438 ZONES AND REGIONS [Pt. Ill, Sect. II 



of a period of rest that is independent of external influences. These experi- 

 ments have also determined the optimum temperatures for the develop- 

 ment of the flowers and fruits of temperate trees, and though not adequately 

 appreciated they have put an end to a chaos of false statements regarding 

 the connexion between temperature and periodicity. 



Most fruit-trees are suitable for forcing only after a few frosts, which 

 accelerate the transition to the active condition, so that, for instance, the 

 forcing of the peach-tree cannot be commenced before January ; in Belgium, 

 however, it succeeds from November onwards. The vine, on the other 

 hand, comes into a condition in which it can be forced at temperatures some- 

 what above zero. 



When forcing is commenced the temperature need by no means be high, 

 and, at most, should not exceed 6° to 8° C. It is gradually raised, but 

 never actually above the temperatures prevailing in nature during the 

 corresponding stages of development. Higher temperatures cause ab- 

 normal features, such as excessively long shoots, defective formation of 

 wood, atrophy of flowers, and so forth. It is especially necessary that 

 the temperature be kept low during the flowering time, and, in the case 

 of stone-fruits, during the stoning, otherwise the flowers or young fruits 

 drop off. The growth of the fruit, however, demands higher temperatures 

 than does the development of the flower-buds, but for late autumnal fruits 

 (grapes) moderate final temperatures are again necessary. 



Night-temperatures must be kept from 2 to 4 centigrade degrees lower 

 than day-temperatures ; otherwise hypertrophic phenomena set in. 



It follows from the above-mentioned and other phenomena that the opti- 

 mum temperatures for the processes of growth in length are higher than 

 those for other processes — such as the formation of flower, the development 

 of wood — so that higher temperatures favour the former at the expense of the 

 latter. Experience in the cultivation of temperate woody plants in warmer 

 zones completely confirms this ; as there also growth is greatly accelerated 1 . 

 Light retards growth, and it is possibly owing to this fact that forced 

 plants withstand higher temperatures by day and in the sun than by 

 night or on a cloudy day. 



The general results of the experience of practical men amount to this, 

 that in the case of our fruit-trees the degrees of temperature naturally 

 prevailing during the vegetative season approximately correspond to the 

 optimum for the formation of flower and fruit, as well as for the formation 

 of wood, but are below the optimum for the processes of growth of foliage- 

 shoots. Hence the forcing of fruit-trees is generally confined, in the first 

 place, to the abolition of that portion of the period of rest that is due 

 to low temperatures, and, in the second place, to imitating as closely 

 as possible the temperatures prevailing during the different stages of 



1 See p. 48 ff. 



