184 SECTIONAL ADDRESSES 
certain critical observations, which enable me to say with conviction 
things relating to Australasian species which otherwise had not been avail- 
able in the living form and whose peculiarities well-nigh defied analysis. 
I refer to U. dichotoma and related species, and to Polypompholyx. 
The species of Utricularia (I include Polypompholyx for the purpose of 
description from now on) fall into two major ecological groups: the 
submersed, floating forms, of which the familiar U. vulgaris and less 
familiar U. purpurea are good examples, and the so-called terrestrial forms, 
of which the frequently cultivated U. reniformis and U. cerulea may serve 
as illustrations. ‘This statement leads me to emphasise the fact that the 
structure of the bladders, or traps (as I prefer to call them), is far more 
uniform, almost to monotony, within the submersed forms, while the 
contrary is true of the terrestrial. ‘That this should be the case seems 
natural, as the environment of the submersed kinds is more uniform. 
But that the diversity of the remainder should be due to the lack of 
uniformity of environment is not so clear, since the lack is, I imagine, more 
apparent than real. ‘The terrestrial species are all confined to a very wet 
substratum, and grow usually as much submersed as the floating forms ; 
and the species which appear to be the least limited in their need of water 
are those which, like U. reniformis, U. longifolia, etc., grow in wet moss as 
epiphytes, etc., and in spite of this are most like the submersed U. vulgaris 
or U. gibba. Whatever the explanation, it is my present purpose rather 
to display the variety of the traps and to attempt to explain their workings. 
In order to approach immediately to this purpose I shall clear the way 
by summarising those properties which are common to all. 
1. Nature of Action—The trap is a snap-action mechanism—that is, it 
acts with extreme suddenness, much to the surprise of Darwin when he 
examined U. vulgaris.2, So swift is it that the whole action falls within 
the limits of ;'; second, and by means of superspeed cinephotomicro- 
graphy, taking 160 pictures a second, I have found that the opening 
phase of the door falls within ;4, second, while the closing phase is 
completed in four pictures, or #7, second. All the species which I 
have examined act similarly. During this brief moment the side walls 
of the trap spring out, the shape of the periphery as seen in lateral view 
alters correspondingly, the door opens fully and closes, falling into a 
semi-relaxed position, during which a column of water, carrying with 
it any luckless animals if small enough, rushes into the interior of 
the trap. The closing movement of the door shuts off the water 
before the walls have completely relaxed, so that, on closure being 
completed, there remains still some degree of ‘ negative’ pressure. As 
Merl (1922) showed, complete relaxation may be procured experimentally, 
either by puncturing a wall of the trap or by holding the door open for 
a moment, thus throwing it out of action. I have shown also that cutting 
the velum has the same result. 
2 The action of the trap in U. vulgaris, U. gibba and U. purpurea is demon- 
strated by means of motion pictures. By means of animated diagrams the 
particulars of behaviour of the door are shown for U. gibba, U. vulgaris, U. cornuta 
and U. c@yulea. The rate of movement of the door of U. vulgaris is demon- 
strated with superspeed motion picture taken at the rate of 160 frames per 
second. The question of irritability has been dealt with by me elsewhere (1932). 
