1322 A TEXTBOOK OF THEORETICAL BOTANY 



When an insect visits the flower it ahghts on the wing petals. Since these 

 petals are adherent, at the base, to the keel petals, its weight causes the wing 

 and keel petals to be depressed simultaneously. The bee now thrusts its 

 proboscis into the flower in search of the nectar, which is copiously dis- 

 charged into the cavity between the ovary and the staminal tube. In so 

 doing the keel is further depressed and the young stamens discharge their 

 pollen underneath the body of the bee. In a later stage of anthesis, when the 

 stigma is ripe, it occupies a position similar to the anthers, but the tip 

 projects above them so that it is the first organ to touch the body of the 

 visiting insect. It will therefore tend to receive foreign pollen. After the 

 insect has left the flower, the weight being removed from the wing petals, 

 the keel closes up again over the stamens and once more protects them. 

 Both keel and vexillum efficiently protect the pollen from rain or dew, as 

 well as preventing small insects from robbing the flower of its nectar. 



(b) Humble Bee Flowers 



These flowers have longer corolla tubes than those of the bee flowers, 

 that is to say the tube is more than 7 mm. long, thus precluding any shorter- 

 tongued insects from reaching the nectar. Such flowers may however be 

 pollinated by Butterflies and Moths, while some Hover Flies seem to visit the 

 flowers as well. Bees with short probosces, however, often rob the flowers 

 by biting through the corolla tube and reaching the nectar that way. 

 The Hive Bee will sometimes rob Red Clover, making use of holes already 

 cut by short-tongued Bees who are excluded from the flowers. In many 

 instances Lepidoptera with very long tongues can reach the nectar through 

 the natural entrances but fail to cause pollination. In some instances pol- 

 lination is restricted to a single species of Humble Bee, owing to the depth 

 at which the nectar is secreted, and such flowers are restricted in their dis- 

 tribution to those parts of the world where the natural pollinator occurs. 



Among the common Humble Bee flowers we may mention the following: 

 Trifolium pratense (Red Clover); Aqiiilegia vulgaris (Columbine); Antir- 

 rhinum majus (Snap Dragon); Digitalis purpurea (Foxglove); Atropa bella- 

 donna (Deadly Nightshade); Pedicularis sylvatica (Lousewort); Salvia 

 pratensis (Sage) and many others. 



The Red Clover is by far the most valuable fodder plant for store cattle. 

 About 40 per cent, of the bee visitors to this flower are seeking pollen. It 

 can be worked for nectar by the Hive Bee only when the nectar has accumu- 

 lated to a depth of i -7 mm., which may happen after nights of heavy dew. 

 In spite of this limitation and the old belief that the Humble Bee is the most 

 important pollinator, careful observation shows that about 80 per cent, of the 

 flowers are Hive Bee pollinated during pollen-collecting flights, and only 

 about 15 per cent, by the Humble Bees, which are much less energetic. 



Butler after careful investigation found only about 7 per cent, of the 

 pollen loads of the Hive Bee containing mixed pollens, so the bees must 

 be remarkably species-constant on their journeys and hence are most 

 efi"ective as cross-pollinators. 



