INTRODUCTION 



La Physiologie est une des sciences les plu 

 dignes de l'attention des esprits eleves par 

 l'importance des questions, qu'elle traite, et 

 de toute la Sympathie des hommes de progres 

 par l'influence, qu'elle est destinee ä exercer 

 sur le bienetre de l'humanitö. 



— Claude Bernard. 



The aim of plant physiology is to gain a complete and thorough knowledge 

 of all the phenomena occurring in plants, to analyze the complex life processes 

 so as to interpret them in terms of simpler ones and to reduce them finally to 

 the principles of physics and chemistry. It is evident from this statement that 

 physiology is dependent upon physics and chemistry, and that progress in 

 physiology depends, in great measure, upon progress in these two other sciences. 

 Only since the end of the eighteenth century, when the principle of the con- 

 servation of mass was formulated by Lavoisier, and chemistry became an exact 

 science, did it become possible for physiology also to begin to assume this 

 character. Since that time it has been possible to employ the balance in pre- 

 cise studies of the materials that enter and leave plants. The well-known 

 experiment of van Helmont (15 7 7-1 644), performed long before those of Lavoi- 

 sier, may be cited as an early though but partially successful attempt to use the 

 balance for determining the source of the materials found in the plant body. 

 A willow branch weighing 5 pounds was potted in 200 pounds of dry soil and 

 watered with rain-water. After five years the weight of the rooted branch 

 was estimated to be 164 pounds, while the dried earth showed a loss in weight of 

 only 2 ounces. Van Helmont concluded from this that the material of the plant 

 was formed from water, but this inference is incorrect, since the surrounding air 

 was not considered. He would have been justified in concluding, however, that 

 the greater part of the non-aqueous material of plants does not come from the soil. 



Besides the discoveries of Lavoisier, another important event in the history 

 of chemistry must be alluded to here, the synthesis of urea, accomplished by 

 Wöhler in 1828. Up to that time organic compounds had been obtained only 

 from living organisms, and the idea prevailed that the synthetic preparation of 

 such compounds from inorganic materials was impossible and that their forma- 

 tion presupposed the participation of a special vital activity. Wöhler's dis- 

 covery, together with subsequently successful syntheses of various other organic 

 compounds, have shown that no vital force is essential to the formation of 

 such substances. 



The organic and inorganic compounds of carbon are often combined in a 

 single group, but there is an essential difference between them for the physi- 

 ologist; all organic substances contain a store of energy, since they give off heat 



