As we watch the nectar-feeding insects at work, we not only observe that the flowers they visit possess the general characters given above as common to the insect-pollinated species, but also that, in many instances, the structure of the flower is such that the transfer of pollen from anthers to stigma could only be accomplished by the particular kind of insect which it feeds. Various contrivances are also adopted by many flowers to attract the insects which are most useful to them, and to exclude those species which would deprive them of nectar and pollen without aiding in the work of pollination. Thus, some flowers are best pollinated by the aid of certain nocturnal insects, which they attract at night by the expansion of their pale-coloured corollas and by the emission of fragrant perfumes. These close their petals by day in order to economise their stores and protect their parts from injury while their helpers are at rest. Others require the help of day-flying insects: these are expanded while their fertilisers are on the wing, and sleep throughout the night. We do not propose to give detailed accounts of the various stratagems by which flowers secure the aid of insects in this short chapter. Several examples are given in connexion with the descriptions of flowers in subsequent pages, but a few typical instances, briefly outlined here, will give the reader some idea of features which should be observed as flowers are being examined. In many flowers the anthers and the stigma are not mature at the same time, and consequently self-pollination is quite impossible. With these it often happens that the anthers and stigma alternately occupy the same position, so that the same part of the body of an insect which becomes dusted with pollen in one flower rubs against the stigma of another. Other flowers, such as the Forget-me-not, in which both stamens and stigma are ripe together, project their stigmas above the stamens at first, in order that an insect from another flower might touch the stigma before it reaches the stamens, and thus cross-pollinate them; and their stamens are afterwards raised by the lengthening of the corolla until they touch the stigma. Thus the flowers attempt to secure cross-pollination; but, failing this, pollinate themselves. In the Common Arum or Cuckoo Pint, described on p. 106, we have an example of a flower of peculiar construction, surrounded by a very large bract in which insects are imprisoned and fed until the anthers are mature, and then set free in order that they might carry the pollen to another flower of which the stigmas are ripe.