THE ANGIOSPERMAE 1261 



(thrum-eyed Primula and species of Erica) or by folding of the petals 

 {Antirrhinum and the alae in Papilionaceae). The result of all such contri- 

 vances is that only strong and long-tongued insects can force an entrance 

 to the nectar, and in doing so cause pollination. Horizontal zygomorphic 

 flowers may provide sufficient shelter for their nectar by means of the 

 perianth alone, especially if this is sympetalous, the tube forming a natural 

 concealment. Even in these flowers there may be additional protection 

 afforded, sometimes by stiff hairs in the tube, sometimes by the infolding 

 of the petals, as we have already mentioned, and often by spurs, bags or 

 pouches either formed from parts of the perianth, or sunk in the tissue of the 

 receptacle, in which the nectar collects until an insect of the right type comes 

 to find it. 



Hermann von Miiller in his observations on the pollination of alpine 

 plants, published in 1881, sought to establish a hierarchy of colours among 

 flowers, as related to pollinating visitors. He regarded yellow^ and white as 

 relatively primitive colours and blue as the most highly evolved. There is 

 much in this which is perhaps too fanciful, but Loew, working in the 

 environment of the Berlin Botanic Gardens, where a great variety of flowers 

 and insects, many of them strangers, are brought together, determined that 

 pollinating insects visited mostly the darker-coloured flowers, while 

 " unbidden guests " such as flies preferred the lighter colours. 



Investigations on the Hive Bee show that it has no innate colour pre- 

 ferences, but that it quickly learns to associate a particular colour with 

 nectar and can select that colour from a number of others. Bees have, how- 

 ever, very short sight, and it seems probable that they are only guided by 

 colour at short range. On the other hand many insects have an extraordin- 

 arilv keen sense of smell, which is probably the guiding influence at long 

 range. Some flowers have markings on the corolla which have been known 

 as " honey guides " ever since they were first pointed out by Sprengel, 

 and it has been claimed that these markings have scents differing from those 

 of the rest of the corolla. If this is true their smell may also be important 

 at short range. 



It is not only the colours themselves but their contrasts w^hich lend 

 conspicuousness to a flower and many striking contrasts exist. Such are, 

 for example, the brilliant colour-patterns on the petals of Tigridia, the 

 differently coloured standard and fall petals in many Irises, the contrasting 

 colours of disc and ray-florets in many Composites, or the contrast of green 

 flowers and coloured bracts as in Poinsettia and some Euphorbias. Even a 

 dimlv coloured flower may be made to stand out strikingly if provided with 

 the right background, and this may frequently be noticed, as for example 

 in the widely spread Composite genus Helichrysum, where the disc-flowers 

 are weakly coloured, but the many bracts of the involucre are white or 

 brilliantly tinted with red or yellow. 



Among Butterflies, some similarity between the colour of the insect 

 and the colour of the flowers it visits was noted by Hermann Miiller in the 

 Alps. Many alpine Butterflies are red-coloured and appear to choose red 



