1897] PHYLOGENY AND TAXONOMY OF THE ANGIOSPERMS 163 
2. The leaves of dicotyledons are entire or more commonly 
dentate or lobed, usually broad, netted-veined blades, opposite, 
alternate, or scattered upon the stem, to which they are usually 
attached indirectly (petiolate) by a narrow base (rarely by a 
broad base), which is commonly supplied with stipules. 
These structural differences are mainly due to differences 
in development. The parallelism of venation and the general 
absence of lobing in the leaves of monocotyledons result from 
the localization of growth at the base of the blade or in definite 
bands on each side of its axis, and commonly the netted venation 
in the leaves of dicotyledons results from the longer continued 
and more or less irregular growth of all parts of the blade; and 
it is to this irregularity of growth, also (especially in the peri- 
pheral portions), that the serrations, dentations, lobings, etc., are 
due. The development of a petiole is correlated with the 
assimilatory function of the leaf, and in both sub-classes is less 
or more, according to the degree of its illumination. The broad 
basal attachment in monocotyledons may depend upon the looser 
disposition of the fibrovascular bundles in the stems, or possibly 
it may indicate that leaf and stem are not yet as fully differen- 
tiated as they are in dicotyledons, a view which receives some 
confirmatory suggestion from the presence of an articulation at 
the base of the leaf in most dicotyledons, while it is absent from 
most monocotyledons. The significance of the stipules is not so 
obvious ; probably their more frequent occurrence in dicotyledons 
is correlated with the more common development of the petiole 
in this sub-class. 
The particular morphology of leaves is commonly indicative 
of relationship between species and genera, and now and then it 
has a broader significance. In the monocotyledons the common 
type of leaf is particularly modified in the sedges and grasses, 
this modified type being maintained with great constancy through- 
out the two great families. Among dicotyledons the greatly 
branched (“compound”) leaves of mimosas (Mimosacez), 
brasilettos (Czsalpiniacee), sumachs (Anacardiacez), walnuts 
(Juglandacez), and umbellifers (Umbellifere) are characteristic 
