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PHARMACEUTICAL BOTANY 
collateral vascular bundles of the stem extend, these showing xylem 
uppermost and phloem beneath; but in the Palmacece, Aracece, 
Dioscoreacece and Musaceee 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 parenchyme 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 Umbel- 
liferce where the flattened sheathing leaf stalk is known as the peri- 
cladium. Such a structure is not peculiar to the Umbelliferce for in 
many Ranunculacece, etc., a similar sheathing development is ob- 
served. The stalk may bear the laminar tissue on its extremity. 
This is most commonly the rule, but when the plant is exposed to 
xerophytic conditions, as the Acacias of Australia, the stalk instead 
of being cylindric or sub-cylindric becomes flattened from side to 
side until there is produced a bifacial vertically placed petiole with 
a large green surface that wholly takes the place of the lamina. 
The petiolar structure in primitive types of Dicotyls resembles 
that seen in Monocotyls except that the bundles are more con- 
densed side by side. In these the petiole is somewhat dorsiventral, 
shows an external epidermis, a flattened cortex with a set of parallel 
vascular bundles each with xylem uppermost and phloem below. 
