32 
HOW PLANTS EMPLOY INSECTS TO WORK FOR THEM, 
blossom and entering as before, as the insect makes its way onward, it can hardly 
fail to rub the pollen-covered top of its head against the large stigma which forms 
the roof of the passage. The stigma of every other Orchid is smooth and glu- 
tinous. This is merely moist and finely roughened : the roughness comes from 
very minute projections, all pointing forwards, so that the surface may be likened 
to that of a wool-card or of a rasp on a very fine scale. So, as the insect passes 
under, the film of pollen is carded or rasped olf its head by the stigma and left 
upon it ; and when the fly passes out it takes a fresh load of pollen on its head 
'with which to fertilize the next flower. This mode of action we first predicted 
from an inspection of the flower and a simple experiment. It has since been con- 
firmed by repeated observations. The early-flowering and purple Stemless Lady’s- 
Slipper differs from the others in having its larger slipper or sac pendent, and 
with a long slit in front, instead of a round open orifice ; the two lips of the slit 
are mostly in contact, but the fly may readily push its way in ; the way of exit 
is more open than in the other species. 
64. In Asclepias or Milkweed. Now and then the rough legs of butterflies and 
bees are found to be encumbered with bodies sticking to them which resemble the 
pollen-masses of Orchids ; but there is always a pair of them, of waxy 
A appearance, hanging by a curved stalk from a dark-colored disk, if it 
may be so called, which is not button-shaped. These are the pollen- 
masses of Milkweed, carried off by insects alighting on the flower to 
suck the nectar from five little cups, and, sticking fast to their legs or 
feet, are so carried from flower to flower. Fig. 24 shows a pair of 
Fl poiien-masses th em - Milkweeds are like Orchids in this respect only. Their flow- 
of Milkweed. e rs are very different and peculiar, not readily to be explained ex- 
cept with the plant itself in hand; but insects are equally necessary to fertilize 
them. 
65. How ordinary blossoms are cross-fertilized by insects passing continually 
from flower to flower will be obvious enough after these explanations. But ob- 
serving eyes will detect many curious arrangements in the commonest plants, now 
that the way is pointed out. A few may be described. 
66. In Barberry-blossoms there is a remarkable peculiarity. We have learned, in 
the first chapter, that certain plants are endowed with the power of moving some 
part freely in order that they may climb. Barberry-blossoms have a movement 
upon irritation, which has long been familiar as a mere curiosity, but which we 
