INTRODUCTION 



THE study of the physiological processes of the plant 

 underwent a great change at about the year 1860, a fact 

 which makes the date stand out almost as an epoch in 

 the history of the science. The change can be traced to 

 two especial causes, and can be estimated only by noticing 

 the direction of inquiry before and after their operation. 



The first of these was the discovery of the practical 

 identity of the living substance of the two kingdoms of 

 Nature. Protoplasm had been to a certain extent investi- 

 gated by Von Mohl and other writers, sarcode was well 

 known to many zoologists. When the two were compared 

 and found to be identical, an impetus was given to study 

 and research on the botanical side, which had till then 

 been lacking. The study of animal protoplasm was further 

 advanced, a fact which opened up long lines of inquiry 

 into the extent to which identity could be established. 

 Soon the questions of respiration and nutrition were re- 

 examined, and knowledge slowly grew till they were found 

 to be comparable in all respects and to the smallest details. 



The puzzle that was so prominent at the outset, the 

 apparent differences between the methods of supplying 

 food to animals and plants and between the actual 

 nutritive substances in the two cases, opened out under 

 new aspects the old question of the relations involved in 

 the gaseous interchanges between the air and the green 

 plant. The problems of Priestley and his immediate suc- 

 cessors turned very largely on such points as the purifica- 

 tion or contamination of the air ; the effects upon the 

 organization or well-being of the plant were put in quite 

 subordinate positions. All this was changed when the 



