The shape of the nectary and of the
adjoining parts are likewise related to the particular kinds of insects
which habitually visit the flowers; this has been well shown by Hermann
Muller by his comparison of lowland species which are chiefly visited by
bees, with alpine species belonging to the same genera which are visited
by butterflies. (10/5. 'Nature' 1874 page 110, 1875 page 190, 1876 pages
210, 289.) Flowers may also be adapted to certain kinds of insects, by
secreting nectar particularly attractive to them, and unattractive to
other kinds; of which fact Epipactis latifolia offers the most striking
instance known to me, as it is visited exclusively by wasps. Structures
also exist, such as the hairs within the corolla of the fox glove
(Digitalis), which apparently serve to exclude insects that are not well
fitted to bring pollen from one flower to another. (10/6. Belt 'The
Naturalist in Nicaragua' 1874 page 132.) I need say nothing here of the
endless contrivances, such as the viscid glands attached to the
pollen-masses of the Orchideae and Asclepiadae, or the viscid or
roughened state of the pollen-grains of many plants, or the irritability
of their stamens which move when touched by insects etc.--as all these
contrivances evidently favour or ensure cross-fertilisation.
All ordinary flowers are so far open that insects can force an entrance
into them, notwithstanding that some, like the Snapdragon (Antirrhinum),
various Papilionaceous and Fumariaceous flowers, are in appearance
closed.
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