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368 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 conception of the questions or to new experimental determinations.

2. About the year 1760 new life was infused into the consideration 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 pollination, 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 significance fully appreciated till quite modern times and in connection 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,