218 THE FERTILISATION OF FLOWERS. [PART IIT, 
incloses the reproductive organs and protects them from rain and 
from pollen-feeding insects. The two lateral petals (ale) have 
a threefold function, serving (1) as a platform for bees to alight 
on; (2) as a lever to depress the carina; (3) to keep the carina in 
its place as regards the reproductive organs, and to bring it back to 
—_— 
its place after depression if repeated insect-visits are necessary for — 
fertilisation. In order to serve as levers for depressing the carina, 
the ale must be united firmly therewith. This is either accom- ~ 
plished by certain areas of the alar laminz bulging out and being 
received in hollows of the carina, or by interdigitation of processes 
on the contiguous surfaces of both; and such union is the firmer 
the more frequently the carina has to be depresséd and again 
brought back to its original position in order to ensure cross- 
fertilisation. 
The carina is mainly kept in its proper place and brought back 
to it after depression by basal lobes of the alee which embrace 
the column; sometimes they form swollen pouches (Trifolium), — 
sometimes they are produced into long processes (Melilotus, 
Medicago, ete. ). 
The vexillum, with its large, erect, coloured surface, is the 
chief agent in rendering the flower conspicuous; it also serves 
as a fixed point or fulcrum against which a bee may place its head 
while it pushes the carina with its feet resting upon the ale. 
The stigma and pollen must be applied to the ventral surface 
of the bee if cross-fertilisation is to result. The necessary 
arrangement is attained by the filaments cohering to form a ~ 
cylinder round the pistil. In all Papilionacese which contain 
honey, the honey is secreted on the inner sides of the bases of the 
filaments, and it accumulates in the space between the stamens 
and pistil. Since the reproductive organs have to come in contact 
with the under side of the bee, it is clear that access to the honey 
must only be permitted above the reproductive organs. We find, — 
accordingly, that in all Papilionaceee which contain honey the 
coherent filaments leave a passage superiorly. One stamen here 
is separate from the rest and leaves two entrances to the honey 
free on either side of its base, either by curving upwards at its 
base, or by the neighbouring filaments curving outwards there, 
or in both modes combined. 
The arrangement and action of the various parts of the flower 
in which all the Papilionaceze (except Onobrychis) agree, cause the 
bees to act and move in a perfectly definite way, such as to ensure 
the stigma and pollen coming in contact with their under surfaces. 
