Went — 151 — 



ing that it is hard to see what physiological process could be responsible for 

 the breaking of the dormancy. A similar phenomenon is known for Gladio- 

 lus corms (Denny 1942). When these are stored at warm soil temper- 

 atures, no development takes places. But short periods, of 24 hours or less, 

 at 0-5 °C., will break their dormancy. These cases may be comparable with 

 the chilling requirements of some seeds, which will not germinate until they 

 have been subjected to freezing (Crocker 1916). In this case hard-seeded- 

 ness has been held responsible for the dormancy, the freezing softening the 

 seed coat. 



Most deciduous trees of the temperate zone pass through a period of 

 dormancy, during which time the buds cannot, or only with great difficulty, 

 be made to develop. This is a secondary induced dormancy. In spring, 

 soon after the buds have broken, axillary buds enlarge on the young shoots, 

 and in early summer they have reached full size. By that time these buds 

 are not dormant, but defoliation, or placing cut branches in water in the 

 greenhouse, will cause almost immediate development into shoots. Two 

 months later the buds, when subjected to the same treatment, will not de- 

 velop even under favorable conditions. At the time of leaf fall the buds 

 have reached the stage of deepest dormancy. In trees growing out of doors 

 this dormancy decreases, until it has completely disappeared in spring. When 

 branches or small trees are kept in the greenhouse, the dormancy does not 

 disappear. In nature the dormancy may not be broken by the time of spring 

 when the winter has been exceptionally warm. This gives rise to the phe- 

 nomenon of "delayed foliation" (Chandler et al. 1937). Consequently 

 many of these deciduous plants requiring a cold winter cannot be grown in 

 climates with an equitable climate, such as subtropical regions and higher 

 altitudes in the tropics. 



Precise laboratory experiments on the chilling requirements of de- 

 ciduous trees have not been carried out as yet. Numerous observations in 

 nature have led to the following conclusions : 



Only temperatures below 5-8° C, seem to be effective in breaking dor- 

 mancy. These low temperatures must last for a sufficient number of hours, 

 so that for each species and variety of plant a minimal number of hours be- 

 low 5°C., can be assigned, which are required before the tree will leaf out. 

 These hours have not necessarily to be consecutive, but the effect is cumu- 

 lative. Varieties native to colder climates have a longer chilling require- 

 ment than those from climates with warmer winters. Therefore northern 

 varieties grown in the south usually show delayed foliation (late and erratic 

 breaking of buds in spring), whereas southern varieties may be killed farther 

 north when their chilling requirements have been met before the danger of 

 late frosts is past. Although the purely factual description of chilling re- 

 quirements is very incomplete as yet, the physiology of the buds is better 

 investigated. No reference will be made to the numerous papers describing 

 the chemical composition of dormant tissues and of tissues whose dormancy 

 has been broken, naturally or by artificial means. Nor will the many arti- 

 ficial methods employed to break dormancy be enumerated. 



Bennett and Skoog (1938) found that buds kept dormant by leaving 

 pear trees during winter in a heated greenhouse, could be made to sprout 

 by application of yeast extract. Guthrie (1940) suggested that the glu- 



