INSECTIVOROUS PLANTS 131 
with multitudes of flying insects, they become veritable 
cemeteries. 
Both of these plants live in bogs, the butterwort, on 
the whole, preferring the sweeter parts, while the sundew 
is indifferent. In bogs the water is sour and deficient in 
nitrates. There is plenty of nitrogen present, but, as it is 
mostly in the form of humous acids or ammonium-com- 
pounds, little use can be made of it. The insectivorous 
habit in these bog-plants is correlated with the need for 
nitrogen in a place where it is deficient. Not being able 
to get sufficient from the soil, the plants entrap living 
insects, and secure a supply of 
protein from the dissolving bodies 
of their prey. 
(c) Utricularia vulgaris, bladder- 
wort (Fig. 46), a submerged free- 
floating aquatic found in ditches. 
It has no roots. The leaves are 
large and much-divided, floating in 
suspension just below the surface 
of the water. In summer the 
flowering shoots emerge from the 
water, lifting into the air a spike 
of conspicuous flowers, pollinated 
by insects. Upon the submerged 
leaves, at the base of some of the 
green, thread-like segments, occur 
small, bladder - like structures, or 
which give the plant its name 4, 45 prnquicula vul- 
(Lat. utriculum, a bladder). Each Gi Ga). 
of these hollow chambers is pro- 
vided with a hinged door, which is easily opened 
from without, but cannot be opened by pushing from 
within. Over the entrance is a brush of bristling hairs, 
and round the mouth of the bladder occurs a number 
of glandular hairs. Small crustacea, such as Cyclops 
(water - fleas), poking about among these hairs for 
food, and possibly attracted by the secretions of the 
glandular hairs, push open the trap-door and enter the 
chamber. The door closes behind them, and they are 
entrapped. Inside, they soon die either of suffocation or 
starvation, and the products of their decaying bodies are 
absorbed. The interior walls of the bladder are covered 
