SUNFLOWER LEAF 123 



phyll-containing parenchyma with intercellular spaces 

 Note beneath each stoma an enlarged intercellular 

 space the air-chamber. 



3. In the number and arrangement of the vascular 

 bundles. In the petiole there are three main bundles, 

 besides several smaller ones (compare the rough dis- 

 section of the stem, p. 56). 



4. In the absence of interfascicular cambium : the 

 larger bundles are, for a short time at least, open 

 bundles, having an active cambium, while the smaller 

 ones are closed, having no secondary thickening by 

 cambium. 



5. No general bundle-sheath is present, though each 

 bundle is surrounded by a layer of colourless cells 

 without intercellular spaces, which may be regarded as 

 representing the bundle-sheath. 



On the petiole of some leaves, both of Dicotyledons and Mono- 

 cotyledons, a cylindrical swelling or pulvinus is found : it is 

 usually at the base of the petiole, and smaller ones may also be 

 found at the foot of each pinna : this may be clearly seen in 

 Phaseolus, and other Leguminosse. Leaves provided with a pul- 

 vinus are frequently capable of change of position, turning upon 

 it as upon a hinge : observe the difference of position of the 

 leaves of Phaseolus at midday, and in the evening. 



Cut transverse sections through the pulvinus of Phaseolus : 

 mount in glycerine or chlor-zinc-iodine, and observe 



1. The epidermis, with a well-marked cuticle, and conical, 

 often hooked hairs. 



2. A broad band of cortex, consisting of cells with thin 

 cellulose walls, and very small intercellular spaces. 



3. This is limited internally by a starch-sheath, which sur- 

 rounds a central cylindrical vascular mass : this is composed of 



4. A peripheral band of phloem, with considerable collen- 

 chymatous thickening of the walls of the outer portion. 





