122 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



Note that the cork of the stem does not run on to the 

 petiole (which has collenchyma below its epidermis), and 

 that where it stops short a cork-layer runs across the base 

 of the petiole. On the outer side of this cork-layer is the 

 absciss-layer, a zone of loose rounded yellowish cells. The 

 cork-layer is at first interrupted by the bundles that pass 

 into the petiole, but on the disorganisation of the absciss- 



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SB. Longitudinal Section of Node of Sycamore Stem, showing the Absciss 

 Layer across the base of the Leaf on each side. Above each leaf-base an 

 axillary bud is seen. 



layer (which is continued through the parenchyma of the 

 bundle) the cork-layer is completed by the formation of 

 cork over the projecting stumps of the bundles, and then 

 the leaf is separated, the vessels of the exposed bundles 

 being compressed and closed while the cork-layer is left 

 covering the leaf-scar. The cells in the petiole contain, as 

 a rule, abundant calcium oxalate crystals, while those in 

 the stem cortex usually contain starch. 



151. Isobilateral Leaves, Pliyllodes, Centric Leaves. 

 In some leaves which grow nearly erect, e.g. Hyacinth, the tissues 

 have much the same arrangement on the two sides, but the bundles 

 have the xylem facing the upper surface ; in Hyacinth itself, for 

 instance, both sides have stomata, and there is no distinct palisade 

 tissue. In other cases, however, isobilateral structure is shown, 

 e.g. in Iris, where palisade tissue occurs within the stoma-bearing 



