148 ARRANGEMENT OF LEAVES. 
Thistles, it is decurrent (fig. 283); when the two sides of the 
base of a leaf project beyond the stem, and unite, as in the 
Hare’s-ear (fig. 284), it is said to be perfoliate, because the 
stem then appears to pass through the blade ; or when two 
leaves placed at the same level on opposite sides of the stem 
unite more or less by their bases, they are said to be connate, 
as in the Teasels and some species of Honeysuckle (fig. 285). 
2. ARRANGEMENT OF LEAVES ON THE STEM OR PHYLLO- 
TAXIS.—The term phyllotaxis is used in a general sense to 
indicate the various modes in which leaves are arranged on the 
stem or branches. The following are the more important 
varieties. Thus, when only one leaf arises from a node, the 
leaves as they succeed each other are placed alternately on 
different sides of the stem, and are then said to be alternate 
(fig. 289). When two leaves are produced at a node, they are 
usually situated on opposite sides of the stem, in which case 
Fic. 286. 
Fig. 286. Whorled leaves of a species of Galium.—Fig. 287, Decussate 
leaves of Pimelea decussata. 
they are described as opposite (fig. 287) ; or when three or more 
leaves arise from the stem so as to be arranged around it at the 
same level in the form of a circle, they are called verticillate or 
whorled (fig. 286), and each circle is termed a verticil or whorl. 
When leaves are opposite, the pairs as they succeed each other 
usually cross at right angles, in which case they*are said to 
decussute (fig. 287), and the arrangement is called decussation. 
When different whorls succeed each other it also frequently 
happens that a somewhat similar arrangement occurs ; thus the 
leaves of one whorl correspond to the intervals of the whorl 
below it. There are, however, commonly great irregularities 
in this respect, and in some cases the number of leaves in the 
successive whorls vary, by which their arrangement becomes 
still more complicated. This is the case, for instance, in Lysi- 
machia vulgaris. 
