1 90S.] Supposed Degeneracy of the Potato. 



3-i 



been converted into something — a sugar — which is soluble. 

 This conversion is effected by a ferment or enzyme called 

 diastase. It can be extracted from the potato tuber and it 

 will do its work quite as effectively outside as inside ; it has 

 been found ; however, that if an extract be made from an 

 ordinary healthy potato it has, at first, no action on starch ; 

 but, after standing for a few days, attacks it energetically. 

 (Vines, Physiology of Plants, p. 192.) 



The explanation is that diastase is not present as such in a 

 potato tuber but is the result of the decomposition of another 

 substance called a zymogen, a word which merely means 

 " ferment producer." Before the starch in a potato tuber 

 can be made available to support growth two things have to 

 take place : (1) Decomposition of the zymogen to produce 

 diastase ; (2) conversion of the starch into sugar by its action. 

 Until they do take place there will be no growth from the tuber. 



When, therefore, a tuber fails to grow it is not a sufficient 

 explanation of the failure to say that the plant is degenerate, 

 or that diastase is wanting. Both explanations are con- 

 clusively disproved by the interesting experiment of the 

 Curator of Kew in exposing the sluggish tubers to a higher 

 temperature. They then sprouted vigorously, and, after 

 planting, produced a normal crop. It is clear that what 

 was really amiss was that the diastase remained locked up in 

 the zymogen and was not available to do its work. It is known 

 that the production of diastase requires the access of oxygen 

 from the air ; it also needs some amount of warmth and is 

 probably stimulated by light. 



When a sluggish tuber is planted, it either remains com- 

 pletely dormant or sends up a feeble growth which utilises 

 little of the accumulated starch. On the other hand, the 

 shoot manufactures more, and this is transferred to the tuber 

 which actually increases in size. What is now complained 

 of seems to be only an extreme form of a state of tilings 

 which has long been known to cultivators. In Gloucestershire 

 the remedy recommended is " greening " the tubers. The 

 method is described in Leaflet No. 173, pp. 4, 5, issued by the 

 Board. The tubers " are placed in the autumn in shallow 

 boxes containing no soil or other material, and stored through- 

 out the winter in tiers in a cool, well-ventilated and well- 



