No. 4.] PLANT DISEASES. 31 



just at the close of blossoming time a condition which I term 

 the critical period in the life of the potato plant, during 

 which the continued health of the plant, if not its very 

 life, hangs, as it were, in the balance. This is the turning 

 point, the crisis, in the life of the plant. Before this time 

 it is a plant of remarkable vegetative vigor, which will 

 quickly recover from very severe ravages of insects or other 

 adverse conditions ; a little later it will do the same ; but 

 serious injury to the foliage or arrest of development by 

 unfavorable soil conditions at this period will start the plant 

 upon a decline which continues to its death. No subsequent 

 treatment, therefore, can make amends for neglect during 

 this critical period. If, however, the plant is carried in full 

 vigor through this crisis, it starts upon what is virtually a 

 new lease of life, — a vegetative period which with the more 

 viirorous varieties in far northern climates seems to have no 

 clearly defined natural terminus. It is during this second 

 or vegetative period that the marketal^le crop is developed. 

 It is for this that we have grown the plant. It is therefore 

 Avorth while to know exactly Avhat happens during it, — to 

 trace the rate of growth of the real potato crop, the tubers. 

 This has been done at the Vermont Experiment Station 

 during three seasons, by beginning just after the blossom- 

 ing period and making partial diggings at ten-day intervals. 

 The plants were of course sprayed, to maintain life to full 

 maturity. The results were surprising to us at first, and are 

 of great practical importance. They show that with our 

 vigorous main crop varieties, even in northern Vermont, 

 where properly protected there is a possible period of from 

 eight to ten weeks for the tuber formation and growth ; and, 

 moreover, that the last half of this period is the most valua- 

 ble part. The folloAvang table shows the results we obtained 

 one year, which may illustrate this important point more 

 clearly : — 



