MONOCOTYLEDONOUS ROOT. 165 



tinguished with certainty in cross-section from the sieve-tubes. 

 From the endodermis the wood and bast are separated by a 

 single layer of cells, the pericycle (p). This pericycle, therefore, 

 occupies the same relative position in roots as in stems, and 

 constitutes the outermost layer of the central cylinder or stele. 

 In concentrated sulphuric acid the entire cross-section is dis- 

 solved, with the exception of the epidermis, the layer of cells 

 underlying it, the endodermis and the vessels. These last have 

 stained a beautiful yellow. The endodermis, which from the 

 action of the sulphuric acid will have partially turned over, shows 

 the middle band in the radial walls beautifully undulated. In 

 the outermost cortical layer also, adjoining the epidermis, a 

 similar appearance is, however, to be observed ; and if we 

 examine again our earlier preparation, we shall assure ourselves 

 that in this case also the radial walls show a dark shading. 

 The cells in question are also firmly united together, and form, 

 therefore, a kind of outer endodermis, which has been termed 

 suberose layer or exodermis, 



The longitudinal section shows the vessels with the thickenings 

 already referred to ; and with aniline blue the sieve-plates of the 

 sieve-tubes can easily be made visible through staining sky-blue. 

 From the sieve-tubes their companion-cells can now be distin- 

 guished by their abundant contents and their smaller length. 

 The wavy middle band of the radial walls of the endodermis, 

 seen now in surface view, shows as a ladder-like thickening. 



Boot of Hyacinth. The roots of the Hyacinth (Hyacintlms 

 orientalis), similarly grown in water, may be used as an alterna- 

 tive to those of the Onion, with which they agree in all essential 

 points. The number of vascular bundles usually varies between 

 six and eight. 



Acorns Calamus. The root of the Sweet Flag, (not uncommon 

 by the sides of streams and ditches in the eastern and midland 

 counties, and the rhizome of which is used in perfumery), will 

 serve to further inform us as to root-structure. The cross-section 

 of a piece of a fully-developed root (Fig. 66) shows that here the 

 xylem rays (s) of the central cylinder, do not coalesce in the 

 centre of the cylinder, but are usually, to the number of eight, 

 arranged in an interrupted ring, while the middle is occupied by 

 a pith, The large vessels lie, as in Allium, towards the interior, 

 the small ones towards the periphery. The bast strings (v) 

 alternate, as usual, with the wood rays. They are separated 



