BACTERIA AND ENZYMES IN AGRICULTURE 265 



placed in water, in a cylindrical vessel, with a little earth at 

 the bottom for root attachments, and the whole set in the 

 sun ; bubbles of gas soon arise from the leaves and may be 

 readily collected. If a glowing splinter of wood be held in 

 the gas, it will burst into flame, showing that the gas con- 

 sists, for the most part at any rate, of oxygen. 



On the other hand, if the plant is placed in darkness, and 

 air, freed from C0 2 by passing through potash solution, is led 

 over the plant, and then passed into baryta water, the latter 

 will become turbid from formation of barium carbonate. 



From these experiments, it is clear that two main processes 

 go on in the leaves, the evolution of oxygen in sunlight, and 

 of carbon dioxide in darkness. These two changes, as a matter 

 of fact, take place at all times, but the preponderance of one 

 over the other depends on the presence or absence of sunlight. 

 The evolution of oxygen is a building up, or anabolic process, 

 arising from the decomposition of the carbon dioxide in the air, 

 the plant utilising the carbon and giving off the oxygen; on 

 the other hand, the evolution of carbon dioxide is essentially 

 a process of respiration, or a catabolic process, where the 

 carbonaceous constituents of the plant are broken down, with 

 production of carbon dioxide and water. The volume of the 

 oxygen given out in the assimilation process is practically 

 equal to the volume of carbon dioxide taken in, sufficiently 

 indicating that the changes involved are of a fairly simple 

 order. The problem to the chemist is to discover how the 

 carbon, taken in by the plant as C0 2 , is built up into starch and 

 cellulose, and by what stages these latter are reconverted into 

 carbon dioxide, and thus the life cycle maintained. Micro- 

 scopic observation indicates that starch is the first visible pro- 

 duct appearing in the leaf cell ; but, of course, between a simple 

 substance such as carbon dioxide and starch, the chemical 

 steps must be very numerous. 



In 1870 von Baeyer put forward a very suggestive hypothesis 

 in regard to the first of these steps ; the simplest carbohydrate, 



