FLORAL MECHANISM, ETC . 
97 
extinction in bad seasons ; they also set a vast number of 
seeds in every pollinated ovary. It is instructive to compare 
this order with the Compositae (see Part II.). 
A somewhat similar state of things is found in the 
Asclepiadaceae, but these flowers are more visited by insects 
and do not set so many seeds. There are also other 
flowers in which self-pollination is fully prevented, but such 
cases are rare, and autogamy may occur if an insect visit 
the same flower twice. Such are Mimulus, Martynia, &c. 
(sensitive stigmas), Iris, Viola sp. (the open flowers), &c., 
and the fully dichogamous flowers above mentioned. 
The most simple and successful mode of ensuring the 
best chance of a cross for the longest time, with the cer- 
tainty (in most cases) of autogamy if the cross fails, is the 
piston-mechanism found in Compositae and some Campanu- 
laceae, where the style acts as pollen-presenter with closed 
stigmas, and afterwards the latter open out. An instructive 
series of flowers is found in Campanulaceae, leading up to 
the perfected mechanism of Compositae (see Part II.). 
A second type of the piston-mechanism is that seen in 
Leguminosae (q.v.). Here autogamy is prevented in many 
cases by the fact of the stigma only becoming receptive 
when rubbed, so that no fertilisation can take place till the 
flower has been visited by an insect. We may also mention 
here the lever-mechanisms of Salvia and Roscoea, the pollen- 
cup of Goodeniaceae, &c. The mechanism of Leguminosae 
is curiously repeated in Collinsia, Schizanthus, Phlomis, &c. 
The style also acts as pollen-presenter in many Proteaceae. 
Many of the Compositae (Cynareae) have the mechanism 
varied in detail by a sort of explosive arrangement, and 
this is even more marked in many Leguminosae, e.g. Genista. 
Similar phenomena are seen in some Orchids, e.g. Ptero- 
stylis, in Caladenia sp., Candollea, Kalmia, Posoqueria, &c. 
This leads naturally to the mention of such other cases 
of sensitive and motile organs in flowers as the stamens 
of Berberis, Sparmannia, Portulaca, &c. (see also above, 
Mimulus, &c.). 
A very common arrangement is for the style to project 
a little beyond the anthers, so as to be touched first by the 
insect visitor. The flower in these cases is usually nearly 
homogamous. Of course autogamy is nearly certain to 
w. 
7 
