GENERAL MORPHOLOGY OF THE PLANT 83 



leaf will therefore pass through the base of every fifth leaf above 

 and below it. These lines that can be drawn vertically down 

 the stem through the leaf-bases are known as orthostichies, and 

 the imaginary spiral line is called the genetic spiral. If we 

 refer again to the arrangement of the leaves in the Cherry or 

 Apple, we shall find that before we arrive at the sixth leaf (fig. 

 164), which is over the first, the string or line used to connect 

 the base of the leaves will have passed twice round the circum- 

 ference of the branch. The point where a leaf is thus found, 

 which is placed perpendicularly over the first, shows the com- 

 pletion of a series or cycle, and thus in the Cherry and Apple 

 the cycle consists of five leaves. As the five leaves are 

 equidistant from each other, and as the line which connects 

 them passes twice round the stem, the distance of one leaf from 



Fig. 163. 



Fig. 164. 



Fig. 163. Fascicled or tufted leaves of the 



Larch. t'ig. 164. A portion of a branch 



of the Cherry-tree with six leaves, the sixth 

 of which is placed vertically over the first. 

 The right-hand figure is the same branch 

 magnified, the leaves having been removed, 

 and numbers placed to indicate the points 

 of their insertion. 



the other will be f of its circumference. The fraction f expresses 

 the angular divergence, or the length of the arc interposed 

 between the insertion of two successive leaves. If a circle be 

 drawn to represent the circumference of the stem, and the position 

 of the orthostichies which bear two consecutive leaves be marked 

 upon it, and radii be drawn from them to the centre of the circle, 

 the angle so formed will show the divergence, and will be | of 360°, 

 or 144°. The successive leaves as they are produced on the 

 stem, as we have seen, are also arranged in similar cycles. This 

 arrangement in cycles of five is common in Dicotyledons. It is 

 termed the xientasticTious or five-ranTced arrangement. 



A second variety of arrangement in alternate leaves is that 

 which is called distichous, or two-ranhed. Here the second leaf 

 on the spiral is directly opposite to the first {fig. 165), and the 



