68 THE PLANT CliLL. 



be seen protoplasmic contents and starch. Wood-parenchyma 

 is, in fact, a tissue set apart for the supplementary storing of 

 carbohydrates, and is analogous to the phloem-parenchyma. 



All these elements of the xylem, with the exception of the 

 wood-parenchyma, are functional in the upward conduction of 

 raw unelaborated sap from the root to the leaves, the moving 

 forces being the transpiration current and root-pressure (see 

 Chap. X.). The tracheides are especially useful in this process, 

 it being probable that the sap passes partly by means of the cell- 

 cavities and partly through the cell-walls. The various forms of 

 " pits " occurring in the walls may possibly be of use in sap- 

 conduction, but, as a matter of fact, these pits function more as 

 a means of exit for the protoplasm after it has finished its work 

 in the xylem-elements. Those elements of the central cylinder 

 of Dicotyledons and Conifers which now remain to be studied — 

 viz., the medullary rays, the endodermis, the pericycle, and the 

 medulla — are not conducting tissues, but are, nevertheless, of 

 importance from several points of view. 



Note. — To facilitate tlie practical examination of the xylem those plants 

 in which the various elements may be studied will now be mentioned. In 

 all cases both longitudinal and transverse sections should be made of the 

 stems or roots : — 



Pitted and reticulate vessels and tracheides, Bog-oak, Hazel, Sicinus, 

 and Lime (stems of all these). 



Tracheides with bordered-pits : Pinim (stem or root). 



Scalariform (reticulate) vessels : Pteris and other Filicinese. 



Spiral and annular elements : protoxylem of Dicotyledons and Conifers, 

 and xylem of Monocotyledons and Ferns. Also the bundles in many leaves. 



Wood-parenchyma : Hazel (stem). 



In showing up wood-elements either solution of aniline sulphate or 

 iodine solution may be used. The latter reagent also shows up the endo- 

 dermis (starch-sheath) wood-parench}'ma and medullary rays, since the 

 starch in the cells of these tissues turns dark blue. 



In the examination of individual separate elements wood may be 

 macerated in Schulze's macerating mixture,* which dissolves the middle 

 lamellae, and the resulting mass teased out, washed in distilled water, and 

 the separate elements stained with methyl-green solution. 



C. The Medullary Rays (see Figs. 52, 53, and 54). 

 Medullary rays are of two kinds, viz. : — 



(a) Primary or ground-tlssue rays. 

 (h) Secondary or true medullary rays. 



* Dilute nitric acid (20 per cent.), to which 2 to 3 per cent, of a. 

 saturated solution of chlorate of potash has been added. 



