CLEISTOGAMOUS FLOWERS (from Gk. iaelark, closed, and yaps', marriage). In most of our familiar plants the flower is open to the air, so that there is a possibility of cross-fertilization. Indeed, in many plants, such as the primrose, the gentian, etc., self-pollination is rendered difficult or impossible by the struc ture and development of the flower. In other plants, however, among which are the dog violet, Impatiens fulva, Impatiens noli-me tangere, etc., the reproduction of the plant is carned on in part, at least, by flowers that never open, and are hence of necessity self pollinated. These flowers are called cleistoga mous. They are more or less abortive: their petals are absent or stunted and colorless, and they lack the nectar and scent that attract in sects in other blossoms.
No plants are lcnown which possess only cleistogamous flowers. For example, in the dog-violet we find in addition to the fertile cleistogamous flowers the showy but usually sterile blossoms which alone are known to the casual observer. The sterility of the ordinary flowers of the dog-violet is due, not to any in trinsic defect of the pistil or the germs in cluded therein, but to the fact that the anthers often contain no pollen, and still more to the fact that they are not frequented by bees.
When artificially fertilized the open flowers yield as much seed as the cleistogamous ones.
A very interesting fact about some cleis togamous flowers, such as that of the dog violet (Viola canine) is that the pollen-grains give rise to their tubes in situ. These must find their way from the anthers across the cavity of the flower to the stigma before they can fertilize the ovules. The mechanism of this peculiar process is not at all understood.
According to Darwin, the cleistogamous flowers are not merely ordinary flowers in a state of arrested development, as the existence in certain cases of a very complete series of transitional forms between the cleistogamous and tbe perfect flowers might seem to show, but have undergone modifications which fit them especially for self-fertilization. They form a lcind of insurance to the plant against interruption of the more customary, modes of fertilization, and this may have caused their evolution in plants which already had a ten dency to mature some of their flowers before they were completely open. Consult Danvin, C, 'Forms of Flowers' (London 11377).