1 88 FRESH-WATER BIOLOGY 



very different composition from the slime which occurs on the lower 

 surface of a floating leaf. The occurrence of the latter may be 

 accidental so far as the plant is concerned and have little im- 

 portance in its welfare. In the algae and even with delicate parts 



of higher plants such a coating 

 may serve to retard the ex- 

 change of liquids, thus pre- 

 venting plasmolysis, or in like 

 manner it may enable the 

 plant to maintain a cell sap 

 of much greater density than 

 that of the surrounding water 



(After Gobel.) (Fig. 2 6 3 ). 



Quite a number of terrestrial species are especially adapted for 

 retaining and digesting as food small animals which are so unfor- 

 tunate as to wander into the traps borne by the plant. Few 

 aquatic species have acquired this habit but there are some mem- 

 bers of the genus Utricu- 

 laria remarkable for the 

 special organs developed to 

 secure animal food. The 

 bladders are generally re- 

 garded as modified leaves, 

 and structures resembling 

 stomata have been found 

 on them in some cases. 

 The bladders have small 



FIG. 264. Utricularia minor . Numerous bladders on the leaves. 



Openings guarded by hairS A, enlarged bladders. L, flower-stalk rising above the water. 



The main leaf axis or stem is to be imagined as horizontal 



and closed by a sort of trap- in the water - ( After Glack -> 



door which permits small animals in the water to enter but which 



prevents any escape for the victims (Fig. 264). 



These plants may float free, so far as roots are concerned, but, as 

 with Ceratophyllum, accidental attachment or rather anchorage fre- 

 quently occurs through entanglement with other plants or by being 

 partly buried in the mud. 



All of the species raise the inflorescence above the water and 

 Utricularia inflata sends out whorls of leaves with inflated petioles 



