LEAVES AND THEIR FUNCTIONS 



139 



from the axils of the true, very tiny, scalelike leaves. The spines noted in the 

 cactus are examples of reduced leaves. 



Leaves as Insect Traps. — Most curious of all are the modifications of the 

 leaf into insect traps. It frequently happens that the habitat of a plant will 

 not furnish the raw food materials necessary to form proteid food and to 

 build protoplasm. Nitrogen is the lacking element. The plant has become 

 adapted to these conditions and obtains nitrogenous food from the bodies of 

 insects which it catches. Examples of insect traps are the common bladder- 

 wort (Utricularia) , the Venus's flytrap (Dioncea muscipula), the sundew 

 (Drosera rotundifolia) , and certain of the pitcher plants. 



Bladderwort, showing finely dissected submerged leaves bearing blades which capture 



animalcula. 



Bladderwort. — The simplest contrivance for the taking of animal food 

 by the leaf is seen in the bladderwort. Here certain of the leaves are modi- 

 fied into little bladders provided with trapdoors w^hich open inwards. Small 

 water-swimming crustaceans (as water fleas, etc.) push their way into the 

 trap and there die, perhaps of starvation. Bacteria, causing decay, soon 

 break down their bodies into soluble substances, the nitrogenous portion of 

 which is absorbed by the inner surface of the bladders and used by the plant 

 as food. 



Venus's Flytrap. — In the Venus's Kytrap, a curious plant found in our 

 Southern states, the apex of the leaf is peculiarly modified to form an insect 



