INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 259 



These observations show that the plants at Petrograd came to flowering with 

 a smaller temperature summation than did those at Brussels. It is also note- 

 worthy that the date of flowering at the northern station is very markedly later 

 than that at the southern only in case of the early-flowering forms, and that the 

 difference between the two dates decreases as the date for Brussels becomes later; 

 the difference is only eleven days in the case of the latest-flowering forms here 

 considered, the linden being one of this group. This last observation may 

 be explained by pointing out that the period with temperatures below the 

 freezing point of water is also important in the development of perennials. This 

 is a period of low activity, but not one of complete inactivity, and various 

 chemical transformations are completed during the cold winter, which prepare 

 the plant for the active growth of spring. These transformations are accelerated 

 only very slightly by higher temperatures, as may be seen in the case of the 

 sixth group considered above. The linden began to flower at Brussels only 

 eleven days earlier than at Petrograd, although the temperature at the southern 

 station was already above zero by the middle of January, and zero was not 

 passed as Petrograd until early April. Direct experiment shows that higher 

 temperature alone is not sufficient to bring plants out of the resting condition 

 into active growth. In an experiment in this connection twigs were removed 

 from a cherry tree at intervals throughout the winter and placed in a green- 

 house with a temperature of from 20 to 2 5°C. Twigs cut in the autumn failed to 

 produce leaves or flowers and finally died, while those cut during the winter and 

 early spring flowered after they had been exposed to the greenhouse temperature 

 for a certain time, this period becoming shorter with the advance of the season. 

 The number of days of greenhouse conditions required to produce flowers on 

 these twigs is shown below, for twigs cut at various dates. In spite of the 

 favorable temperature of the greenhouse, the earlier the twigs were cut, the 



longer was the period before flowering. 



Period Required to 

 Date of Cutting and Placing in Greenhouse Produce Flowers 



days 



Dec. 14 27 



Jan. 10 18 



Feb. 2 17 



Mar. 2 12 



Mar. 23 8 



Apr. 3 S 



This experiment shows that, in making an estimate of the amount of heat 

 necessary for development of the plant, it is necessary to consider the resting 

 period which may continue, or even begin, in spite of temperature conditions 

 generally favorable to active growth. Certain trees and shrubs, when trans- 

 ferred from temperate to warm climates and thus removed from the conditions 

 of their winter resting period, although adequately supplied with moisture and 

 heat (so that vital activity need not be directly retarded), still retain their 

 earlier habit for a long time, losing their leaves and passing over into the resting 

 condition for a part of the year. The life of the plant is thus not governed 



