110 PLANT PHYSIOLOGY 



itself alone, and had been deprived of that great share which has 

 to-day been afforded it by science and practice. 



Science and practice! How often is their advantageous reciprocity 

 misjudged! But it is just in the realm of plant physiology that their 

 reciprocal advancement becomes clear to the unprejudiced, in spite 

 of the errors which have proved a hindrance to their union for so 

 long. Starting from a purely theoretical point of view, Liebig entered 

 the sphere of practical agriculture, and, after an activity full of 

 vicissitudes in the practical and theoretical trends of thought, led by 

 practical ideas, worked especially at his experimental farm at Bechel- 

 bronn in Alsace, and Boussingault worked in the field of plant nutri- 

 tion, advancing, with the most refined means of research, toward 

 the solution of plant physiology. What these two accomplished 

 for agriculture through their researches, especially in regard to 

 the nitrogenous and mineral foodstuffs of crops, must remain as 

 difficult to estimate as the great advance which plant physiology 

 owes to these two men who established the most intimate bond 

 between chemistry and physiology through the founding of agri- 

 cultural chemistry. 



At the end of the sixties the condition of things \vas thus. Plant 

 physiology had not only come into relation with botany, but had 

 become, indeed, an integral part of it. Further, plant anatomy, physics, 

 chemistry, and practical agricultural chemistry had come to her 

 assistance. Even animal physiology, now and then at least, came 

 into a relation to physiology mutually beneficial to both, since the 

 interrelation of animal and plant life had been clarified by Ingen- 

 Housz and Saussure. I shall take occasion to return to this question 

 later. 



In spite of the efforts of Unger and others, who sought to portray 

 plant physiology in comprehensive works, this knowledge, which 

 had been derived from so many sources, was not yet welded into a 

 real unit. There appeared in 1865 the Experimental Physiologie der 

 Pflanzen of J. Sachs, in which was drawn up a critical summary of 

 the sum total of the knowledge gained up to this time. This work 

 gave a great impulse to new research. It was a most seasonable under- 

 taking, which, not only by its rich contents, but also by its incom- 

 parably clear and illuminating presentation, did not fail to exert 

 a great effect in the further development of plant physiology. 

 Unger's researches and his scholarly activity and Sachs' Experimental 

 Physiologie contributed the most to the fruition of plant physi- 

 ology in Germany in the second half of the last century. 



Thus, although Hales laid down the first guiding principles of 

 plant physiology, we have seen that its further development took 

 place in France and Germany. If we except the discovery made first 

 by Priestley, that oxygen is excreted by plants, a conclusion 



