Stimulation of Root Growth in Cuttings 93 



developed at approximately the same rate whether the twigs were taken 

 before or after the end of the rest period. In both cases the roots were 

 about equally developed forty-five days after the cuttings were made. 

 Very sim lar results were obtained with cuttings of Prunus cerasifera, 

 Comus stolonifera Michx., and Evonymus europaea, and with regard to 

 callus formation in Pyrus malus. 



Cuttings of Evonymiis taken in the latter part of September had a 

 strong root system developed by March 1, yet the buds did not break 

 until May 1.; while the buds on cuttings taken from the same bush on 

 December 29 started growth within ten days, but as no root system was 

 developed a large number of the twigs withered. Root and callus forma- 

 tion in cuttings, therefore, appear to be independent of the rest period, 

 as stated above. There is a possibihty that the wound shock at the 

 cut end brings that part out of the rest period; Howard (1915 b) and 

 other investigators have found that such a stimulus may shorten the 

 rest period. But in some cases the roots develop at some distance from 

 this- wounded tissue, and therefore this explanation is insufficient. The 

 cut of course exposes the adjacent tissue to better aeration, resulting 

 in a greater oxygen supply as well as a more rapid loss of carbon dioxide, 

 which might serve to start growth in the dormant cambium. When the 

 twig is immersed in water any inhibiting substances may be washed- 

 out of the tissues or the ready access of oxygen may oxidize them. Such 

 an explanation is not entirely satisfactory, however, for if the twig is 

 kept in a moist chamber, not only will both cut ends develop a callus 

 but also cork cells will be formed at the lenticels, indicating cellular activity 

 in regions far from the wounded part; yet in no cases do the buds develop, 

 even when submerged in water or when situated very close to the cut end. 



Simon (1906) has shown that root and callus formation proceed when 

 the buds are in a resting state. Howard, in his work on shortening the 

 rest period of cut twigs, makes no mention of any effect on their rooting 

 ability. In a report on experiments with several species of potted woody 

 plants (Howard, 1915 b), he states that treatments which stimulated 

 the tops had no effect on root growth. He does not say, however, how 

 he determined this, and he gives no data. 



Possibility that a treatment affecting the rest period may at the same tim£ 

 affect root growth — On the other hand, the fact that the stimulation 

 obtained was independent of an effect on the rest period is no proof 



