368 Introduction. [BOOK in. 



the other hand, the predilection for mechanics and mechanical 

 explanation of organic processes in Newton's age bore fair fruit 

 in Hales' enquiries into the movement of sap in plants ; his 

 ' Statical Essays' of 1727 connect closely with the works before 

 mentioned which had laid the foundations of the science, and 

 with this important performance the first period of its history 

 reaches a distinctly marked conclusion. 



This time of vigorous advance was followed by many years, 

 in which no notable work was done and no great discovery 

 effected ; there was active disputation on what had been 

 already ascertained, but it did not lead to any deeper concep- 

 tion of the questions or to new experimental determinations. 



2. About the year 1760 new life was infused into the con- 

 sideration of various branches of vegetable physiology. Du 

 Hamel's 'Physique des arbres ' (1758) gave a summary of 

 former knowledge and added a number of new observations, 

 and from that time till the beginning of the present century a 

 series of important discoveries was made. The doctrine of 

 sexual propagation, which had scarcely been advanced since 

 the time of Camerarius, and was disfigured by the theory of 

 evolution, found an observer of the first rank in Koelreuter 

 (1760-1770), who threw new light upon the nature of sexuality 

 by his experiments on the artificial production of hybrids ; he 

 was the first who carefully studied the arrangements for polli- 

 nation, and pointed out the remarkable connection between 

 them and insect-life. These relations were afterwards (1793) 

 examined in greater detail by Konrad Sprengel, who arrived at 

 such astonishing and far-reaching results, that they were not 

 even understood by his contemporaries, nor was their signifi- 

 cance fully appreciated till quite modern times and in connec- 

 tion with the theory of descent. 



No less important was the advance made in the doctrine of 

 the nourishment of plants. Between 1780 and 1790 Ingen 

 Houss proved, that the green parts of plants absorb carbon 

 dioxide under the influence of light and eliminate the oxygen, 



