150 PRACTICAL COURSE IN BOTANY 
Fic. 188.— Whorled 
leaves of Indian cucum- 
ber. 
The different forms are characterized by 
the angular distance between the points 
of leaf insertion around the stem. In the 
elm, basswood, and most grasses, they are 
distributed in two rows or ranks on op- 
posite sides of the stem, each just half 
way round the circumference from the 
one next in succession (Fig. 189), the 
third in vertical order standing directly 
over the first. In most of our common 
trees and shrubs five leaves are passed 
in making two turns round the stem, 
the sixth leaf in vertical order stand- 
ing over the first. This is called the five-ranked arrange- 
ment, and is the most 
common order among 
dicotyls. 
169. Relation be- 
tween the shape and 
arrangement of leaves. 
— Phyllotaxy is of im- 
portance chiefly on ac- 
count of its influence 
on the light relation of 
leaves. A compact, 
close-ranked arrange- 
ment tends to shut off 
the light from thelower * 
nodes, and hence, in 
plants where it pre- 
vails, the leaves are apt 
to be long and narrow 
in proportion to the 
frequency of the ver- 
tical rows. The yucca, 
oleander, Canada flea- 
Fic. 189. — Twig of a hackberry (Celtis cinerea), 
showing the two-ranked arrangement. Notice how 
the position of the stems and branches of the main 
axis corresponds to that of the leaves. 
