1324 A TEXTBOOK OF THEORETICAL BOTANY 



scatter their pollen on the lower side of its thorax. When the insect with- 

 draws its head, the alae and carina return to their original position. 



Though this type has been selected because of its economic importance, 

 all Humble Bee flowers do not work in the same way. Even in the family 

 Papilionaceae there are a number of different mechanisms, such as the 

 piston and explosive mechanisms, which are operated by Humble Bees, and 

 further mechanisms can be seen in other families. Reference to some of 

 these will be found in the account of the Families of Angiosperms in Chap- 

 ters XXVHI, XXIX, XXX. 



The corollas of certain flowers are so closely folded or the petals so 

 adpressed that only a Humble Bee is strong enough to force an entry. The 

 case of the Snapdragon {Antirrhinum) is very familiar in this connection. 

 Less known is that of Pediciilaris, where the stamens are closely covered by 

 the infolded upper lip of the flower and the lower lip affords a landing- 

 stage. As the bee presses its head into the flower, the edges of the upper lip 

 are forced apart and the pollen is showered down on to the insect. 



Pediciilaris lanata, a species in which these conditions obtain, whereby 

 only a Humble Bee could effect pollination, grows nevertheless in Spitz- 

 bergen, where there are no Humble Bees. Self-pollination is regularly 

 ensured by the backward bending of the style, so that the stigma makes 

 direct contact with the anthers. The same thing occurs in Euphrasia minima 

 and other small-flowered high-alpine species of that genus. No cross- 

 pollination is possible, yet these species do not seem to have suffered, either 

 in seed production or in the vitality of their offspring (see also under 

 Cleistogamy, p. 135 1). 



[c) Bee-Humble Bee Flowers 



Several kinds of intermediate condition exist in which flowers have 

 become adapted to visits of more than one type of insect, which differ in the 

 length of their probosces. These are mostly exceptional cases and of con- 

 siderable interest. There are two distinct principles involved. The first 

 is illustrated by Calamintha alpina, in which there are two distinct forms, 

 differing in the length of the floral tube. The long-tubed form is regularly 

 visited by Humble Bees, while the short-tubed form is pollinated by Hive 

 Bees. Both stocks are alike in other respects and both produce herma- 

 phrodite, protandrous flowers. 



(d) Bee-Butterfly Flowers 



The second case is that illustrated by species of Rhinanthus of the 

 R. hirsutus group (Fig. 1239). These are alpine species, the flowers of which 

 are adapted to pollination both by Humble Bees and by Butterflies. There 

 is a very narrow opening in the corolla immediately under the stigma, 

 through which a butterfly can insert its proboscis and obtain nectar, in the 

 course of which it brings about pollination. Lower down the corolla tube 

 is a second, wider opening, which is so situated that a shorter-tongued 

 insect, like a Humble Bee, can reach the nectar. The anthers are pendent 



