74 



THE LEAF 



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

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

 warlilie ants defend the Acacia frojn animal foes, in particular from 



leaf-cuttill^■ insects. 



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

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

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

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

 merely by its length, since the space 

 available for all the leaves around the 

 stem is increased in proportion to the 

 liuigth of the petioles. 1 But further 



57. An erect shoot of 

 (xaliuin. The 

 whorlei) leaves 

 spread in radi- 

 ati!i,i< (lirei^tioiis 

 aboiil eijiially 

 ou all sides. 



A prostrate shoot of Galium. The leaves now dis- 

 pose themselves in horizontal positions, and ^vith- 

 out much over-shading ot one by another. 



than this the petiole, by its own move- 

 ment, so disposes the blade that it 

 i-eceives the best illumination possible 

 under any given circumstances (Figs. 

 57, 58). If a potted plant, not t(jo 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 tiirns 

 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. If the petioles are doubled in length, the space avail- 

 able for the blades becomes quadrupled. 



