In the family of the composites we find a group of genera with two forms of florets on each flower-head. The hermaphrodite ones are tubular with 5, or rarely 4, equal teeth, and occupy the center of the head. These are often called the flosculous florets or disk-florets. Those of the circumference are ligulate and ordinarily unisexual, without stamens. In many cases they are sterile, having only an imperfect ovary. They are large and brightly colored and are generally designated as ray-florets. As instances we may cite the camomile (_Anthemis nobilis_), the wild camomile (_Matricaria Chamomilla_), [131] the yarrow (_Achillea Millefolium_), the daisies, the Dahlia and many others. Species occur in this group of plants from time to time that lack the ray-florets, as in the tansy (_Tanacetum vulgare_) and some _artemisias_. And the genus of the marigolds or _Bidens_ is noted for containing both of these types. The smaller and the three-toothed marigold (_B. cernua_ and _B. tripartita_) are very common plants of wet soil and swamps, ordinarily lacking the ray-florets, and in some countries they are very abundant and wholly constant in this respect, never forming radiate flower-heads. On the other hand the white-flowered and the purple marigold (_B. leucantha_ and _B. atropurpurea_) are cultivated species of our gardens, prized for their showy flower-heads with large white or deeply colored, nearly black-purple florets. Here we have opportunity to observe positive and negative varieties of the same character. The smaller, and the three-toothed marigold occur from time to time, provided with ray florets, showing a positive variation. And the white marigold has produced in our gardens a variety without rays. Such varieties are quite constant, never returning to the old species. Positive and negative varieties of this kind are by no means rare among the compositae. [132] In systematic works the positive ones are as a rule called "radiate," and the negative ones "discoid." Discoid forms of the ordinary camomile, of the daisy, of some asters (_Aster Tripolium_), and of some centauries have been described. Radiate forms have been observed in the tansy (_Tanacetum vulgare_), the common horse-weed or Canada fleabane (_Erigeron canadensis_) and the common groundsel (_Senecio vulgaris_). Taken broadly the negative varieties seem to be somewhat more numerous than the positive ones, but it is very difficult to come to a definite conclusion on this point. Quite the contrary is the case with regard to the color-varieties of red and blue flowers. Here the loss of color is so common that every one could give long lists of examples of it. Linnaeus himself supposed that no blue or red-colored wild species would be without a white variety. It is well known that he founded his often criticized prescript never to trust to color in recognizing or describing a species, on this belief.