28 FIELD AND WOODLAND PLANTS 



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 polUnated 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 thek helpers are at rest. Others 

 require the help of day-flying insects : these are expanded while 

 their fertilisers are on the wing, and sleep tliroughout 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 outHned 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 - 

 poUinate 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 -polUnation ; but, faihng this, polli- 

 nate themselves. 



In the Common Arum or Cuckoo Pint, described on p. 106, 

 we have an example of a flower of j)ecuhar 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. 



Sometimes the flowers of the same species assume two or tlu'ee 

 different forms as far as the lengths of the stamens and pistils are 

 concerned, the anthers of one being of just the same height as the 

 stigma of another, so that the pollen from the former will dust that 

 portion of the body of the insect which rubs against the latter 



