OUTLINE OF STRUCTURAL BOTANY 13 
So the leaf may consist of the blade and the base without the 
petiole, as we see it in the long and narrow blade of grass with its 
base clasping the stem without an intermediate part representing a 
petiole. 
Beyond these three very general forms there are various other 
modifications such as the formation of a hollow tube by the petiole, 
as in the Sarracenia, or of the simple awl-like or needle-like forms 
of the pine leaves or the thread-like leaves of many aquatic plants, 
and in some plants the form and general structure of the leaf is 
modified according to the medium in which it exists. Thus some 
species of Sagittaria have thread-like leaves which are entirely 
submersed, living wholly below the surface of the water ; broad, 
rounded blades floating upon the surface of water and sharply 
angled arrow-like leaves which rise above the water’s surface. . 
Many instances of this polymorphism might be mentioned, the 
modifications being due primarily to the adaptation of the leaf 
to divers circumstances. 
A great, and in classification, a very important, division in the 
general structure of leaves is that which distinguishes the so-called 
parallel-veined leaves from the net-veined leaves. 
With the former, the parallel-veined leaf, is commonly associated 
the seed of a single cotyledon or lobe; while with the net-veined leaf 
is usually associated the two-lobed or dicotyledoned seed and upon 
these characters are founded the first great division in the class of 
flowering plants, the monocotyledonous and the dicotyledonous 
angiosperms, that is, the one-lobed and the two-lobed sub-groups of 
the class of plants having the ovules within an enclosing ovary. 
These striking characteristics in the construction of the leaves 
permit us, in a great majority of cases, to judge without further 
consideration whether a given plant belongs to one or the other of 
these two great groups. 
Exceptions, as to most rules, occur here, for there are plants 
with net-veined leaves belonging to the great group of monocoty- 
ledons, for example; the trilliums, which are members of the lily 
family, have net veins. Then also a few plants belonging to the 
group of dicotyledons have apparently parallel veins. 
Notwithstanding the few exceptions it becomes easy after a little 
observation to determine with which of the great sub-classes we 
have to do. 
Returning to our ‘buttercup leaf, we find that not only do the 
veins of the blade diverge and cross among themselves, but that 
the blade is deeply cut into several segments or lobes and that 
starting from the point of attachment of the petiole there radiate, 
