Modified Parts. 



H5 



where the pitchers stand open. Insect-catching, at any 

 rate, appears to be one of their important functions. 



It is not easy to conceive how the modifications into 

 pitchers have come about. We know that there is in 

 plants a capacity to vary which is apparently stimulated to 

 activity by both internal and external causes. 

 Useful variations would be likely to persist 

 and become more pronounced by the produc- 

 tion of more numerous and stronger offspring, 

 of which those having the most useful varia- 

 tions would finally predominate, because bet- 

 ter fitted to contend for soil, air, 



and sunlight. But how, in this case, 

 the variation became started along 

 the line of pitcher formation lead- 

 ing finally to the win- 

 dowed dome, slip- 

 pery surfaces, and 

 detaining down- 

 ward-pointing hairs 

 can only be an- 

 swered with conjec- 

 tures. 



115.Bladderworts. 

 — Not less wonder- 

 ful are the modified 

 leaves of the common Utrictilaria or bladderwort. This 

 is an immersed, floating water plant (see Fig. 73, A). 

 Some of its leaves are much divided and threadlike, and 

 others are modified in the form of little bladderlike traps 

 adapted to catching very minute water animals. The en- 

 trance into the cavity of the bladder is provided with a 

 door which swings inward, but never outward, because its 



Fig. 73, 



Utricularia grafiana. A, showing flower rising above 

 the water, and below the water finely divided foliage 

 leaves, and bladderlike leaves. After Kerner. 

 B, longitudinal diagram of a bladder leaf of Utricu- 

 laria vulgaris, showing trap door opening inwards. 



