54 MISSOURI AGR. EXP. STA. RESEARCH BULLETIN NO. 21 



sition stage between the main, deep state of rest and the first begin- 

 ning of growth has been aptly termed the "after rest." During this 

 period there appears to be considerable enzyme activity. Also at 

 this time rest breaking treatments have little or no effect in hastening 

 growth. Indeed, treatments which, during the middle rest, were 

 capable of starting growth in a short time, now often injure the 

 plants and actually hinder growth. Just as in the beginning of the 

 rest period, certain of the plant parts end their rest before other 

 parts. The enzymic activity in one twig may be more rapid than 

 in another but as soon as the excess supply of the products of their 

 work is reduced to a certain point, growth begins. When this 

 equilibrium has been established or is about to be reached, treatments 

 not only fail to hasten but may even retard the starting of growth. 



Briefly then the rest period first begins to set in on account of 

 the inhibition of the enzymes by the accumulation of the products of 

 their work. This is the early rest of mid or late summer. In the 

 fall, excess supplies of carbohydrates continue to be accumulated 

 and the further inhibition of the enzymes is actively aided by the 

 approach of cool weather. These factors acting together bring 

 about the main or middle state of rest. However, since dormant trees 

 are never completely at rest respiration continuing all the time, 

 and doubtless enzyme activity too the over-accumulation of 

 carbohydrates is gradually reduced. Toward the last of this period 

 occurs the after-rest which, as the enzymes become more and more 

 active, gives place to the beginning of growth. 



The length of the rest (the total time occupied by the three rest 

 phases mentioned), varies greatly with the different species. In 

 some the rest lasts from June or July to March or April, while 

 others may be ready to grow again after two or three weeks of 

 dormancy. 



ACKNOWLEDGMENT 



Too much credit cannot be given to A. J. Heinicke, research 

 scholar in horticulture, 1913-14, but now instructor in pomological 

 research in Cornell University, for valuable services rendered in 

 connection with this investigation. Mr. Heinicke displayed great 

 originality in devising apparatus and methods for conducting the 

 various tests described in this bulletin. A great many of the tests 

 were very tedious and exacting, and required great devotion to the 

 work in order that they might be carried out in detail. Furthermore, 

 it should be added that Mr. Heinicke conducted the tests and 

 recorded his observations with care and accuracy. In recognition 



