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. [Illustration: 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.”