THE ARRANGEMENT OF LEAVES. 149 



gation of the petiole ; as in the pea, vetch, agrimony, 

 quince, etc. 



IV. THE ARRANGEMENT OF LEAVES (PHYLLOTAXIS). 



197. Leaves are disposed on stems in various ways : 



(1.) They may be in whorls of three or more encircling 

 the stem at intervals. In this case each whorl was formed as 

 a ring of rudimentary leaves about the punctum vegetationis.* 

 The leaves of each succeeding whorl usually appear just 

 above and between the preceding ones, so that the whorls 

 alternate with one another. 



(2.) Where two leaves originate on exactly opposite sides 

 of, and at the same height on, the punctum vegetationis, the 

 opposite arrangement is produced. Here, as in whorled 

 leaves, the new ones usually arise in the intervals between 

 the previously formed ones, so that the pairs of leaves decus- 

 sate. 



(3.) If the leaves originate singly (scattered or alternate 

 leaves), the simplest case is that in which each succeeding 

 leaf appears a little above the preceding and on the opposite 

 side of the punctum vegetationis. In this case, where the 

 stems elongate, the leaves are arranged in two opposite lon- 

 gitudinal rows or ranks (orthostichies), f hence this is called 

 a two-ranked arrangement. 



(4.) If, instead of each new leaf forming at a point half 

 of the circumference of the punctum vegetationis from the 

 last, it appears at a point distant (always in the same direc- 

 tion) one third of the circumference, there will be three ver- 

 tical rows of leaves upon the stem ; this is the three-ranked 

 arrangement. 



(5.) In rare cases the succeeding leaf is in each case distant 

 one fourth of the circumference from the last, always meas- 

 uring in the same direction ; this, gives rise to the four 1 

 ranked arrangement. 



* There are some cases of false whorls, in which the leaves are first 

 formed at different heights, and only later by irregularities in the 

 growth of the stem become whorled. 



f From the Greek optfoS, straight, and ar/^oS, a row. 



