fig. 450]). The corolla usually gives the characteristic form to the flower, and the name is usually applied to the form of the corolla.
Fig. 450.
Several forms of flowers. Regular flowers. wh, wheel-shaped corolla; sa, salver-shaped; tub, tubular-shaped. Irregular flowers. pa, butterfly or papilionaceous; per, personate or masked flower; lab, gaping or ringent corolla. The two latter are called bilabiate flowers.
Some of the different forms are wheel-shaped or rotate corolla when the petals spread out at once like the spokes of a wheel, as in the potato, tomato, or bittersweet; salver-shaped when the petals spread out at right angles from the end of a corolla tube, as in the phlox; bell-shaped, or campanulate, as in the harebell or campanula; funnel-shaped, as in the morning glory; tubular, when the ends of the petals spread but little or none from the end of the corolla tube, as in the turnip flower or in the disk florets of the composites. The butterfly, or papilionaceous corolla is peculiar as in the pea or bean. The upper petal is the “banner,” the two lateral ones the “wings,” and the two lower the “keel.”