DEVELOPMENT OF PLANT PHYSIOLOGY 121 



cess of its solution a reflection of the history of our science, ever 

 showing how changeable its limits are and from what various di- 

 rections, often unexpectedly, its help comes. A pertinent example 

 has been already advanced by me, that of leaf-fall. Allow me to 

 give as briefly as possible two other examples of great illustrative 

 power. 



The problem of leaf position has been till recently purely descrip- 

 tive, although treated in part with great mathematical and geo- 

 metrical precision. Later, by Schwendener, it was brought, as a 

 mechanical problem, out of the field of morphology into that of 

 physiology. Quite recently it has been shown that, with reference 

 to illumination, the simplest positions for lateral and for vertical 

 axes, the approximation of leaf positions to the irrational limits of 

 value are the most purposeful. Thus the problem of leaf position 

 was at first morphological, then physiological, and finally biological, 1 

 or, as we may more precisely say, ecological, whereby it is, however, 

 not said that it cannot be further advanced from the side of morpho- 

 logy and physiology. 



In the second place, our great problem of photosynthesis (carbon 

 assimilation). Priestley discovered the excretion of oxygen by the 

 plant, Scheele that of carbon dioxid. But neither was able to say 

 under what conditions these took place. Ingen-Housz first showed 

 that the photosynthesis correlated with oxygen secretion takes 

 place only in the green organs of plants under the influence of light. 

 The explanations of the chemistry involved which obtained from 

 the time of Th. de Saussure to Boussingault are well known. Ana- 

 tomy now took a hand, and showed us, in the living body of the 

 chlorophyll grain, the place where photosynthesis takes place. The 

 knowledge of the spectrum of chlorophyll, contributed by the physi- 

 cist, led to the attempt to study the absorption of light by chloro- 

 phyll from the physiological point of view. First it was shown how 

 the pigment chlorophyll by light absorption influences the process 

 of transpiration 2 and then the same in regard to photosynthesis. 3 

 The reference of fermentation to an enzymatic process has raised 

 the question whether photosynthesis may not be a process of this 

 kind. As you know, we are in the midst of a strife of opinions as to 

 whether or not photosynthesis is a matter bound up with the living 

 condition or has to do merely with an enzymatic process. And now 

 the chlorophyll question wanders into the realm of cosmic physics, 

 in that on the one hand the view is set forth that the correlations 

 between photosynthesis and the life of plants and animals presents 

 itself not as a struggle for the elements or for energy, but as a struggle 



1 Zur Biologie der Blattstellung , Biol. Centralblatt, 1903, p. 209 ff. 



2 Wiesner, Untersuchungen uber den Einfluss des Lichtes auf die Transpiration. 

 Sitzungsbericht der Wiener. Akad. d. Wissensch., 1876. 



3 The well-known works of Engelmann, Reinke, and TimirjazefT. 



