PART IY. | GENERAL RETROSPECT. ‘585 
as yet also insufficient to show us how the circle of visitors to 
any particular plant is determined by the time of flowering, the 
nature of the locality, the competition of other flowers, and the 
peculiarities of structure in the flower itself. 
It is obvious that flowers which only expand at night are 
thereby protected from the visits of all merely diurnal insects, 
but among our native flowers we have scarcely a distinct example 
of this condition. Of the flowers mentioned above as adapted for 
hawk-moths and nocturnal Lepidoptera, Lychnis vespertina expands 
in the evening, without, however, being completely closed by day ; 
while the others exclude diurnal insects from their honey only by 
their long, narrow tubes, and specially attract hawk-moths and 
night-flying Lepidoptera by their colour, and by exhaling their 
odour most powerfully at night. 
2. STRUCTURAL CHARACTERS WHICH AID IN FERTILISATION. 
Adaptive Modifications of the Pollen and Stigma. 
All the characters of flowers so far discussed can only be of 
advantage to the plant in so much as they contribute indirectly to 
the conveyance of pollen by insect-visitors to the stigmas of other 
flowers. But this result is only possible if the pollen is of such a 
nature as to adhere to the insect, and if the stigma is fitted to 
remove it from the insect in turn. 
While in anemophilous plants the pollen consists very generally 
of loose, smooth, and easily scattered grains, in entomophilous 
flowers it presents a great variety of characters, all such as to enable 
it to adhere to the bodies of insect-visitors; and in all cases the 
nature of the stigma stands in the closest relation to the nature 
of the pollen, and the stigma is always fitted for attaching the 
pollen to itself by a viscid surface or by projecting papille. 
In flowers furnished with a contrivance fur scattering their pollen 
(several Scrophulariacee, Hvricacew, etc.) the pollen is usually 
smooth and loose as in anemophilous plants; but it is inclosed in 
receptacles from which it is dislodged by a touch of the. insect, 
and it is often directed in its fall by special hairs. In Syringa 
and Symphoricarpus the insect’s head or proboscis is first moistened 
with honey, to which the pollen then adheres; in Vinca and 
Polygala viscid matter is secreted by the stigma, and in Bryonia, 
Marrubiwm, Sideritis, etc., by spherical cells of the anthers, and 
fulfils the same function. In the great majority of cases, that is to 
