120 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



the bundles are arranged in a ring (e.g. Ivy, Horse Chestnut), but 

 as a rule the petiole is more or less flattened, or grooved, on its 

 upper surface, and if several bundles are present they are usually 

 arranged in a curved band, the xylems being on the concave side, 

 which faces upwards. Collenchyma is generally present below 

 the epidermis, and in Dicotyledonous petioles there is a rudi- 

 mentary or for some time functional cambium between the xylem 

 and phloem of the bundles. 



147. Vertical Transverse Section of Leaf Blade. 



Remove a strip of Laurel or of Ivy leaf by making a cut 

 down each side of the midrib, including a portion of the 

 thin wing. Hold the strip in pith, and cut thin sections 

 at right angles to the midrib. The midrib projects on the 

 lower side of the leaf, hence it is easy to distinguish the 

 upper and lower sides of the section when mounted. In 

 the midrib note the large bundle, or curved band of 

 bundles, with the xylem facing upwards ; in Laurel there 

 are numerous brown cells around the bundles ; within 

 the epidermis on both sides there is a zone of collen- 

 chyma. 



In the thin lateral parts, on each side of the midrib, 

 note 



(1) the upper epidermis, the upper and lower cell- 

 walls usually convex, and the upper wall covered with 

 cuticle ; 



(2) the palisade mesophyll, consisting of cells elon- 

 gated vertically and containing abundant chloroplasts 

 on being traced downwards the cells of the palisade layers 

 (often two or three in number) become shorter and less 

 closely packed and pass into 



(3) the spongy mesophyll, in which the cells are of 

 irregular shape, contain chloroplasts, and are loosely 

 arranged, large intercellular air-spaces being present ; 



(4) the small bundles or veins, lying between the pali- 

 sade and spongy zones of the mesophyll ; 



(5) small cells here and there in the mesophyll, con- 

 taining either single crystals or spherical crystal clusters 

 consisting of calcium oxalate; 



(6) the lower epidermis, showing at places the two 

 small guard-cells of a stoma. 



