513 



Thes-e reserve substances are protected so far as is possible against frost. 

 Part of the starch wanders into the protected central portion of the trunk 

 and branches (pith, medullary rays and parenchyma wood), and part is 

 transverted into sugar or occurs instead as a fatty oil. In the needles of 

 Alpine spruces, the substance of the chloroplasts is found to flow away and 

 the cell content in winter forms a homogeneous cytoplasmic mass with 

 abundant oil drops. Lidforss' has proved this transformation for all the 

 green cells of evergreen plants ; in the spring the starch is reformed. 



This removal of solid bodies from the cell with the appearance oi 

 winter takes place, according to Mez, as an advantageous arrangement in 

 plants resistant to freezing. He calls the fluid substances "thermally active," 

 for, in cr^'stallization, they set free heat. The solid elements, on the other 

 hand, follow retardingly the temperature of the fluids ; they are "thermally 

 passive" and absorb heat, since, with the formation of ice, the change of 

 temperature from the point of supercooling towards zero, they must again 

 give up this heat relatively rapidly. This circumstance acts in such a way 

 that, with the accumulation of solid bodies in the cell, the melting point 

 of the cell sap cannot be reached after supercooling has taken place. A 

 great number of thermally passive elements consequently form a menace 

 for the plant, while the fluid, thermally active bodies are proved advan- 

 tageous as producers of heat. Profiting by the experiments of A. Fischer-, 

 we will distinguish between oil trees and starch trees, according to whether 

 they change their starch into oil or let it pass into the interior of their trunks 

 and branches and convert it into sugar in the bark. The fatty oil of oil 

 trees (conifers, birches), which we have learned to recognize from Jonescu 

 as a protection against lightning, besides this peculiarity of preventing 

 supercooling, like sugar, is thermally active, i. e., stores up heat to be given 

 out in crystallization. The trees which transform all their starch into oil, 

 conifers, may be fitted to survive a higher degree of cold than those in which 

 a part of the starch is left free and becomes sugar only in the bark (the ma- 

 jority of deciduous trees). This circumstance surely explains the phe- 

 nomenon that conifers and birches extend farther up into cold regions. 



Disturbance due to Chilling. 



Cases occur in potted plants in greenhouses, in which the plants suffer 

 when carried from one house to another, in case they are thus exposed to a 

 temperature below zero degrees at times for only a few minutes. Practical 

 gardeners maintain that the plants have "taken cold." 



Moebius^ has studied this statement very recently, and has been able to 

 confirm the above assertion. For example, he took a Begonia mctallica 

 from a warm house, kept it one or two minutes out of doors in a tempera- 

 ture of 5 degrees C below zero and then put it again in its former place. 



1 Lidforss, Zur Physiologie und Biologie der wintergriinen Flora. Bot. 

 CentralbL 1896, p. 33. 



-' Jjihrb. f. wiss. Bot. 1891, p. 155, cit. by Pfeffer loc. cit.. p. 137. 



3 Mobius, M., Die Erkaltung der Pflanzen. Ber. d. D. Bot. Ges. 1907, Vol. XXV 

 pt. 2, p. 67. 



