Au«. s, 19x8 Anatomy of the Potato Plant 251 
connection occurs between the outer and the inner phloem through the 
leaf and branch gaps (PI. 40, B). 
Through such a connection the inner and outer phloem become inter¬ 
dependent, just as do the individual groups of either region through 
branching and anastomosis, as stated above. The physiological impor¬ 
tance of the connection of the phloem groups becomes self-evident and 
must be taken into account when interpreting pathological conditions of 
the conducting system of the plant. 
Secondary growth is quite extensive, and it is necessary to understand 
the relation of the primary growth to secondary tissues in order to judge 
correctly their relative importance. It is obvious that there is a great 
amount of secondary wood formed, but the importance of the secondary 
phloem seems to have escaped attention; at least it has often been held 
that secondary phloem does not play a great r 61 e in the transport of 
plastic materials. But since the large amount of secondary phloem 
formed consists chiefly of sieve tubes, it seems self-evident that it is of 
primary importance in translocation. Before the time of tuberization 
the movement of plastic materials is localized, most of the organic food 
being used in the building up of new tissues and for respiration. For 
the comparatively small downward movement the primary phloem is 
sufficient, and little or no secondary phloem develops. At the time of 
flowering, when tuber formation is under way, secondary sieve tubes are 
formed in large numbers. But while the secondary phloem is formed 
and takes part in the translocation of plastic materials, the primary 
groups remain active until the plant is mature. Of course, the delicate 
walls of the phloem elements become somewhat thickened, but this is a 
condition to be expected in older structures. 
The process of tuber formation has been treated by Reed, whose 
observations this study confirms and extends. But while Reed believes 
most of the tuber tissue to be formed by the pith and the perimedullary 
zone, the writer is led to conclude that the pith does not contribute 
much to the formation of new tissues, but that it is especially the peri¬ 
medullary zone which forms most of the tuber. There is further a 
divergence of opinion in regard to the origin of the periderm. De Vries 
(7, 8 ), states that the periderm is formed by the epidermis, whereas 
Reed (11) shows figures to prove that it arises from the hypodermis. 
A study of the series of photographs (PI. 39, A-F) shows that, though 
most of the periderm is formed from the hypodermis, a superficial peri¬ 
derm several cells thick is formed by the epidermis. 
LITERATURE CITED 
(1) Schacht, Hermann. 
1856. BERICHT an DAS k6NIGEICHE LANDES-OEKONOMIE-KOEIvEGIUM ObER DIE 
KARTOFEELPEEANZB UND DEREN KRANKHEITEN. 29 p., 10 pi. Berlin. 
(2) Hanstein, J. L. E. R. von. 
1864. DIE MIBCHSAETGEEASSE UND DIE VERWANDTEN ORGANE DER RINDE. . ♦ 
92 p., 10 pi. Berlin. 
