PART IIL. | THE MECHANISMS OF FLOWERS. 459 
5 mm., the tube is so wide that a humble-bee’s head enters it 
easily; a proboscis 10 to 11 mm. long therefore suffices, and 
only B. terrestris and small workers of some other species are 
excluded. 
In its narrower part the tube is three-sided, the lower wall 
standing horizontal, the sides first bending sharply inwards then 
rising almost perpendicularly, and holding the style in their upper 
angle. All three sides widen out gradually for a space of 8 to 10 
mm. from the base of the tube, then, on a sudden, more rapidly, 
the sides rising up into a hood for the pollen-reservoir, and the 
floor increasing from 3 mm. to 5 mm. in breadth. In this wide 
part of the tube the lower part of the side walls is doubled down 
upon the floor by a re-entrant fold (a, 2, 3), so that the breadth 
of the entrance is reduced to scarcely 3 mm.; and this is further 
reduced in height to 1 to 2 mm. by two dark-yellow pouches in the 
under lip (pathfinders, b, 3), and by a fold close behind the free 
edge of the hood-like upper lip; if, however, the folds in the side 
walls and in the upper lip are opened out by a bee thrusting in its 
head, there is plenty of room in the broad part of the tube for the 
bee’s head, which is about 5 mm. broad, and 3 mm. high. So by 
this peculiar conformation of the mouth, weak, short-lipped insects 
are excluded, and at the same time humble-bees are permitted 
to insert their heads. There are yet other peculiarities in the 
mouth of the flower which complete this result. The filaments 
which, in the narrow part of the tube, are thin and adhere to the 
corolla, become free from it in the wide part, and rise obliquely 
upwards as stiff, broad rods beset with teeth on their inner sides, 
towards the hooded upper lip, which incloses the stamens. The 
two anterior stamens lie, in their ascending portions, so close 
behind the entrance of the flower that they block it for a further 
space of } mm. on either side. So in looking from the front into 
the mouth of the flower we see an opening only 2 mm. broad and 
scarcely so high, bounded on the right and left by the prickly 
margins of the filaments, above by the soft. tufts of hairs on the 
upper lip, and by the weak hairs projecting downwards from the 
pollen-receptacle (d, 3). The bee in inserting the delicate tip of 
its proboscis carefully avoids any rough contact ; therefore, in this 
flower, it directs it neither to the right or left, where it would 
meet the sharp points upon the stamens, but upwards where it 
rubs against the soft hairs of the upper lip and the pollen- 
receptacle in passing into the tube. This course is essential for 
| eross-fertilisation, 
