THE LEAF 243 
bundles. In all of these the petiole shows distinct closed col- 
lateral to concentric bundles embedded in parenchyma which in 
turn is surrounded by epidermis. In the Monocotyl genus 
Maranta of the Arrowroot family, a special swelling is found at 
the apex of the petiole which is termed a Pulvinus. 
In DicoryLepons, the PETIOLE attains its most perfect devel- 
opment and here usually shows differentiation into a short basal 
part called the Putvinus or leaf 
cushion and a longer part, the 
STALK portion. (See Fig. 18.) 
The pulvinus is sensitive to 
environal stimuli and in some 
groups, as Oxalidacee and Legu- end \ 
minosé, 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. Fic. 164.—Barley, (a member of 
The stalk portion of the petiole the Grass Family). 4, portion of 
: . : leaf at juncture of blade and decurrent 
In Dicotyledons 1S usually plano- sheath; B, stem cut in median longi- 
convex or nearly to quite Cir- tudinal section. 214. (Robbins.) 
cular in outline; rarely in certain 
families does it resemble 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 Parsley family (Umbellifere) where the flattened 
sheathing leaf stalk is known as the PericLaprum. Such a 
structure is not peculiar to the Umbellifere, for in many members 
of the Crowfoot family (Ranunculacee), etc., a similar sheathing 
development is observed. The stalk may bear the laminar tissue 
on its extremity. This is most commonly the rule, but when the 
