POTATO 461 



increasing regularly in thickness when active growth begins 

 again, grow out from the ends or about the lateral 'eyes.' The 

 new growths may form irregular lumps or even smaller tubers on 

 the older ones; this is known as super-tuberation or second 

 growth, and is most common in kidney and oval varieties. 



The anatomy of the tuber in its young state resembles that of 

 the rhizome, to which it belongs, and like all similar stems 

 possesses epidermis, cortex, and vascular cylinder with its cam- 

 bium-ring and central medulla. The disposition of the tissues 

 is readily seen in a young tuber (Fig. 143). 



In a fully developed tuber the epidermis is replaced by peri- 

 derm, the outer layer of which consists of cork-cells; the latter 

 afford protection against excessive loss of water from the interior 

 of the tuber. Beneath the 

 ' skin ' or periderm is the 

 cortex, and in its outer cells 

 the cell sap is frequently 

 coloured, giving a charac- 

 teristic tint to the different 

 varieties of potatoes. The 



ramhinm in its PTOwth nro- ^'°- i43- — Longitudinal section of a young 



camuium m lis growill pro potato tuber. <: cortex; V vascular bundle; m 



duces much wood, and it is ">=f uUa ; ^ scale leaf in the axil of which is a 

 ' bud ; i terminal bud. 



this tissue which forms the 



main bulk of the tuber; instead of the wood, however, con- 

 sisting of lignified tissue it is almost entirely made up of 

 parenchymatous thin-walled cells, with only a few isolated 

 groups of lignified elements, and cannot therefore be readily 

 distinguished from the medulla and cortex. 



The chief reserve-food stored in the tuber is starch, the 

 largest amount being found in the innermost parts of the cortex, 

 the degenerate wood-tissue, and part of the medulla. In thin 

 slices of the potato the bast, cambium and centre of the medulla 

 appear semi-translucent, and contain little or no starch. 



Germination of the Tuber. — Ripe potatoes cannot be 



