282 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1919. 
went into a condition of complete dormancy. They did this at a 
greenhouse temperature which in spring and summer would have kept 
the plants in a condition of luxuriant growth. The completeness of 
the condition of dormancy which such plants reach can be best ap- 
preciated from photographs. (See pi. 1.) 
Since 1910 this experiment has been repeated many times, and 
with many species of plants, and without exception those trees and 
shrubs native of our northern cold-winter region which were tested 
went dormant in fall or winter regardless of temperature. In com- 
paring outdoor plants with indoor plants of the same species the 
most that can be said in favor of outdoor conditions is that dormancy 
progresses a little faster in outdoor plants, evidently because their 
foliage is injured by freezing weather, and they drop their leaves 
somewhat earlier than indoor plants. 
2. Trees and shrubs that are kept contiTWUsly warm during the 
winter start into growth mmich later in spring than those that have 
teen subjected to a period of chilling. 
In the late winter and early spring of 1910 I waited patiently, 
and then impatiently, for my indoor plants to bloom, and at last 
I was forced to realize that they never would bloom. When com- 
pared with plants of the same kind that had been outdoors during 
the winter and had been brought into the greenhouse in early spring, 
the difference was astonishing. The outdoor plants burst into leaf 
and flower luxuriantly, while the indoor plants remained com- 
pletely dormant and naked. The experiment was repeated many 
times and with various species of plants, some of which may be used 
in illustration. (See pis. 2 to 5.) 
At first it was supposed that the plants needed to be frozen to 
start them into growth, but a single freezing proved not to be effec- 
tive. And then it was found that the dormant plants would start 
into growth without any freezing whatever. It was necessary only 
that they be subjected to a period of prolonged chilling, usually two 
to three months, at a temperature a few degrees above freezing. 
If plants are kept continuously in a warm place without chilling, 
the dormant condition often continues for an extraordinaiy length 
of time. In some instances plants have remained dormant for a 
whole year under conditions of heat, light, and moisture that ordi- 
narily would make the same plant grow with the greatest luxuriance. 
3. The stimulating effect of cold is limited to such portions of the 
plant as are subjected to tlie chilling. 
The conspicuous difference in spring growth between chilled plants 
and plants not chilled has already been shown. These differences, 
furthermore, can be produced experimentally upon different parts 
