THE PLANT BODY 



15 



Blade 



FIG. 5. Body plan of the lilac and apple 



a, lilac twig with cyclic leaf arrangement ; b, apple 



twig with spiral leaf arrangement. Compare Fig. 5, 



and b, with Fig. 7, a and b 



The, leaves of plants 

 (Fig. 5) spring from 

 definite points on the 

 main axis, which are 

 called nodes, and the 

 nodes are separated by 

 definite spaces, or in- 

 tervals of stem, called 

 internodes. The leaves 

 also grow from definite 

 points at the nodes and 

 are so arranged on adja- 

 cent nodes that a leaf at 

 one node never stands 

 immediately above the 



] ea f at t ] ie no( J e ^-^ 

 J 



below it. By this ar- 

 rangement the shading 



of the green tissues of one leaf by another above it is avoided. 



The leaves on a stem are therefore arranged in mathematical 



order, which usually conforms to one 



of two types of arrangement, namely, 



the cyclic arrangement and the spiral 



arrangement. 



In the cyclic leaf arrangement two 



or more leaves occur at each node, 



and the leaves of adjacent nodes 



alternate with each other. In such 



plants as the lilac and catnip two 



leaves are placed opposite each other 



at each node, alternating with the FlG 6 Leaf arrangem ent on 



pairs of leaves at the nodes immedi- a lilac twig seen from above 



ately above and below them (FigS. Note the alternating leaf pairs, 



5, ', and 6). The entire leafage of 



such a stem is thus arranged in four 



vertical rows of leaves up and down the stem, each row being 



separated from its neighbor by an angle of 90. Each leaf in 



