194 



FIRST YEAR SCIENCE 



A striking example of this is the compass plant, the 



leaves of which arrange themselves so that the sun's rays 

 strike the broad* surface of the leaves at 

 night and morning when the rays are not 

 very strong, but at noon the edge of the 

 leaf is toward the sun, the leaf thus main- 

 taining a nearly vertical position all day, 

 with its greatest length extending in a 

 nearly north and south line. It is the 

 effort to regulate the amount of light fal- 

 ling on the leaf, and not any magnetic 

 influence, which causes the leaf to point in 



the direction of the compass needle. 



The shapes of the leaves vary greatly in different plants. 



Sometimes they assume very 



singular forms, as in the pitcher 



plant (Fig. 92) and Jack-in- 



the-pulpit. Sometimes they 



even become carnivorous, as 



in the sundew and Venus fly- 

 trap. 



Around the margin of the 



sundew leaf and on the inner 



surface are a number of short 



bristles each having at the end 



a knob which secretes a sticky 



liquid. As soon as an insect 



touches one of these knobs, it 



sticks to the knob and the 



other bristles begin to close in 



upon the inject and hold it fast. 



Soon the insect dies and the leaf Fi - 93< 



+ 1 



secretes a juice which digests its soluble parts. 



In the Venus flytrap (Fig. 93) the leaf terminates in 



