202 XV. THE LEAF VASCULAR BUNDLE ENDINGS. 



layers of cells, and joins on to the periderm of the twig with its 

 margins. Its phellogen lies on the side towards the stem. This 

 cork layer is traversed by the vascular bundles of the leaf. 

 Separated by some layers of cells from this periderm, and on the 

 leaf-stalk side of it, the absciss layer, only a few cell-rows thick r 

 passes amongst the roundish cells of the leaf-stalk, recognisable 

 by its yellow colour, the newly-intercalated dividing walls, and 

 the more copious contents of its cells, which likewise contain 

 small starch grains. It is first formed shortly before the fall of 

 the leaf (while the periderm was already present much earlier), 

 and is continued through the living elements of the vascular 

 bundle. Elsewhere, the cells of the leaf-stalk are almost com- 

 pletely emptied of reserve food-materials ; they contain, as treat- 

 ment with iodine shows, mere traces of starch. In the same way 

 starch is wanting, alike in leaf and cortex, within the vascular 

 bundle, although in the cortex it is very abundantly represented 

 in the vicinity of the bundle. The thin-walled elements of the 

 bundle are, on the other hand, filled with highly refractive 

 masses, which give fat and tannin reactions. 



If fresh sections are examined in water, this latter very quickly 

 shows blue fluorescence, from the sesculin which comes out of 

 the stem. Numerous cells of the leaf- stalk contain clusters of 

 crystals, or a single crystal, of oxalate of lime. Preparations 

 treated with acetic methyl-green show in the cells of the leaf-stalk 

 remains of the cytoplasmic sac, nucleus and chlorophyll grains. 

 The yellow grains, into which the chlorophyll bodies break up,, 

 give to the leaves their autumn tint. 



The fall of the leaf takes place within the absciss layer the 

 cells of which become rounded, and so disunited ; the vascular 

 bundle is torn through in the corresponding part. The leaf-scar 

 is covered by the roundish parenchyrnatous cells, which lie 

 between the absciss layer and the cork layer, and therefore at 

 first appears greenish. These cells become brown, and dry up 

 quickly in air. At the same time the vessels] become filled with 

 brown masses of wound-mucilage, which has come from the 

 neighbouring cells into the cavities of the vessels, and plugs them 

 up. Soon the formation of periderm begins in the living ele- 

 ments of the vascular bundle of the scar, only the vessels and 

 sieve-tubes being excluded in the process, and this periderm joins 

 on to that already present, so that the scar is quickly cut off by 

 a complete periderm layer, which continues to develop just as 



