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



the conclusion of the swelling period. The later rapid and vigorous growth 

 up to the completion of maturity takes place at the expense of the glucose 

 carried thither by the vessels from the branches and stem. 



The opening of the buds consumes a considerable part of the carbo- 

 hydrates that are stored in the axes. The starch, which just before was 

 so abundant, undergoes an extensive diminution, which however is in part 

 accounted for by conversion of starch into glucose that is not used. This 

 spring-minimum of the carbohydrates, especially of starch, is of short 

 duration, as very soon fresh quantities of assimilated material are produced 

 by the young leaves and conducted to the food-reservoirs. Thus com- 

 mences the accumulation that culminates in the autumnal maximum. 



In the above paragraphs, processes of metabolism have been described 

 that come into play partly in the period of rest, during the cool and cold 

 seasons. The question arises how far they are directly dependent on the 

 temperature^ or how far determined by inherent hereditary characters. 

 Experiments show that hotli tcvipcraturc and inlicriiaucc co-cperatc. 



The disappearance of starch from the cortex at the commencement 

 of winter is a direct effect of low temperatures, for it does not occur in 

 the twigs of trees that are exposed during the period in question to higher 

 temperatures, in rooms or in plant-houses. The reformation of the starch 

 is likewise a function of the temperature, for when the temperature is 

 sufficiently high (minimum 5° C, optimum 25°-30°), it commences in 

 a few hours, and even in the smallest pieces of cortex, so long as they 

 possess uninjured cells. The cooling down to 2° C. of such branches 

 in which reformation of starch has taken place causes the starch to dis- 

 appear again. 



The connexion between the phenomena just described and the tem- 

 perature is evident, but the temperature is not the sole controlling factor, 

 for if it were so the starch would disappear, even in summer, after an 

 artificial reduction of temperature. This however is not the case. 



ii. STARCH-TREES AND FAT-TREES. 



The wood)' plants of the cold temperate zones, so far as is known, all 

 behave in the main like the gean-tree. In details however they exhibit 

 many differences. Independent of the generally known external differences 

 in periodicity, a group of starch-trees and another of fat-trees have been 

 established on the basis of their respective conditions in winter, as revealed 

 in Russow's investigations. In the starch-trees, to which chiefly hard- 

 woods — among others the gean-tree — belong, at the commencement 

 of winter only very little fat is produced at the expense of the starch, 

 which in the cortex is converted into glucose and unknown bodies, but 



1 See p. 48. 



