400 



DISTRIBUTION OF THE GREEN LEAVES ON THE STEM. 



another, and accordingly eight rectilineal lines (orthostichies) run down the stem. 

 Each line is separated from its neighbour by ^ of the circumference. This arrange- 

 ment, which occurs in roses, raspberries, pears, and poplars, in laburnuns, and in the 

 barberry, is called the three-eighths phyllotaxis. 



Yet a fifth case is very often to be found in trees and bushes with narrow- 

 leaves, viz. in the Almond-tree, in the Goat's-thorn, in the Sweet Willow, in the 

 Sea Buckthorn, and many Spiraea bushes. Each story contains thirteen leaves. 



Fig. 99.— Plan for Spiral Phyllotaxis. 



1 One-lialf Phyllotaxis. 2 One-third Phyllotaxis. » Two-fifths Phyllotaxis. * Three-eighths Phyllotaxis. The conical steii) 

 horizontally projected ; the points of insertion of the leaves on the circumference of the stem marked by a dot. 



which may be connected by a spiral line, i.e. a " genetic spiral ", with five revolu- 

 tions. The number of the straight lines here amounts to thirteen, and the distance 

 between two leaves following one another in age is -^ of the circumference, i.e. 

 138° (see fig. 100). 



Not so common, or rather not demonstrable with the same precision, are 

 instances in which one story shows twenty-one leaves which are connected by a 

 genetic spiral with eight revolutions; and where a story includes thirty- four 

 leaves which are connected by a genetic spiral with thirteen revolutions. In the 

 one case any two leaves next one another in age in a story are separated from one 



