563 



make up for the evaporation of the foliage', has the temperature of the soil 

 layers. R. Hartig- furnishes very clear proof of this. The branches were 

 removed from one of two similar trees, on which the sun shone, so that in 

 the current of evaporation they almost came to a standstill. The ther- 

 mometer then proved a temperature of about lO degrees lower in the tree 

 on which the leaves had been left than in that from which the branches 

 had been removed. After the removal of the branches of the second 

 specimen, its temperature at once increased about lo degrees. 



Since, in spring, the air body warms up very quickly, it soon increases 

 the direct action of the sun's rays on the branches" and keeps them at a 

 temperature at which they can grow. The more intense and lasting the 

 warmth in the air, the more the cambial ring is stimulated so that its pro- 

 duction of new wood and bark elements extends from the crown down- 

 ward until, in April and May, it reaches the roots and then finally causes 

 the production of a new wood ring. The time of the awakening, the 

 thickness of the new wood ring and its maturation diifer in different tree 

 species. In fact,^ an individual difference disappears inasmuch as all speci- 

 mens are not able every year to produce so much plastic material in the 

 tree top that it will sufhce for the nutrition of the cambial mantel of the 

 roots. It then happens that the thickening ring, in such a lean year, 

 extends from the top only to the base of the trunk where it tapers out to 

 nothing, so that the roots in this year do not become any thicker. 



The heat wa\c and therefore the acti\ity of the cambial ring gradually 

 disappear in autumn from abo\e downward, just as they had advanced. 

 Since the soil remains warm longer than the air, the root has still oppor- 

 tunity to continue its growth e\en if no longer so intensively. This 

 explains v. Mohl's observation tliat roots in December, January and Febru- 

 ary are still active in thickening the cell walls of the last formed annual 

 ring. 



Definite figures will give the clearest idea of this. v. Mohl found in 

 the winter of 1861-62 that the root formation in a sweet cherry tree had 

 not stopped by the 4th of April. In this the branch buds had already 

 developed a length of more than 2 cm. and the new wood ring in the parent 

 branch had so far matured its new ducts that their pitting was recognizable. 



1 Ebermaj'er, Die phy.sikalischen Einwirkungen des Waldes auf Luft und Boden. 

 I, Aschaffenburg- 1873, p. 119-39. Measurements show that no essential difference 

 exists between the temperature of the trees (breast hig-h) and of forest soil. With 

 increasing depth of soil and height of tree, however, the differences become marked. 

 In general it is found that, from October to March, forest trees are colder than 

 forest soil. "The roots in this period are the warmest part of the tree; the mean 

 tree temperature decreases with increasing- height and is lowest in the branches and 

 twigs." "In the summer half of the year (April up to and including September), 

 conversely, forest trees are warmer than the soil, i. e., the temperature of the tree 

 decreases from above downward and. during the day, is highest in the twigs and 

 branches, lowest in the roots." The mean annual temperature of the tree fluctuates 

 between 3.9 to 6.7 degrees according to the elevation in the plane of growth. It is 

 less than the mean air temperature and higher than the mean soil temperature of 

 the forest. 



2 Lehrbuch der Baumkrankheiten 1882, p. 177. 



3 Compare Krutsch, Untersuchung liber die Tcmperatur der Baume, etc. Jahrb. 

 d. Kgl. Sachsischen Akad. zu Tharand, v. X, 1854. 



