112 THE ESSEX NATURALIST.
The view that Monocotyledons are derived from Dicotyle-
donous ancestors is one for whose support a considerable mass
of evidence has been obtained. The typical leaves of Monocoty-
ledons, however, with their narrow shape, parallel veins and
sheathing bases, are remarkably different from what occur in any
large group of Dicotyledons. According to the "Phyllode Theory
of the Monocotyledonous Leaf," a paper recently published
by Mrs. Arber, all Monocotyledonous leaves are to be inter-
preted as representing either the leaf-stalk or leaf-base of the
Dicotyledonous leaf ; and where a blade is developed, this does
not correspond to the characteristic leaf-blade of Dicotyledons,
but is to be regarded as a new secondary structure formed by
the spreading out of the petiole, in response to special require-
ments. This theory has received strong additional support
from anatomical evidence, namely, in the discovery that many
Monocotyledons possess "inverted" vascular bundles in their
leaves. I will try to explain briefly what this means :—
A vascular bundle contains two kinds of conducting tissues,
wood and bast; the wood conducts water, the bast food-stuffs
The cylindrical petioles of Dicotyledons have a complete ring
of bundles, in which the wood lies on the inside, the bast on the
outside: Where the petiole meets the blade the bundle ring opens
out, with the result that all the bundles or veins in the blade
have the Wood on the upper side of the leaf and bast on the lower
In the leaves of many Monocotyledons it has now been found
that there occur, besides the normally oriented bundles, a series
of small inverted bundles near the upper leaf surface, with the
bast directed to the upper side and the wood to the lower. Such
an arrangement is readily accounted for if these so-called leaf-
blades are regarded as cylindrical petioles, that have become
flattened out and extended.
As an example we may take a leaf that seems to have been
little changed from a petiolar structure, like that of a Daffodil.
This is flat and of nearly equal width throughout; a cross section
shows that on the outer side (away from the axis of the plant),
is an arc of normal bundles, that is with bast on the outside and
wood inside; on the inner side of the leaf (facing the axis of the
plant) is a series of small inverted bundles, with a reverse arrange-
ment of wood and bast. It is just as if a ring of bundles had been
pressed so that the two sides came close together.