FIELD BOOK OF PONDS AND STREAMS 



pressed sac (Fig. 82, 2) with a slit-shaped opening; this open- 

 ing is guarded by a valve and the rim of the vestibule is 

 armed with teeth and bristling hairs. Within the bladder 



Fig. 82. — Bladderwort, Utricularia vulgaris-, i, 

 branch with bladders; 2, bladder more enlarged. 



there are more branched hairs, and a digestive secretion is 

 produced from the walls. When little organisms enter the 

 vestibule their movements stimulate the valve to open. 

 Ordinarily the sides of the sac are dented inward (Fig. S2) 

 but as soon as the valve opens the entering water presses its 

 walls from within and at once pushes them outward. This 

 outward movement starts a suction that pulls water and 

 animals with it into the bladder; thus making it at least in- 

 directly the active captor of its prey. By first counting all 

 the animals in a certain number of bladders and then all of 

 the bladders on a stem, Hegner has estimated that all stems 

 taken together on one large plant contained 150,000 living 

 organisms. The bladders which he studied were indented 

 again and "set" for another capture in about 20 minutes 

 after one had taken place. 



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