408 DR C. L. WITHYCOMBE ON THE FUNCTION OF 
(insensitive) condition. That the pricking is not sufficient stimulus to * fire " 
the bladder is shown by the fact that the bladder may be pricked superficially 
in any part, or even severed from its footstalk without “firing,” or the 
bladder may often be actually penetrated by the needle; if the latter remains 
filling the puncture and no water enters from outside, the bladder remains 
concave. It is when water is allowed to enter through the puncture that the 
walls spring outwards. 
Merl (6) states that there can be no negative pressure, since bubbles often 
occur in bladders and such bladders still act. It would seem more correct 
to say that bladders act, but to a feebler extent when containing air, and also 
that if a bladder encloses a large bubble, it does not act. It is true that one 
would expect a bubble to increase in size with decrease in pressure, and this 
does not appear to occur to any noticeable extent, although some quite definite 
increases have been observed. Probably the reason why no very appreciable 
Fic. 6. 
Motor tissue. x 400. 
increase in size can be detected is that the bubble moves its position as the 
walls decrease the bladder cavity posteriorly, and that it moves to a position 
where the cavity is broader. Assuming that the pressure within a sensitive 
bladder is half that within a recently “fired ” bladder, then a bubble would 
only increase to double its volume. Such an increase would be approximately 
the amount which seems to occur, and it is, of course, not very noticeable 
when allowed for in three dimensions. The suction of a bladder containing 
air is never as great as that of one which contains only water, and the larger 
the bubble contained, the less the suction, a large bubble causing a bladder 
to function entirely. It should be noted that it is not normal for bladders to 
contain any gas. Plants grown from winter buds and continuously in water 
have bladders without air. When plantsare removed from water, the sensory 
hairs are stimulated and air enters the bladders. This occurrence is denoted 
by a popping sound as the plants are removed, 
