398 FOUNDATIONS OF BIOLOGY 



remained until the biologist had recourse to further data 

 from the chemist, in 1779, PRIESTLEY (1733-1804) of Eng- 

 land, the discoverer of oxygen, showed that this gas under 

 certain conditions is liberated by plants. This fact was 

 seized upon by a native of Holland, INGENHOUSZ (1730- 

 1799), who demonstrated that carbon dioxide from the air 

 is reduced to its component elements in the leaf during 

 exposure to sunlight. The plant retains the carbon and 

 returns the oxygen this process of carbon-getting being 

 quite distinct from that of respiration in which carbon 

 dioxide is eliminated. It remained then for DE SAUSSURK 

 (1767-1845) in Geneva to show that, in addition to the 

 fixation of carbon, the elements of water are also employed, 

 while from the soil various salts, including combinations of 

 nitrogen, are obtained. But it was nearly the middle of 

 the last century before the influence and work of LIEBIG 

 (1803-1873) at Giessen led to a clear realization of the 

 fundamental part played by the chlorophyll of the green 

 leaf in making certain chemical elements available to animals. 

 The establishment of the cosmical function of green plants 

 the link they supply in the circulation of the elements in 

 nature is a landmark in biological progress. 



4. Histology 



Studies on the physiology of plants and animals natural^ 

 involved the progressive analysis of the physical basis of 

 the phenomena under consideration, but the Aristotelian 

 classification of the materials of the body as unorganized 

 substance, homogeneous parts or tissues, and heterogeneous 

 parts or organs, practically represented the level of analysis 

 until the beginning of the eighteenth century. In fact it was 

 not .until the revival of interest in embryology early in 

 the last century that the cell became a particular object of 



