CHAPTER XII. 



THE CENTRAL CYLINDER (STELE) AND SECONDARY 

 THICKENING OF ROOTS. 



PKINCIPAL MATERIALS USED. 



Roots of Onion (Allium Cepa], or of Hyacinth, or of Acorus Calamus ; fresh 



or in alcohol. 



Roots of Flag Iris ; alcohol material. 

 Roots of Ranunculus repens : in alcohol. 

 Roots of Taxus baccata, e.g., from seedlings ; alcohol material, softened with 



alcohol-glycerine. 



PRINCIPAL REAGENTS USED. 



Concentrated sulphuric acid Potash Aniline blue. 



Root of Allium Cepa. The structure of the central cylinder 

 of roots we will first study in the root of Allium Cepa, the garden 

 Onion. Abundant material for investigation can be at any time 

 secured by allowing an onion to grow in water in a hyacinth 

 glass. Figure 65 shows us a cross-section from the base of a 

 strong adventitious root thus obtained. The epidermis, or 

 piliferous layer (so called from bearing the root-hairs), and the 

 very thick primary cortex, are left out of the drawing, but of the 

 latter we see the cells (c) which bound the bundle-sheath, or 

 endodermis (e). This endodermis shows a characteristic dark 

 shading upon the radial walls, which is produced by the wavy 

 flexure of a median strip of the walls, which is also suberised. 

 Such an endodermis is always unilamellar ; it arises from the 

 innermost layer of the cortex. In old roots the endodermic cells 

 are thickened on their inner and radial walls in U-form, and these 

 thickening layers disguise the original structure of the radial 

 walls. The centre of the vascular cylinder is occupied by broad 

 scalariform vessels (sc). If the root is not old enough the 



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