32 HOW PLANTS EMPLOY INSECTS TO WOKK FOE 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 poiuting 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 off 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 

 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 

 *^poikn-mi"I them. Milkweeds are like Orchids in this respect only. Their flow- 

 ofMUkweed. ers 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 inay be described. 



66. In Barberry-blossoms there is a remarkable peculiarity. We have learned, in 

 the first chapter, that certain plants are endowed with tlie 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 



