210 SECTIONAL ADDRESSES 
the middle area thereof, fade into a somewhat different form of cell, in 
which the corrugations are very prominent and regular and have given 
rise to some misrepresentation. The corrugations are continuous from 
cell to cell, and are very regularly concentric, giving the impression that 
they are the walls of isodiametric cells. The centre thus indicated lies 
just at a point where the door is at its thinnest, the central hinge, and 
immediately below this thin spot issue four (sometimes abnormally more) 
gracefully curved and pointed, stiff, three-celled trichomes. These are 
the tripping hairs, and constitute mechanically a latch lever. At their 
bases they are inserted close together in the upper part of a thickened mass 
of tissue, with courses of cells of equal thickness, the middle piece. This 
is slightly thinner along its middle line, that is, in the sagittal plane. 
When the door is opened the middle piece folds inwardly along this line. 
It acts as a mechanical unit with the latch lever, any movement of which 
disturbs the edge of the door one way or another, in any case effecting its 
release from a slightly outwardly turned surface of the threshold against 
which it rests. On either side the middle piece merges quickly into the 
outer hinge tissue. 
The threshold, supported on a strong, upturned bolster of tissue, is 
nearly circular in axial view, its arc length being shorter by about 10 per 
cent. than the edge of the door (Lloyd, 1932). Since it is the door edge 
which lies in contact with the threshold, its curve lies obliquely from its 
point of attachment—the inner angle of the threshold—to a point in the 
middle in front of the middle zone. This is raised slightly, affording 
a resting-place for the door edge, resisting its inswing. At the sides the 
front surface of the door rests on triangular areas of the threshold at each 
end, these areas facing obliquely outwardly, so that the door under the 
pressure of the outer water is tightly cramped into place. The outer 
zone, wider at the sides than in the middle, carries a velum, consisting of 
several rows of bladdery cuticles, to which is attached a loose membrane 
arising from the cells of the middle zone and, to some extent, of the outer 
zone. ‘The total threshold surface is shaped in correlation with the 
emplacement of the door edge. Important is the angle of divergence 
between the plane of the door and that of the threshold, which approaches 
go degrees, whereas in the series previously considered (U. globulariefolia 
excepted) that angle is a narrow one (Fig. 12). That is, (a) the position 
of the surface of resistance for the emplacement of the door edge is in 
front of the threshold when the angle of divergence is great and in the 
back when that angle is small ; (6) in the latter case also the thrust of the 
sides of the door is directly down on to the threshold, while, when the angle 
of divergence is large, this thrust is directed obliquely forward, toward 
the middle point of the door edge ; (c) the outer zone is wide, furnishing © 
a very wide and ample velum when the angle of divergence is small, or 
narrow when large. 
Actuation of the door occurs when the latch lever is touched in any 
direction. Many trials have failed to convince me that the sensitivity of 
the mechanism is greater for one direction than another. This can be 
understood when one sees that an upward swing of the middle piece frees 
it from the resisting threshold ridge, while a downward swing releases 
