82 PLANT PHYSIOLOGY 



Droseia, Pinguicula, and by Sarracenia, Darlingtonia, the 

 Nepenthes, and Utricularia. Of these Drosera, Sarracenia, 

 and Utricularia are most familiar and will sufficiently illus- 

 trate the principles concerned. 



Drosera, the '" Sun-dew" of northern bogs, is a low annual, 

 with nearly horizontally expanded, round, or elongated 

 leaves, characterized by peculiar columnar outgrowths 

 from the upper surface. The chlorophyll of the leaves is 

 usually masked by the red cell-sap of the superficial cells and 

 by the slender outgrowths. These last are multicellular, 

 traversed for about half their length by a single vertical 

 vascular bundle, and covered except on the top by ordinary 

 epidermal cells with cutinized walls. The free ends of the 

 columnar structures are enlarged and globular, glandular, 

 and covered by a glistening, sticky, syrupy, more or less 

 sweet secretion. Attracted by the unusual color and the 

 glistening surface of these leaves, small insects alight upon 

 them, taste the sweet secretion, and while they feed upon it 

 are detained by its stickiness. The weight and movements 

 of the insect induce movements in the hairs adjacent but 

 untouched and in the blade of the leaf itself, as well as in 

 the hairs with which it is in contact. Unless the insect 

 is powerful enough to break away from the sticky surface, 

 it presently comes into contact with other hairs and sticks 

 to them also. Finally the blade of the leaf bends upward 

 and the prey becomes enclosed between it and the many hair- 

 like tentacles bending over upon it from all sides. The 

 continued mechanical irritation of the hairs causes a more 

 abundant secretion from the glandular ends ; but, as Darwin 

 has shown, * besides the mechanical there is also a chemical 

 irritation, and this latter induces a change in the composi- 

 tion of the secretion. Non-nitrogenous material — a stone or 

 a piece of wood — of similar size and weight will not induce 

 the same response as an insect, a piece of meat or of hard- 

 boiled egg, plainly showing that upon the chemical composi- 

 tion of the irritating body depends in part the nature of 

 the irritation and of the response. 



The continued contact of highly elaborated organic nitro- 

 * Darwin. Charles. L. c. 



