410 DR. C. L. WITHYCOMBE ON THE FUNCTION OF 
gradual steps, bladders can even be imbedded in wax for sectioning while 
atill in the concave condition. The bladders do not allow of penetration by 
fixatives very rapidly, and the usual time of immersion in Bouin’s Picro- 
formol (full strength) was twelve to eighteen hours. Several cases have been 
encountered of bladders responding to rough stimulus of the valve after 
having been in the above fixative for half-an-hour. I cannot believe that 
these were still living. 
If stains are added to the water in which plants are growing, these 
do not enter the bladders for a considerable period. Even methylene blue, 
which is such a useful intra vitam stain, does not enter for a day or two, and 
if then, it is probably due to death or injury of the bladder. Such facts as 
these would appear to prove conclusively that the sealing of the bladder is 
very perfect. 
The mode of action of the bladder may therefore be summarized as follows. 
The bladder is hermetically sealed by the springing back of the valve into a 
groove, and the sealing is rendered more complete by the secretion of mucus 
by certain of the collar zone cells, Water is absorbed from within the 
bladder, probably by the quadrifid hairs. This produces a negative pressure 
within, so that the lateral walls are drawn inwards, often so near as almost to 
touch each other. They tend always to spring back to the previous slightly 
convex position, and no doubt therefore exert a pull. When either of the 
four sensory hairs upon the valve is touched, certain motor tissue, mainly 
ventrally to these hairs, contracts, thus withdrawing the valve margin from 
its groove and allowing an opening of the valve with its consequent influx of 
water. The valve immediately springs forward to its former position, fitting 
into the collar zone. 
No part of the bladder is sensitive to touching except the four sensory 
hairs upon the valves. Other hairs, whatever their form, are insensitive. 
As regards the nature of prey captured by Utricularia, various records 
appear of small crustacea, insect larvæ, worms, etc. Moseley (7) mentions 
that Mr. G. E. Simms had observed numbers of young fish caught in the 
bladders by head or tail or even by the yolk-sac. I have seen a tadpole (8) 
held firmly with the head in one bladder and the tailin another. Also small 
molluses (Planorbis lineatus) and a colonial Polyzoan (Cristatella mucedo) 
have been observed held at the mouth of a bladder. 
Whether the animals caught are digested, as in Pinguicula, or whether 
they merely decay, is still somewhat of an open question. ‘The chemical 
secretions of the bladders have been fully investigated by von Luetzelburg (5), 
who brings forward much evidence in favour of true digestion. 
TECHNIQUE. 
It may be useful to add a note on the method of imbedding bladders for 
sectioning with as little alteration in form as possibie. The following gave 
best results. 
