Mar. 15, 1924 
Studies on the Potato Tuber 
813 
and of the pith gradually disappear until it becomes impossible to differ¬ 
entiate between these two tissues. 
Between the vascular tissue and the cortex, there is differentiated in 
close proximity to the apex a special layer of parenchymatous cells— 
the endodermis. The cells of this layer are smaller than those of the 
cortex, are more regular, and lack intercellular spaces. On the radial 
walls there are hyaline swellings which stain red with phloroglucin and 
hydrochloric acid. These—the Casparian strips —though usually found 
as a narrow band along the radial walls often broaden out and cover a 
part of the tangential surface of the cell. At a later stage in the develop¬ 
ment of the endodermis a suberin lamella is deposited over the inner 
wall, which makes the cells more or less impermeable to dissolved sub¬ 
stances and gases. As the young tuber enlarges, the endodermis is less 
readily detected. The cells with the radial dots become separated by 
cells resembling those of the cortex, and gradually this layer disappears 
altogether, and only vestiges of it are seen in the basal part of the tuber. 
By virtue of its structure the endodermis may be considered as a layer 
of cells which regulates diffusion between two regions of different pres¬ 
sure, and may be the cause of different pressure. Its primary function 
is probably to restrict passage of w r ater and soluble substances to certain 
channels and to aid in the absorption of water (r j) . 2 
THE MATURE TUBER 
A median longitudinal cut through a tuber shows two zones of tissue— 
a central pith with its lateral branches, and the vascular ring surrounded 
on either side by thick storage parenchyma, in which numerous small 
islets of phloem are embedded. The outer covering of the tuber is formed 
by the periderm, a new tissue which has replaced the epidermis of the 
tuber primordium. 
The pith forms the central part of the tuber. It is broadest near the 
middle. In its course it gives off at intervals branches which in the form 
of laterally compressed hollow cylinders communicate with the lateral 
branches of the tuber—the eyes. The relative lengths of these branches 
and the angle which they form with the pith varies greatly, but is, never¬ 
theless, closely related to the phyllotaxy and the steepness of the spiral 
formed by the eyes. The general appearance and the size pf the pith 
and its branches also varies, but is more or less constant for a given 
variety. The pith either terminates abruptly in the region of the apical 
bud, or it branches out some distance below the apex, each branch com¬ 
municating with one of the eyes of the apical eye cluster. In either case, 
however, the terminal bud forms a broad connection with the pith tissue— 
a fact which seems to be related to the time of renewal of growth. 
The cortex forms a narrow band of. tissue limited internally by an 
almost uninterrupted circle of phloem groups, externally by the periderm. 
In the young stolon, as has been previously noted, the cortex occupied a 
large area compared with the organ as a whole, but in the later develop¬ 
ment of the tuber the cortex adds but little new tissue, and hardly more 
than doubles the number of rows of cells in the radial extent. 
The mature cells of the cortex, like those of the pith, are polyhedral, 
with a median diameter of about 180 microns. The periclinal walls are 
larger, because of the great peripheral growth of the organ. Compared 
* Reference is made by number (italic) to ** Literature cited,” pp. 834 - 835 . 
