The External Form of Plants. | 89 
manner on a branch which has remained undeveloped, as 
in the larch. | : | 
Since the secondary axes (branches) generally spring 
from axillary buds, it follows that their arrangement on the 
stem is dependent on the phyllotaxis of those leaves in the ~ 
axils of which they arise, as is seen at once in any labiate 
plant. It is also easily understood that an apparently alto- 
gether irregular arrangement of the branches on the stem 
may be the result of the abortion of a number of buds. 
A leaf of the most perfect development may be divided 
into its sheath, its petiole or leaf stalk, and its lamina or 
flatly expanded blade, although one or other of these parts | 
Fic. 141.—Diagram of the 3/8 phyllotaxis | Fic. 142.—a Split leaf-sheath of a 
of Plantago. grass; 4 ligule; d@ part of the 
lamina of the leaf; c node of the 
culm. 
-is most often wanting. Thus the leaves of grasses have a 
sheath, but no petiole ; those of the apple and pear a petiole, 
but no sheath ; in those of many Umbelliferee, such as Azge- — 
lica, all three parts occur. 
The eaf-sheath embraces the stem in the form of a hol- 
low tube. In grasses it is always split in front (Fig. 142) ; 
