638 GERMINATION AND DORMANCY 



and that the buds of many such species are in a quiescent rather than a dor- 

 mant state during much of the winter. 



Dormancy of buds is sometimes due to correlative effects, many of which 

 probably have a hormonal mechanism. A familiar example is the phenomenon 

 of apical dominance (Chap. XXXIV), in which lateral buds remain dormant 

 as long as the terminal growing region remains intact, but usually resume 

 growth upon its injury or removal. 



Likewise the buds of many kinds of tubers, rhizomes, corms, and bulbs 

 often remain dormant for periods of greater or less duration while environ- 

 mental conditions are favorable for their development. 



Methods of Breaking the Dormancy of Buds. — As the previous dis- 

 cussion has shown the buds on many species of woody plants remain dormant 

 through the autumn and at least the forepart of the winter. This dormancy 

 is not caused by low temperatures as it is also exhibited by such species when 

 kept in a greenhouse where the temperature is continuously maintained at 

 levels typical of midsummer weather. The dormant condition often persists 

 as long as the temperatures are high but may be broken by subjecting the 

 plants to temperatures near the freezing point (Coville, 1920). The buds 

 of some species such as peach will begin to grow, if environmental conditions 

 are favorable, after only a few days of chilling, but the buds of other kinds 

 of plants such as the blueberry (Vaccinium corymbostim) fail to grow well 

 unless exposed to temperatures near the freezing point for several weeks. 

 Coville has demonstrated that the effect of the low temperatures is restricted 

 to the tissues exposed. When single branches are chilled while the rest of 

 the plant is kept warm, only the buds on the chilled branch are able to grow, 

 all of the others remaining dormant (Fig. 140). 



The dormant buds of many species of plants may be induced to grow by 

 immersing them in warm water for several hours. Molisch, who discovered 

 this method of forcing dormant buds, found that submerging the branches in 

 a water bath at 30 to 35° C. for 9 to 12 hours was effective with many 

 species. The success of the treatment varies with the kind of plant and with 

 the time of the year at which the treatment is applied. Buds of some species 

 fail to react to the warm bath treatment in September but grow readily if 

 treated in the same way in October or November. Like the low temperature 

 treatment just described the effect of the warm bath is local, only those buds 

 that are brought into contact with the warm water developing. 



Early in the Twentieth Century Johannsen discovered that dormant buds 

 of many kinds of plants will begin to grow after being exposed to vapors of 

 ether or chloroform for a day or two. The interval of time between the ether 

 treatment and the beginning of growth differs widely with the time of the 



