162 BOTANICAL GAZETTE | SEPIEMBER 
again. Here we have as a temporary condition in dicotyledons 
the structure which is permanent in monocotyledons. 
Stems.—The young stems of monocotyledons and dicotyle- 
dons differ less than do the old stems of these sub-classes. In 
young stems of dicotyledons the skeletal system is composed of 
separate fibrovascular bundles which traverse the parenchymatous 
ground tissue, and at this stage the hypodermal tissues are not 
unlike, either in composition or arrangement. In herbaceous 
stems this similarity is maintained much longer than in woody 
stems, where the dissimilarity eventually becomes extreme. The 
important difference between these two types of stems is that 
the skeletal tissues combine to form a single solid column in the 
dicotyledons, while they do not in the monocotyledons. Now 
when to this we add the fact that the bundles of dicotyledons 
have fused in such a manner that their continued growth adds 
to the mass of the skeletal column, thus giving to the stem the 
possibility of indefinite increase in mass, we have again an indi- 
cation of the higher rank of this sub-class. 
In regard to external morphology it may be remarked that in 
monocotyledons there are two well defined modifications of the 
normal type of vegetative stem, as seen in lilies, naiads, orchids, 
etc. One extreme of this modification occurs in the grasses and 
sedges in which the internodes are greatly elongated, and the 
other in palms and screw-pines, in which the internodes are 
usually so short as to be scarcely recognizable. The suggestion 
which these stem modifications offer as to the relationship of 
grasses and sedges on the one hand, and palms and screw-pines 
on the other, to the lilies is obvious. 
Leaves.—In general structure the leaves of angiosperms are 
essentially alike. The significant differences may be enumerated 
as follows: 
1. The leaves of monocotyledons are usually entire, elongated, 
parallel-veined blades, placed alternately or scattered upon the 
stem, to which they are attached directly (in sessile leaves) oF 
indirectly (in petioled leaves) by a commonly broad _ base 
which is rarely supplied with stipules. 
