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Transactions. 



on both surfaces. There are three vascular bundles — a small one near each 

 margin, and a larger one, which is surrounded by a well-marked endo- 

 dermis, occupying the centre of the petiole. Some of the cells of this 

 endodermis are slightly suberized. The structure of the vascular bundle 

 is the same as in the leaf. 



The mesophyll consists of large, thin-walled, more or less circular cells 

 which are closely packed together. At the margin — that is, in the vicinity 

 of the small vascular bundles — the cells are smaller, and they contain 

 abundant chloroplasts. In the other part of the ground-tissue, especially 

 near the epidermis, there are a few chloroplasts, but most of this tissue 

 is the colourless aqueous tissue. 



Fig. 16. — Cardamine heterophylla. Transverse section of flower- bearing 

 stem ( X 230). a, chlorenchymatous cortex ; b, endodermis ; 

 c, pericycle; rf, phloem; e, xylem- vessel; /, wood-fibre; gr, pith. 



Flowerhtg-stem (fig. 16). — The epidermis consists of small oval or rect- 

 angular cells in which all the cell-walls, but especially the inner and outer, 

 are thickened ; there is a thin cuticle. Stomata are present, the guard-cells 

 being somewhat larger than in the leaf ; they are protected by small 

 guard-cell ridges. The whole of the cortex, with the exception of the endo- 

 dermis, is chlorenchymatous. It consists of oval, thin-waUed cells which 

 vary in size and which contain numerous small chloroplasts. 



