222 



ELEMENTARY STUDIES IN BOTANY 



The " Venus fly-trap " (Dioncea) is one of the most famous 

 and remarkable of insect-trapping plants, being found only 

 in certain sandy swamps near Wilmington, N. C. The leaf- 

 blade is constructed so as to work like a steel trap, the two 

 halves snapping together and the marginal bristles interlock- 

 ing like the teeth of a trap (Fig. 191). A few sensitive hairs, 

 like feelers, are developed on the leaf surface ; and when one 



of these is touched by a 

 small flying or hovering 

 insect, the trap snaps shut 

 and the insect is caught. 

 Only after digestion, which 

 is a slow process, does the 

 trap open again. 



128. Summary. Leaves 

 are essentially expansions 

 of green tissue organized 

 for the work of photosyn- 

 thesis, and therefore exposed 

 to light and air. The vari- 

 ations in the forms and 

 superficial structure of 

 leaves are extremely numer- 

 ous, but the essential struct- 

 ure comprises an epidermis 

 which protects the working 

 cells against excessive loss 



of water, a mesophyll region which is made up of the green 

 working cells, and veins which distribute water among the 

 working cells. The expanded form of the leaf is maintained 

 by the comparatively rigid framework of veins, and by the 

 turgor of the mesophyll cells. ' 



The necessary relations of leaves to light involve adjust- 

 ments to prevent an excessive shading of leaves by one an- 

 other. The most general statement of the situation is that 



FIG. 191. Three leaves of Dioncea: two 

 with the traps open, one with trap shut 

 on an insect. After KERNER. 



