924 



SCIENCE. 



[N. S. Vol. XVI. No. 415. 



the absorption of carbon dioxide from the 

 air takes place by means of solution in the 

 cuticle of the epidermal cells of plants and 

 thence passes by diffusion to the seats of 

 photosynthesis. Only comparatively re- 

 cently has this been shown to be erroneous. 

 If, however, it is once recognized that au- 

 thority is fallible this apparent obstacle 

 becomes the opposite. The more evident 

 questions have not yet been solved, leaving 

 only the more difficult ones for the present- 

 day worker. 



Recognizing the importance of work in 

 this field, and realizing that with the ad- 

 vent of a new century new departures must 

 be taken, I have thought I might venture 

 to direct the thoughts of my hearers, many 

 of whom I may call my colleagues, to the 

 present position of certain problems which 

 have long been the subjects of speculation 

 and which offer the prospect, if not of com- 

 plete solution, at any rate of considerable 

 advance if investigated by modern methods. 



I turn first to a few questions connected 

 with the nutritive problems of plants in 

 general. 



There are several theories abroad as to 

 the progress of events during photosynthe- 

 sis, none of which can be regarded as en- 

 tirely satisfactory. For many reasons it 

 seems desirable that this question shall be 

 thoroughly investigated in the light of the 

 present condition of both chemical and 

 physical science. I may perhaps venture 

 to recall to you the principal hypotheses of 

 carbohydrate formation which have been 

 advanced, so that its present position may 

 be properly appreciated. 



The view that has met with the widest 

 acceptance is that of Baeyer. On his 

 hypothesis the carbon dioxide absorbed is 

 decomposed under normal conditions to 

 yield carbon monoxide and oxygen ; a corre- 

 sponding and coincident decomposition of 

 water leads to the production of free 

 hydrogen and oxygen. The oxygen from 



both sources is exhaled, while the carbon 

 monoxide and hydrogen combine to form 

 formaldehyde. The formaldehyde gives 

 rise by a process of polymerization to some 

 form of sugar. 



A modification of this hypothesis has 

 been advanced, which suggests that the pre- 

 liminary decomposition of the carbon 

 dioxide and the water may not take place, 

 but that by a rather less violent reaction 

 between them the formaldehyde may be 

 formed and the oxygen liberated. 



Erlenmeyer has suggested a somewhat 

 different course of reaction, yielding sub- 

 stantially the same results. He thinks it 

 possible that the first interaction of carbon 

 "dioxide and water leads to the formation 

 of formic acid and hydrogen peroxide, and 

 that these subsequently interact with each 

 other, yielding formaldehyde and water 

 and giving oft" oxygen. 



Many years after the views of Baeyer 

 appeared, a hypothesis of a different nature 

 was proposed by Crato. He suggests that 

 the carbon dioxide after absorption becomes 

 ortho-carbonic acid, and that this remains 

 in solution in the cell sap. This acid has 

 the structure of a closed benzene ring in 

 which six molecules are linked together. 

 This becomes decomposed, liberating six 

 molecules of water and six molecules of 

 oxygen, and forming a hexavalent phenol 

 which subsequently undergoes a molecular 

 rearrangement and becomes glucose. 



Yet another suggestion was made by 

 Bach in 1893. He points out that when 

 sulphurous acid is exposed to light it be- 

 comes transformed to sulphuric acid, sul- 

 phur and water being split off, and he 

 argues that a process analogous with this 

 may take place in a leaf. > The carbon 

 dioxide uniting with water would form 

 carbonic acid, and this might then split up 

 in the same way as the sulphurous acid. 

 The carbon and the water thus split off 

 are on this hypothesis not set free sepa- 



