254 BOTANY PART i 



as about 35 billion kg. The German harvest alone contained in 1912 

 some 9 milliards kilos of assimilated material in the cereals (rye, 

 wheat, spelt, barley). 



There are two methods ( 33 ) in use for determining the amount of assimilation. 

 The method invented by SACHS is as follows. In the morning portions of 

 leaves, usually halves, are removed ; their siiperficial area is measured and they 

 are then dried and weighed. In the evening equally large portions (the remaining 

 halves) of the leaves which have been exposed to light throughout the day are 

 similarly dried and weighed. The increase of weight indicates the gain to the 

 plant by the assimilation of carbon. This is SACHS' half-leaf method. A quite 

 distinct method of quantitatively determining the assimilation of C0 2 is that of 

 KREUSLER which has been used by GILTAY and BROWN. A leaf still attached to 

 the plant is placed in a closed chamber through which a constant current of air 

 passes ; the amount of C0 2 removed from the air by the leaf is determined. The 

 amount of sugar or starch which could be formed from this amount of C0 2 can 

 then be easily calculated. 



2. The Gain in Carbon by Bacteria ( 34 ) 



Certain Bacteria, which will be described in another part of this 

 text-book, are characterised by the power of increasing their substance 

 in a purely inorganic food-solution ; they do this in the dark and 

 without chlorophyll so long as carbonates are present. This has been 

 determined for the Nitrite- and Nitrate-bacteria, the Sulphur-bacteria, 

 and for the Bacteria which oxydise methane and hydrogen. Some of 

 them depend entirely on C0 2 , while others can also utilise organically- 

 combined carbon. 



Nothing is known at present of the products of carbon assimilation 

 in these Bacteria. The gain in organically-combined carbon is slight. 

 Only a quite minimal fraction of the organic carbon compounds which 

 at any moment exist on the earth owes its origin to these Bacteria. 

 The fact of their carbon assimilation remains none the less interesting, 

 especially since it takes place in an essentially different manner to the 

 assimilation of the green plant. Some other source of energy must 

 take the place that sunlight does in assimilation in a green plant to 

 build up the organic substance ; this energy is obtained by oxidation 

 of ammonia, nitrites, oxide of iron, sulphuretted hydrogen, methane, 

 and hydrogen (p. 274). .We may therefore term the formation of 

 organic material in green plants PHOTOSYNTHESIS, and in these Bacteria 



a CHEMOSYNTHESIS. 



3. The Gain in Carbon in Heterotrophie Plants 



While the gain of carbon from carbon dioxide is to be considered 

 as the typical carbon assimilation of plants, it is by no means the only 

 method found in the vegetable kingdom. Since it depends leaving 

 the Bacteria mentioned above out of account on the presence of 



