THE GROWTH OF BIOLOGICAL IDEAS 151 



for von Baer and other great descriptive embryologists of the 

 nineteenth century. 



Scarcely anything was known about the function or sig- 

 nificance of flowers until toward the end of the seventeenth 

 century, when people at last began to realize that the stamens 

 and pollen could be regarded as male and the style, ovary, and 

 ovule as female. This knowledge came from the work of 

 the English medical practitioner Nehemiah Grew and the 

 Tubingen professor, Camerarius. The latter removed the 

 male flowers of plants in w^hich the sexes are borne sepa- 

 rately and found that fruit was not set. It was in the sixties 

 of the eighteenth century that the professional botanist Koel- 

 reuter first showed clearly that certain plants are pollinated 

 by the wind and others by insects. At the end of the century 

 the hermit-like Christian Sprengel made a wonderfully exact 

 study of insect pollination and the devices by which plants 

 escape self-fertilization. 



Understanding of the significance of leaves came later than 

 that of flowers. In the first half of the seventeenth century 

 the mystical chemist van Helmont had made one very con- 

 crete observation: a willow watered only with rain water 

 gained 159 pounds, while the soil contained in the bowl in 

 which it grew lost only three ounces in dry weight. No one 

 followed up this observation until in 1727 Stephen Hales, a 

 Middlesex clergyman, published a work of genius called 

 Vegetable Staticks, in w^hich he showed that plants absorb air 

 through their leaves and that part of their substance is de- 

 rived from the air so absorbed. This work marked the origin 

 of knowledge about the nutritive function of leaves. Hales 

 also measured the transpiration of water through plants and 

 studied root pressure. 



In the second half of the eighteenth century the Unitarian 

 clergyman Joseph Priestley showed that air that had been 

 "injured" by the burning of candles could be made suitable 

 for animal respiration by keeping green plants in it; in fact, 

 green plants give off the gas that we now call oxygen. Jan 

 Ingenhousz, a Dutch doctor, showed in 1779 that plants only 



