THE POTATO TUBER. 
61 
most layer of these cells (Fig. 3 b) constitutes what is known 
as the cork cambium or phellogen ; the special function of 
which is the production of new cork cells. Micro-chemical 
tests indicate that the walls of these cork cells contain 
suberin, the presence of which confers upon them their corky 
character. At least, the application of the customary re- 
agents affords evidence which is usually accepted as de- 
monstrating the presence of this substance in vegetable 
tissues. Possibly no reagent of those so far employed yields 
better results than I ison’s gentian violet-ammonia reagent. 
If thin sections of the skin of the tuber, after passage suc- 
cessively through water and Eau de Javelle, are placed for 
twenty four hours in a dilute solution of gentian violet 
rendered almost colorless by the addition of ammonia, then 
partially discolorised by placing them in an aqueous solution 
of 8% hydrochloric acid, it will be found that the layers of 
cells forming the skin are deeply stained by the dve. Staining 
by this method is particularly useful, in that only the walls 
of the cork cells of the skin retain the stain, and thus enables 
one to differentiate these cork cells from those of the under- 
lying tissue, the walls of which are composed of cellulose. 
There is one exception to which, however, it will be con- 
venient to refer to later. 
Immediately subjacent to the skin, varying in size and 
form in different varieties of tubers, sometimes compressed 
and elongated, at others more or less isodiametric, are cells 
which are known as transitional cells (Fig. 3c). Their par- 
ticular function is not distinctly known, and it is quite an 
open question whether they do not possess a dual function ; 
that is to say, these units may possibly behave either as 
cork cambium cells or as parenchymatous cells. As we 
shall presently see, these transitional cells on mutilation, as 
occurs when a tuber is cut or bruised, are capable of forming 
wound cork, a phenomenon we shall consider in some detail 
when we come to deal with the results induced by mutilation 
of the tuber. The appearance of sections stained with 
Tison’s reagent negatives the presence of suberin in the 
w alls of the transitional cells. 
Turning once more to the exploration of our section 
we find, lying immediately beneath these transitional cells, 
large conspicuous cells, which form by far the greater mas 
of the tuber. These parenchymatous cells, as they are 
termed, constitute a magazine in which carbohydrate 
material in the form of starch is stored. Each parenchy 
matous cell represents a small compartment of this store 
house in which certain of the end products which arise as 
the result of the metabolic activities of the vegetative or 
above ground parts of the plant are stored. 
