DIV. i MORPHOLOGY 115 



rapid passage of the products of assimilation from the assimilatory 

 cells of every part of the leaf to the stem. At the same time it 

 facilitates the most direct supply of water to all parts of the transpir- 

 ing leaf-blade ; the leaf-blade serves for giving off water, while the 

 stem serves for conduction of water. Lastly, the venation increases 

 the rigidity of the lamina. 



It has been seen that the leaves are so arranged on the stem that 

 the leaf-blades, which on erect shoots have a more or less horizontal 

 position, are exposed to the light with the least shading by one 

 another. Many leaves can place their blades at right angles to the 

 incident light by their power of movement. In the case especially of 

 dorsiventral, plagiotropous branches the leaf-blades seen from above 

 are found to fit together 

 more or less closely 

 in a LEAF MOSAIC, the 

 upper surfaces of all 

 being exposed to the 

 light. 



(b) The Leaf-stalk 

 usually resembles a 

 stem, and in its internal 



Construction agrees FIG. 1S2. Imparipinnate leaf of Phaseolus with pulvini. kg, Main 

 with the midrib of the P ul a * ^se of petiole ; fg, pulvinus of one of the pinnae. 

 . (i nat. size.) 



leaf-blade or sometimes 



with the stem. Typical assimilatory tissue is wanting, and the vascular 

 bundles in the case of Angiosperms are usually arranged in an arc 

 open above. The leaf-stalk serves to carry the leaf-blade away from 

 the stem and to place it suitably with respect to the light. 



These movements of adjustment of the leaf to the light are sometimes carried 

 out by special localised swellings at the base or the summit of the leaf-stalk, or 

 in both situations. These LEAF-CUSHIONS or PULVINI work like hinges and occur 

 in many Leguminosae (Fig. 132). 



Stalked leaves, which are more frequent among Dicotyledons than in Monocoty- 

 ledons, either have the lamina sharply marked off from the petiole or the one 

 passes gradually into the other, the petiole appearing more or less winged. When 

 leaves are arranged in a rosette the stalks of the lower leaves are often so long 

 that the laminae borne on them are not shaded by the upper leaves. This is 

 shown very beautifully in the floating rosettes of the Water Nut (Trapa natans). 



(e) The Leaf-base ( 64 ). When the leaf-base of a foliage leaf is 

 specially formed, it usually serves to protect the bud and the younger 

 leaves, enclosing the bud after the leaf-blade has unfolded. 



Stipules are frequently developed from the leaf-base ; they stand 

 one on either side of the leaf to which they belong, forming a pair. 

 They may be inconspicuous (Fig. 124 nb) or larger, and yellow or 

 green in colour. When they serve only to protect the bud they are 



