i3o6 A TEXTBOOK OF THEORETICAL BOTANY 



periodically, curving downwards at night and in bad weather. Other 

 methods of protection are achieved by the presence of a spathe, by the 

 covering of the anthers by the petals or by the complete enclosure of the 

 essential organs, as is so well illustrated by members of the Papilionaceae. 

 Another method exhibited by certain flowers is for the pollen to be dis- 

 charged only in dry weather, while under damp conditions the anthers 

 close up again, thus preserving the pollen for a future occasion. This is 

 well illustrated by such flowers as Sambiiciis, Vttts, Helianthemiim and many 

 others. A possibly unique method is exhibited by Cobaea, in which the 

 pits in the pollen grains are sufficiently deep to prevent the air contained in 

 them from being removed by water, so that the air forms a layer preventing 

 the water from coming into contact with the absorptive surface of the grain. 

 In many plants more than one such protective mechanism may be present, 

 for example the anthers may both close in damp weather and at the same 

 time the flowers may be pendulous. 



There are a number of ways in which a flower may attract and induce 

 insects to visit it, though it would probably be wrong to assume that the 

 visit of an insect is exclusively conditioned through the agency of the flower. 

 Anyone who has watched the activities of an insect striving to discover the 

 means of gaining entry into a flower cannot but conclude that it knows that 

 nectar is present if only it can find the way to reach it. On the other hand 

 it is equally obvious that flowers which are visited by insects exhibit certain 

 characters in common and these features can reasonably be regarded as 

 having been developed to attract the animals to those particular flowers. 

 Among the more important of these features are the following: conspicu- 

 ousness; scent; secretion of nectar; presence of edible sap; provision of a 

 platform upon which the insect may alight; shelter from wind; edible 

 pollen. We have already dealt with these features in general terms (pp. 1253- 

 1262) but further particulars are subjoined, in especial relation to insects. 



Conspiciiousness 



There are two chief ways in which this may be achieved ; either the 

 individual flower may become conspicuous by the development of the 

 perianth or adjacent structures into large and noticeable organs, or the 

 flowers, small in themselves, may be aggregated into closely packed inflores- 

 cences which collectively may become very conspicuous. In some cases 

 the flowers which make up such inflorescences vary in form, the inner 

 flowers remaining small and inconspicuous, while the marginal flowers are 

 considerably larger and increase the conspicuous appearance of the inflores- 

 cence as a whole. These marginal flowers may themselves be actinomorphic, 

 Hke the inner flowers, or they may become zygomorphic, this zygomorphy 

 increasing still more the contrast between the periphery and the centre of 

 the inflorescence. 



Quite apart from the shape of the flower its visibility may be greatly 

 increased according to the colour assumed by these supplementary struc- 

 tures. Colour in flowers is not one of the primary attractions, such as the 



