PROGRESS IN PLANT PHYSIOLOGY 79 



errors. For one thing, he failed to differentiate between 

 vessels proper, latex tubes, and resin passages, and, what 

 was even worse, he thought the secondary thickening in 

 vessels was laid down on the outsides of the walls, although 

 Treviranus had already shown that that was not the 

 case. With all its defects Moldenhawer's paper was 

 a great advance on the productions of Mirbel, Link, 

 Rudolphi, and others whose names I have mentioned 

 to you as among his contemporaries. 



PROGRESS IN PLANT PHYSIOLOGY 



Although substantial progress had thus been made 

 in elucidating the intimate structure of plants during 

 the early years of the nineteenth century, it is again 

 in physiology that the greatest advances are seen, and 

 these advances were in large measure due to the work 

 of two men De Saussure in nutrition and Knight in 

 sensitivity. 



In De Saussure's memoir, Recherches chimiques sur la 

 vegetation, published in 1804, we meet for the first time 

 with quantitative as well as qualitative methods of 

 investigation, and rigid experimentation on what might 

 almost be regarded as modern laboratory lines. I think 

 I cannot do better than give you some of De Saussure's 

 conclusions in his own words. " When green plants," 

 he says, " are exposed in atmospheric air to the successive 

 action of day and -night they inspire and expire alter- 

 nately oxygen mixed with carbonic acid gas. The 

 oxygen which green plants inspire is not directly assimi- 

 lated by them ; it is changed on inspiration into carbonic 

 acid gas. They decompose this gas in the act of expira- 

 tion, and it is only by this decomposition (which is only 

 partial) that they are able to assimilate the oxygen which 

 is present in the atmosphere. Roots, duramen, alburnum, 

 petals, and all parts which in general are not green do 

 not exhibit these successive inspirations and expira- 



