vant ut| THE MECHANISMS OF FLOWERS. 205 
full-grown, and surround the brushes with pollen; they themselves 
are withdrawn from the expanded part of the carina by contraction 
Bot the filaments. As soon as this has taken place, the blade of the 
 vexillum, which is adorned with dark lines, becomes erect, and its 
sides bend backwards ; the ale arch outwards, forming a convenient 
platform for bees, and the flower, which had begun to secrete honey 
abundantly in the later stages of the bud, is now in every respect 
ready for its visitors. 
Though the mechanism of the flower resembles that of V. 
Cracca except in regard to the brushes on the style, its larger size 
and some apparently slight differences in structure, cause a great 
_ difference in the number of visitors, and exclude flies and Lepid- 
_ optera, which in V. Cracea suck the honey without benefiting 
_ the flower. 
- The following conditions produce this result :— 
(1) The flowers are not only considerably larger, but the petals 
_ are notably thicker and stronger, so that much more force is needed 
to push apart the vexillum and alz than in V. Craceca. 
(2) The calyx-tube surrounds the claws of the petals for a 
greater distance, 
| (3) In V. Cracca as well as in V. sepiwm the entrance between 
yexillum and alz is firmly closed by two grooves on the upper 
surface of the vexillum where the claw joins the blade, which 
project on the lower surface as two ridges fitting tightly to the ale. 
n V. Cracca these are thin-walled, and even an Hmpis can insert 
its proboscis beneath the vexillum; but in V. sepium they are much 
thickened. ; ‘ 
(4) The ale in V. sepium form a relatively much shorter lever 
for depressing the carina than in V. Cracca. 
_ Owing to all these circumstances, it is only the most powerful 
bees, viz., Bombus and Anthophora, which can possess themselves of 
the honey in a legitimate way. 
_ Thisis an advantage, in so far as it completely excludes flies and 
Lepidoptera, which in V. Cracca often steal the honey without being 
of any use to the plant ; but it brings with it a disadvantage which 
probably quite neutralises the advantage. For Bombus terrestris, 
finding it difficult to reach the honey legitimately, regularly bites a 
hole in the side of the flower through calyx and corolla, and so 
inserts its proboscis. In many cases it is hard to find a flower 
| which has not been robbed in this manner, and often even unopened 
| buds are robbed. The difficulty of depressing the carina so far as 
} is needful in collecting pollen, prevents those bees which suck 
; 
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