sandwort (Spergula), and some willow-herbs (Epilobium); or they arch over the pistil, as in Galium aparine and Alisma Plantago. The style is also modified to bring it into contact with the anthers, as in the dandelion, groundsel, and many other plants.[1] All these, however, may be occasionally cross-fertilised.
3. In other cases precautions are taken to prevent cross-fertilisation, as in the numerous cleistogamous or closed flowers. These occur in no less than fifty-five different genera, belonging to twenty-four natural orders, and in thirty-two of these genera the normal flowers are irregular, and have therefore been specially modified for insect fertilisation.[2] These flowers appear to be degradations of the normal flowers, and are closed up by various modifications of the petals or other parts, so that it is impossible for insects to reach the interior, yet they produce seed in abundance, and are often the chief means by which the species is continued. Thus, in our common dog-violet the perfect flowers rarely produce seed, while the rudimentary cleistogamic flowers do so in abundance. The sweet violet also produces abundance of seed from its cleistogamic flowers, and few from its perfect flowers; but in Liguria it produces only perfect flowers which seed abundantly. No case appears to be known of a plant which has cleistogamic flowers only, but a small rush (Juncus bufonius) is in this condition in some parts of Russia, while in other parts perfect flowers are also produced.[3] Our common henbit dead-nettle (Lamium amplexicaule) produces cleistogamic flowers, as do also some orchids. The advantage gained by the plant is great economy of specialised material, since with very small flowers and very little expenditure of pollen an abundance of seed is produced.
4. A considerable number of plants which have evidently been specially modified for insect fertilisation have, by further
- ↑ The above examples are taken from Rev. G. Henslow's paper on "Self-Fertilisation of Plants," in Trans. Linn. Soc. Second series, Botany, vol. i. pp. 317-398, with plate. Mr. H. O. Forbes has shown that the same thing occurs among tropical orchids, in his paper "On the Contrivances for insuring Self-Fertilisation in some Tropical Orchids," Journ. Linn. Soc., xxi. p. 538.
- ↑ These are the numbers given by Darwin, but I am informed by Mr. Hemsley that many additions have been since made to the list, and that cleistogamic flowers probably occur in nearly all the natural orders.
- ↑ For a full account of cleistogamic flowers, see Darwin's Forms of Flowers, chap. viii.
