8O EXPERIMENTAL PLANT PHYSIOLOGY. 



while that in the other becomes turbid from the formation of oxa- 

 late of calcium. The heated portion permits the escape of oxalic 

 acid which it contains. 



63. Loss of Heat. On account of the importance of warmth 

 for the. chemical processes in the building up of the plant, 

 many plants possess peculiar adaptations for preventing undue 

 loss of heat. 



EXPERIMENT 107. 



ADAPTATIONS TO PREVENT LOSS OF HEAT. 



Grow seedlings of Helianthus (Sunflower) and Cucurbita 

 (Squash). As soon as the cotyledons are raised above the earth, 

 it may be observed that they are extended during the daytime, and 

 during the coolness of the evening close together above, whereby 

 the loss of heat by radiation is decreased. (See Experiment 75.) 



64. Resting Period. It is known that the winter buds of 

 trees and shrubs can be made to open very early in the spring 

 if they are placed in a warm room or greenhouse. In this 

 way, shoots cut from Syringa vulgaris (Lilac), or the Willow. 

 in February, may be given an early development. It might 

 be inferred that these plants are compelled to rest by the winter 

 cold and need only heat to set in motion their normal develop- 

 ment. This is not, however, entirely true. Experiments have 

 shown that the winter resting period is necessary for the plant, 

 or rather that it has become accustomed to it by thousands 

 of years of habit. It is on account of this acquired habit that 

 buds brought into a warm room in January do not begin to 

 develop before March, and Potato-tubers brought into a warm 

 room in the autumn do not begin to germinate until after 

 a resting period of greater or less duration. Potato-tubers 

 which are placed in a temperature of zero centigrade, for four 

 weeks immediately after digging in August, upon being planted 

 in garden soil and watered, will begin the development of 

 buds. 



