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 Musacea the petiole in part or throughout may be 

 much thickened, strengthened and developed as a semi-cylindric or 

 cylindric structure frequently showing, as in Palmacea, 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 Leguminoscz 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 Umbellifera where the flattened sheathing leaf stalk is known as 

 the peridadium. Such a structure is not peculiar to the Umbelli- 

 fera for in many Ranunculacece, etc., a similar sheathing development 

 is observed. The stalk may bear the laminar tissue on its extremity. 



