Development of Natural History. 131 
As in the previous period the mathematical subjects, 
namely, forest measurement and forest valuation, were 
more systematically developed than the natural history 
basis of forestry practice; the slower progress of the 
latter being caused by the greater difficulties of studying 
natural history and of utilizing direct observation. 
In botanical direction descriptive forest botany was 
first developed and several good books were published by 
Walther, Borkhausen, Bechstein, Reum, the latter 
(1814), of high value, and also by Behlen, Gwinner and 
Hartig. 
In the direction of plant physiology Cotta early and 
creditably attempted (1806) to explain the movement 
and function of sap, but remained unnoticed. Mayer's 
(1805-1808) essay on the influence of the natural forces 
on the growth and nutrition of trees, contains interest- 
ing physiological explanations for advanced silvicultural 
practice. But these sporadic attempts to secure a bio- 
logical basis were soon forgotten. Not until Theodor 
Harttg (1878) published his Anatomy and Physiology 
of Woody Plants was the necessity for exact investi-' 
gation of forest biology as a basis for silvicultural 
practice fully recognized. With the development of 
general biological botany or ecology a new era for silvi- 
culture seems to have arrived. Perhaps in this connec- 
tion there should be mentioned as one of the earlier im- 
portant contributions of much moment, G. Heyer’s 
Verhalten der Baume gegen Licht und Schatten (1856) 
in which the theory of influence of light and shade on 
forest development was elaborated. 
Among those who placed the study of pathology of 
forest trees on a scientific basis should be mentioned first 
Willkomm (1876), followed by R. Hartig. 
