240 RESULTS OF COMPARATIVE OBSERVATIONS: BAVARIA. 



all depths to 2 feet a greater diflFerence between open fields and woods 

 in the former than in the latter, it being 1O.02R. at the surface, O^i.TOat 

 half a foot, 0o.52 at 1 foot, 0O.33 at 2 feet, while at 3 and 4 feet the tem- 

 peratures of autumn are higher than those in the spring by Oo.OO and 

 00.40, respectively. 



The deductions of obgervations are considered separately by months, 

 with a consideration of their iufluence upon vegetation ; but our space 

 does not admit of these details. Their tendency will be inferred from 

 what has already been shown in respect to the seasons. 



As regards the temperature at different hours in the day, it is found 

 that in the open fields the temperature is highest in the afternoon 

 throughout the year. The daily variation is greatest at the surface, and 

 it decreases with the increasing depth. At 3 feet it averages but O^.OL 

 or 0^02 R., and is so slight as to have but very little practical importance. 

 These daily variations in the temperature of the soil differ considerably 

 with the seasons, being greater in summer than in winter. They are 

 about the same in spring as in summer, and become less in autumn. In 

 wooded soil the difference of temperature in morning and evening at all 

 depths and on the surface is less than in the open field. The greatest 

 difference is observed in May. Forests, therefore, diminish the daily 

 variations of temperature and their spreading in the soil. The daily 

 change is less as the elevation above sea-level becomes greater, as is 

 shown by the following table : 



Differences between the temperature of soil in the ^voods and the open fields at different elevations. 



As to the temperature of the earth in the forest and in the open fields to the 

 depth of four feet, or the influence of the forest on the warmth of the 

 earth. 



Experience has proved that a warm soil is more favorable to vegeta- 

 tion than a cold one. But to better understand the influence and im- 

 portance of warmth in the earth to the development of plants, investi- 

 gations and observations have been made by Sachs, and later by Bialo- 

 blocki, which led to the following results : 



1. By increased warmth of the earth, the growth of plants, especially 

 in their first stages of development, is hastened. 



2. Heat operates directly on the development of the roots, and through 

 them indirectly upon the parts of the plant above the surface, thus 

 affecting the growth of the whole plant. The branching out and devel- 

 opment of the root is materially increased by raising the warmth. 



3. The degree of activity in the roots being dependent upon the tem- 

 perature, the absorption of moisture follows in corresponding degree. 

 If the temperature is lessened, the function of the roots is diminished, 

 and at 41° Fahrenheit the absorption by some plants, as, for instance, 

 the tobacco and the cabbage, is insufficient to supply the loss by evapora- 

 tion from the leaves, causing them to wilt, especially if exposed to the 

 direct rays of the sun. A rise of temperature, on the contrary, increases 

 the absorbent power of the roots. 



