126 THE SECOND BOOK OF BOTANY. 



seen in Fig. 268. The leaves in this example are 

 seen to form two rows along the side of the stem, 

 which are separated by half its diameter. 



This is the distichous, two-ranked, or arrange- 

 ment. 



If, in passing from one leaf to another, directly 

 above it, the string goes but once round the stem, 

 and the fourth leaf is over the first, giving a cycle of 

 three leaves, the arrangement is like that shown in 

 Figs. 269 and 270. There are three perpendicular 

 rows of leaves along the stem, separated from each 

 other by its circumference. 



This is the tri-stichous, three-ranked, or J- arrange- 

 ment. 



Again, the string may pass twice around the stem 

 before it reaches the leaf placed just over the first, 

 which, on counting, proves to be the sixth (Fig. 272). 

 There are five longitudinal rows along the stem, sep- 

 arated from each other by f its circumference. 



This is the pentastichous, quincuncial, or f ar- 

 rangement. 



Observe that the numerator in the foregoing frac- 

 tions gives the number of times the string winds 

 around the stem in completing a cycle, while the de- 

 nominator gives the number of leaves in the cycle. 



This fraction is sometimes called the angle of di- 

 vergence of the leaves. In Fig. 268 the angle of di- 

 vergence is -J- the circumference of the stem ; in Fig. 

 269 it is -J-, and in Fig. 271 it is f its circumference. 



In studying some of your specimens, the string 

 may pass three times round the stem in its spiral 

 course before you come to a leaf placed over the first, 

 and this leaf may be the ninth in the upward succes- 



