THE AXGIOSPERMAE 1153 



bouring organs; while in a large number none of these things happen and 

 the petals die from unknown internal causes, attributed by some writers to 

 inherent instability of their protoplasm, which is not very enlightening. 



The general functions of the perianth in anthesis may be summed up 

 under two headings: (i) The attraction of pollinating insects and, to a more 

 limited extent, the regulation of their entry into the flower. (2) The pro- 

 tection of pollen and nectar from the weather and from robbery by non- 

 pollinating insects, such as flies. The first function is naturally limited to 

 insect-pollinated flowers and is associated with bright colours and often 

 with perfume. This function of the perianth as an " advertisement " 

 and as a regulator of insect movement, we shall deal with more particularly 

 when we speak about pollination (see Chapter XXIV). It is no longer a 

 " naive assumption ", as Goebel calls it, that insects are colour sensitive, 

 for much experimental evidence shows that they can have markedly selective 

 preferences in this respect. It is true that diiferential coloration of the 

 reproductive organs is a very widespread phenomenon in the plant world 

 and is found in all groups from the Algae to the Gymnospermae, but there 

 can be little doubt that its remarkable development in the flower is asso- 

 ciated with the habit of insect-pollination, which has been in so many 

 respects the deiis ex machina of floral evolution. 



De CandoUe divided flowers, according to their colours, into a xanthic 

 or vellow series and a cyanic or blue series. This classification does in fact 

 correspond to an important difference between those flowers which carry 

 coloured plastids displaying yellow, orange or red, due to carotenoid pig- 

 ments, and those flowers which carry soluble anthocyanin pigments of red, 

 pink, purple and blue shades. The dehcate yellows of the Primrose type 

 really belong to this latter series, as they are due to the related pigment class 

 of soluble anthoxanthones. Grant Allen suggested that flower colours pre- 

 sented an evolutionar\' sequence, in which yellows and whites predomi- 

 nated among the more primitive polypetalous flowers, especially those 

 pollinated by small flies and beetles, while anthocyanin colours are com- 

 moner among the more advanced sympetalous flowers, which are chiefly 

 pollinated by bees. Lubbock and others having shown that bees have a 

 preference for blue, Allen concluded that natural selection would tend to 

 produce blues as the end of the progression. As a broad general principle 

 this mav be true, but it is an over-simplification of the facts, for the relations 

 of the bees to flowers are very complex and by no means fully known. Some 

 bees are monotropic, that is visit only one type of flower. The individual 

 hive bee may also be monotropic, but the great stores of hive honey demand 

 a wide range of flower-visits and the colony as a whole is polytropic. The 

 choice of flowers is not controlled by colour alone, but even more impor- 

 tantly by the abundance and the concentration of the nectar provided. 

 Humble bees are perhaps more influenced by flower colour in their visits, 

 and most of the large flowers, among which blues and purples are common 

 colours, are pollinated by humble bees, not by the short-tongued hive bees. 



Colour patterns on petals are commonest in zygomorphic flowers, 



