216 SECTIONAL ADDRESSES 
Actuation of the trap can be caused by touching the glandular door- 
trichomes, when the trap is exhausted of water; in these species, 
owing to the thinness of the walls, it proceeds till the two walls are in close 
contact with each other. This is well shown if a trap which has swal- 
lowed a bubble of air is allowed to reset itself—a process which occupies 
about two hours in this species, four to eight times as long as in vulgaris. 
During this period the bubble changes shape in adapting itself to the 
changing contours of the interior, and this is well shown in the motion 
pictures. In whatever direction the trichomes are touched—the traps 
are not all equally (mechanically) sensitive—the actuation takes place. 
The explanation lies in the rotatory movement of the tubercle, and it is 
so poised that in whichever way it is moved, up or down, the effect is to 
raise the door edge a very slight amount, but sufficiently to upset the 
equilibrium. 
SUMMARY. 
The foregoing account is based on the study of about 75 species of 
Utricularia (including Biovularia and Polypompholyx). 
The bladders (called here the traps) appear in a great variety of form. 
The types selected to represent these forms illustrate the whole range of 
variety, so far as known. 
The study of living material of a number of species shows that certain 
properties of the trap heretofore known to us from the study of chiefly 
U. vulgaris are possessed by all. These are, briefly, a watertight door, 
snap action on actuation accompanied by the inrush of a column of water 
carrying with it the prey responsible for the actuation, the immediate 
return of the door to its original position, and the subsequent exhaustion 
of water from the lumen of the trap resulting in resetting it. ‘This reset- 
ting consists in the close adjustment of the door at all points with the 
threshold. The resulting posture of the door enables it to resist the even, 
if considerable, pressure of outside water on it, the watertightness being 
achieved by the sealing along the door edge by the velum. The effective- 
ness of the door posture depends on the shape of the threshold, which is 
always slightly funnel-shaped, the sides converging inwardly. In addition, 
there is a more markedly outfacing ridge or surface against which the 
middle reach of the door edge finds application, resisting the inswing of 
the door. 
The structure of the door is correlated with the function of its various 
regions. It is composed of two courses of cells, the relative depths of which 
vary according to the function. While the whole is remarkable in its 
capacity for bending, we can recognise areas which can bend very freely 
and through a large arc. This is hinge tissue, in which one course of 
cells is thin, the other thick. The thick course always takes the maximum 
compression. The upper part of the door and the regions around the 
sides are hinge tissue. The capacity of compression and extension of the 
deep cells depends on their bellows structure, their periclinal walls being 
corrugated, each corrugation being supported by stiffening rods in the 
anticlinal walls. The occurrence of props in the anticlinal walls is general 
throughout the door tissues, so that the chief characteristic of hinge tissue 
