OcToBER, 1909.] THE ORCHID REVIEW. 291 
Darwin shows that the splitting does not take place spontaneously, and can 
be prevented by excluding insects with a net. 
The Australian Spiranthes australis is markedly different in its 
economy, for Mr. Fitzgerald could never detect any trace of a rostellum or 
of viscid matter, and the flowers are as completely self-fertilising as those 
of Ophrys apifera, even if the visits of insects are prohibited by a bell-glass. 
The pollinia are said to touch the upper edge of the stigma and fertilise it 
at an early stage. The flowers are white, and are arranged on the spike in 
the same spiral manner as in S. autumnalis. 
The Tway-blade, Listera ovata, is a common British Orchid which 
Darwin considers to be one of the most remarkable in the whole Order. It 
has small green flowers, with a long and narrow pendulous lip. The 
rostellum is of large size, thin or foliaceous, and arches over the stigmatic 
surface. Internally it is divided by longitudinal septa into a series of loculi, 
which contain viscid matter and have the power of violently expelling it. 
As soon as the flower opens, if the crest of the rostellum be touched, ever so 
lightly, a large drop of viscid fluid is instantly expelled, which is milky at 
first, but on exposure to the air sets hard in two or three seconds, and 
assumes a purple brown tint. So exquisitely sensitive is the rostellum that 
a touch from the thinnest human hair suffices to cause the explosion. As the 
pointed tips of the pollinia lie on the crest of the rostellum they are always 
caught by the exploded drop. The fertilisation of the flower is thus 
described by Darwin: ‘‘ Small insects alight on the labellum for the sake of 
the nectar copiously secreted by it ; as they lick this they slowly crawl up 
its narrowed surface until their heads stand directly beneath the overarching 
crest of the rostellum ; when they raise their heads they touch the crest; 
this then explodes, and the pollinia are instantly and firmly cemented to 
their heads. As soon as the insect flies away it withdraws the pollinia, 
carries them to another flower, and there leaves masses of the friable pollen 
on the adhesive stigma.” 
On watching a group of the plants Darwin saw numerous small Hymen- 
opterous insects of two species flying about the plants and licking up the 
nectar. The flowers whose pollen had been removed were visited over and 
over again, but at last he saw both species crawl into younger flowers and 
suddenly retreat with a pair of bright yellow pollinia sticking to their fore- 
heads. Darwin’s son watched another bed some miles away, and brought 
home the same two insects with attached pollinia, and also saw some 
Diptera visiting the flowers. He was also struck with the number of 
spiders’ webs spread over these plants, as if the spiders were aware how 
attractive the Listera was to insects. In one case Darwin found an 
extremely minute Hymenopterous insect vainly struggling to escape, with 
its head cemented by the hardened viscid matter to the crest of the 
