462 SOLANACE^ 



made to germinate before a certain time has elapsed. Some 

 varieties need a rest of two months only, while others ripened 

 in autumn do not show signs of growth before January or 

 February, or even later. 



The minimum temperature for germination is about 8° or 

 TO* C, so that tubers planted very early make little or no 

 growth. 



The cause of the resting-period and the chemical changes 

 which go on during that time are not clear. Respiration which 

 is carried on at the expense of the starch can be recognised; 

 at first it is slow, but increases rapidly towards the end of the 

 resting-period. 



When germination commences, the enzyme diastase is formed, 

 whereby the starch is changed into sugar : the latter is trans- 

 ferred to the growing buds, where it is utilised in the formation 

 of new cells. The first development of the shoots is carried on 

 at the expense of the stores of reserve-food within the tuber. 



Rarely do two buds on the same tuber develop equally 

 strongly, the most vigorous being the terminal one, or the 

 central bud in the ' eyes ' near the apex of the tuber. The 

 buds at the base of the tuber are weakest, and often remain 

 dormant altogether. When tubers are cut for ' sets ' so that 

 each piece contains one 'eye,' those pieces from the 'rose' end 

 always produce the most vigorous plants and the best yield. 

 If the main shoot produced from the central bud of an ' eye ' 

 is broken off or otherwise destroyed, the lateral buds in the 

 ' eye ' grow out, but their shoots are never so strong or vigorous 

 as the lost one. 



The shoots produced from the growing buds of potatoes 

 exposed to the light during germination have short internodes 

 and scaly leaves, in the axils of which three lateral buds are 

 usually visible. After planting the tuber, the tip of the main 

 axis of each shoot grows upwards into the open air, where the 

 unfolding leaves carry on ' assimilation.' The food manufac- 



