PLANT ORGANS AND ORGANISMS 
167 
Duration of Leaves.—Leaves vary as to their period of duration. 
They are: Persistent, or evergreen, if they remain green on the tree 
for a year or more. 
Deciduous, if unfolding in spring and falling in autumn. 
Caducous, or fugacious, if falling early in the season. 
Parts of Typical Leaf.—The parts of a typical leaf are petiole or 
leaf stalk, lamina or blade, and stipules. 
Gross Structure and Histology of the Petiole.—The petiole in 
Monocotyledons is usually a broadened, sheathing basal structure 
which connects the lamina to the stem. Into this a set of closed 
collateral vascular bundles of the stem extend, these showing xylem 
uppermost and phloem beneath; but in the Palmacece, Aracece, 
Dioscoreacece and Musacece the petiole in part or throughout may be 
much thickened, strengthened and developed as a semi-cylindric or 
cylindric structure frequently showing, as in Palmacece, generally, 
two sets of bundles. In all of these the petiole shows distinct scat¬ 
tered closed collateral bundles embedded in parenchyma and sur¬ 
rounded by epidermis. In the Monocotyl genus Maranta a special 
swelling is found at the apex of the petiole which is termed a pulvinus. 
In Dicotyledons the petiole attains its most perfect development 
and here usually shows differentiation into a pulvinus or leaf cushion 
and stalk portion. The pulvinus is sensitive to environal stimuli 
and in some groups as Oxalidacece and Leguminosce a gradual increase 
in sensitivity up to a perfect response can be traced. Moreover, in 
these, if we start with the simpler less sensitive pulvini and pass by 
stages to the most complex, we note that a special substance known 
as the aggregation body develops in the pulvinar cortex cells and 
that this substance undergoes rapid molecular change on stimulation 
of the leaf. The stalk portion of the petiole in Dicotyledons is 
usually plano-convex or nearly to quite circular in outline; rarely in 
certain families does it simulate Monocotyledons in becoming 
abruptly or gradually thinned or flattened or widened out so as to 
sheath round the stem. The most striking example of this is seen in 
the Umbelliferce where the flattened sheathing leaf stalk is known as 
the pericladium. Such a structure is not peculiar to the Umbelli - 
ferce for in many Ranunculacece, etc., a similar sheathing development 
is observed. The stalk may bear the laminar tissue on its extremity. 
