REVIEWS. 
105 
touched by an insect convey the excitement of the touch to the anther 
chambers, which instantly discharge the adhesive pollen-clubs at the head 
of the intruder. Thus, and thus alone, at least three species of the genus 
Catasetum are fertilized. 
Mr. Darwin has manifestly spared no labour in collecting facts illustra- 
tive of his subject. He gives a list of twenty-three species of Lepidoptera 
captured with the pollen-clubs of 0. pi/ramidalis attached to their pro- 
bosces. He has himself made observations on nearly all the British species 
of orchids, and upon a large number of exotic species, and he arrives at 
the conclusion that, “ throughout the vast Orchidean order — including, 
according to Bindley, 433 genera, and probably about 6,000 species — the 
act of fertilization is almost invariably left to insects.” 
The chapter on the homologies of Orchids is one of the most interesting 
portions of the book. 
At Torquay, Mr. Darwin watched a number of plants of Spirant lies 
uutumnalis, and saw them visited by humble-bees. “ The bees always 
alighted at the bottom of the spike, and, crawling spirally up it, sucked 
one flower after another.” “ I believe humble-bees generally act thus 
when visiting a dense spike of flowers, as it is most convenient for them ; 
in the same manner as a woodpecker always climbs up a tree in search of 
insects.” This seems a most insignificant observation ; but see the result. 
In the early morning, when the bee starts on her rounds, let us suppose 
that she alighted on the summit of a spike, she would surely extract pollinia 
(pollen-clubs) from the uppermost and last opened flowers, but when 
visiting the next succeeding flower, of which the labellum in all probability 
would not as yet have moved from the column, for this is slowly and very 
gradually effected, the pollen masses ■would often be brushed oft’ her 
proboscis and be wasted. But nature suffers no such waste. The bee goes 
first to the lowest flower, and crawling spirally up the spike, effects 
nothing on the first which she visits till she reaches the upper flowers, 
then she withdraws the pollinia ; she soon flies to another plant, and 
alighting on the lowest and oldest flower, into which there will be a wide 
passage from the greater reflection of the labellum, the pollinia will strike 
the protuberant stigma. If the stigma of the lowest flower has already been 
fully fertilized, little or no pollen will be left on its dried surface ; but on 
the next succeeding flower, of which the stigma is viscid, large sheets of 
pollen will be left. Then, as soon as the bee arrives near the summit of 
the spike, she will again withdraw fresh pollinia, will fly to the lower 
flowers on another plant, and fertilize them ; and thus as she goes her rounds 
and adds to her store of honey, she will continually fertilize fresh flowers, 
and perpetuate the race of our autumnal Spiranthes, which will yield 
honey to future generations of bees. 
