74 



THE LEAF 



rials constitute the food of the thorn-inhabiting ants, for whose sub- 

 sistence the tree seems thus definitely to provide. In return the 

 warlike ants defend the Acacia from animal foes, in particular from 

 leaf -cutting insects. 



113. The petiole. — The petiole is sometimes lacking, 

 and in this case the leaf is said to be sessile. The gen- 

 eral office of the petiole is to aid in securing the best posi- 

 tion for the blade in respect to light. This it would do 

 merely by its length, since the space 

 available for all the leaves around the 

 stem is increased in proportion to the 

 length of the petioles.^ But further 



58. A prostrate shoot of Galium. The leaves now dis- 

 pose themselves in horizontal positions, and with- 

 out much over-shading of one by another. 



57. An erect shoot of 

 Galium. The 

 whorled leaves 

 spread in radi- 

 ating directions 

 about equally 

 on all sides. 



than this the petiole, by its own move- 

 ment, so disposes the blade that it 

 receives the best illumination possible 

 under any given circumstances (Figs. 

 57, 58). If a potted plant, not too old, 

 is taken from a position where it has 

 been lighted from above or on all sides, and placed at a 

 little distance from the window in a room where the light 

 enters only at one side, and the plant is closely watched, 

 it will shortly be seen that nearly all the leaves are very 

 slowly moving. The whole plant indeed seems to be 

 alive to the new direction of light and gradually turns 

 its leaves in that direction. This result is effected by 

 the leaf stalks, though young portions of the stem are 

 pretty sure to take part in the general movement. 



1 Strictly the area in any one plane is proportional to the square of the 

 length of the lines. It' the petioles are doubled in length, the space avail- 

 able for the blades becomes quadrupled. 



