128 



Introduction to Botany. 



the leaf slowly opens and is ready for another victim. 

 However, if the insect digested is large, the leaf may find 

 itself unable to perform its trap function a second time. 



99. Behavior of Sundew. — The leaves of Drosera, or 

 sundew, are orbicular, and bear on their upper surface 

 glandular structures resembling tentacles (see Fig. 65), the 

 tips of which exude a clear viscous fluid. When an insect 

 alights on the leaf it becomes entangled in the viscid ex- 

 cretion, its struggles excite the glands to greater activity, 

 and more and more fluid is exuded ; at the same time the 



tentacles bend 

 down over the 

 insect, and ren- 

 der escape more 

 difficult. The 

 viscous fluid has 

 the nature of a 

 digestive fer- 

 ment, and by it 

 the insect is ren- 

 dered soluble, 

 andinthiscondi- 

 ition is absorbed 

 by the leaf. The 

 glands appear insensible to falling raindrops, but Darwin 

 found that motion was induced when he placed on a ten- 

 tacle a bit of hair weighing only yj^jg of a grain. He 

 found the tentacle to be quite sensitive also to very dilute 

 solutions of nitrogenous salts ; they bent downward com- 

 pletely when the leaves were immersed in a solution of 

 ammonium carbonate so dilute that each gland could 

 absorb no more than ^^^J^^^ of a grain. This experi- 

 ment serves at least to show that the sensibility of plants 



Leaves of Drosera rotundifolia. a, after stimulation by 

 contact with an insect, and b, with all of the tentacles 

 expanded. After Kerner. 



