146 ON THE FERTILIZATION OF DISA GRANDIFLORA. 
How ean insects effect the fertilization of the flower? This is 
effected with marvellous simplicity by a very slight change in the 
form of the two upper petals, and in the position of the viscid 
discs of the pollen-masses. 
The upper sepal is of large size, with the basal margins folded 
inwards, and these, together with the two upper petals which 
overlap each other behind, enclose the column, so that insects, to 
reach the nectar, are compelled to approach the flower in front, in 
precisely the same manner as if the labellum secreted nectar. But 
as the column stands in the way of the nectary, insects must push 
their probosces or heads on either side of it, in order to reach the 
nectar. The flower is manifestly constructed to favour this action ; 
for the two upper petals have narrow bases, which leave a small 
open gateway on each side of the column, as may be seen in the 
drawing (B) of these two petals and of the back of the column. 
In all common Orchids the two viscid discs, to which the pollen- 
masses are attached, stand close together or are some way removed 
from each other ; but they always face either the base or the sides of 
the labellum. Inthe Disa the two discs are widely removed from 
each other, and face outwards from the labellum towards the mar- 
gins of the column, as may be seen in the front view (A) of the 
flower. 
It is impossible to doubt the meaning of this unusual position 
of the discs ; for they are thus seated on the inner margins of the 
two gateways or passages which lead to the nectary. Ifa needle 
be inserted through one of these passages, it inevitably touches 
the extremely viscid dise of that side; and when the needle is 
withdrawn, the pollinium is withdrawn. In figs. A and C the 
position of the medial stigma, seated some way beneath the discs, 
may be seen ; and in fig. D the shape of the elongated pollinium, 
attached to a needle, is shown, with the caudicle bent almost at 
right angles near to the dise. In most British Orchids, when the 
pollinia are removed from their cases, the caudicles undergo a* 
movement of depression, caused, as described by Mr. Darwin, by 
the contraction of the discs ; and at the same time they bend either 
outwards or inwards, always in strict relation to the position of 
the stigma. In the Disa there is no movement of this nature, but 
the end of the much-elongated pollinium bends downwards, from 
its weight, and is brought towards the centre of the flower by the 
crookedness of the caudicle ; so that when a needle, with a polli- 
nium attached to it, is inserted into the passage leading to the 
nectary, the end of the pollen-mass strikes the stigma and leaves 
pollen-grains on its sticky surface. Thus in the Disa, notwith- 
