Chap. II] PERIODIC PHENOMENA IN TEMPERATE ZONES 437 



in the wood remains unaltered. In the group of fat- trees, which are chiefly 

 soft-wooded species, such as conifers, birches, and lime-trees., all the starch 

 in the cortex and wood is converted into fat, and this condition lasts until 

 spring, when the fat is reconverted into starch. 



The formation of fat from starch, and of starch from fat, like the changes 

 mentioned above in the cortex of the gean-tree, are dependent, on the 

 one hand, on inherent characters possessed only during the season of rest, 

 and, on the other hand, on the temperature. The production of fat does 

 not take place while the atmosphere is warm, and any fat that may be 

 present is then converted into starch. 



iii. THEORY OF FORCING. 



The protoplasm of the plants of temperate zones exists in tivo conditions^ 

 one active and one quiescent. The regular periodic alternation of these 

 conditions, as in the tropics, is occasioned by inherent hereditary characters, 

 and they are distinguished by the difference in their behaviour in relation 

 to temperature, as well as by other features. 



In active protoplasm, by means of higher temperatures stimuli are set up 

 that induce the processes of growth, whereas lower degrees of heat result in 

 a general cessation of growth. 



In quiescent protoplasm even optimum temperatures do not call forth 

 phenomena of growth ; on the other hand, changes of temperature produce 

 a reaction in the form of metabolic changes, and these are induced in part 

 by lower and in part by higher temperatures. 



The quiescent condition of the protoplasm is of much shorter duration 

 than its active condition, and by no means persists through the whole 

 normal period of rest. This period of rest is in its second and greater 

 part a direct result of low temperature, and it may accordingly be shortened 

 in this part by a rise in temperature. The forcing of plants depends on 

 this circumstance. On the contrary, the first part of the period of rest, 

 which in the gean-tree lasts from the middle of October to the end of 

 November, but in other woody plants is often shorter (for instance Forsythia 

 viridissima) or longer (Fagus sylvatica), depends exclusively on inherent 

 characters and is not influenced by a rise of temperature. It is quite 

 useless, and even harmful, to try to begin forcing before the end of this 

 necessary period of rest, as buds even under the most favourable tempera- 

 ture remain in their winter condition. Transition from the one condition 

 to the other is slow, and forcing will be in any one case more rapid the 

 more imminent was the completion of the change of the quiescent proto- 

 plasm into active protoplasm. Low temperatures accelerate the transition. 



Horticultural experiments, purely practical in nature but none the less 

 valuable on that account, made particularly on fruit-trees with the object of 

 accelerating development (forcing), most clearly demonstrate the existence 



