16 



GENERAL BOTANY 



any one row is therefore separated from tne leaf immediately 

 above or below it in the same row by two internodes of the stem. 

 If one observes a stem or a branch from above, the four rows of 

 leaves are plainly visible, and the entire upper portion of the 

 shoot has the appearance of Fig. 6. In the diagrammatic draw- 

 ings (Fig. 7), in which the leaves, nodes, and internodes are repre- 

 sented as if projected on a plane, the circles represent nodes and 

 the intervening spaces internodes. The shaded central portion 

 is meant to represent the small leaves and very short internodes 



of the terminal 

 bud. Outside 

 of this central 

 shaded area the 

 interned al spaces 

 widen gradually 

 toward the pe- 

 riphery of each 

 figure to repre- 

 sent the gradual 

 FIG. 7. Diagrams showing cyclic and spiral leaf lengthening; of 



arrangements 



the internodes on 



a, cyclic leaf arrangement of the lilac, reduced to one plane. , ,, 



The circles represent nodes, and the spaces between inter- a s ^em trom its 

 nodes. The leaves are seen to be in four longitudinal rows apex to i^S base, 

 along the stem. 6, spiral leaf arrangement of the apple, ,p, . , , 



reduced to one plane. Nodes and internodes as in a InlS lengthen- 



ing of the lower 



internodes as the leaves grow in surface is of advantage in 

 preventing shading of the lower leaves by large leaves above 

 them. The cyclic leaf arrangement (Fig. 7, a) is thus seen to 

 be well adapted to the exposure of leaves to sunlight without 

 interference or shading by their neighbors on the same shoot. 



In the spiral leaf arrangement (Figs. 5, >, and 8) only one leaf 

 is placed at a node, and the leaves spring from the nodes so as 

 to effect a spiral distribution along the twigs and young shoots. 

 Since any given leaf is always separated from a leaf above or 

 below it in the same straight line by two or more internodes 

 (Fig. 7, 5), according to the type of spiral arrangement in any 

 particular case, the same advantages as regards sun exposure 



