220 BOTANY part i 



the day to carbon dioxide and forms from this sugar or starch its 

 weight must increase, unless other causes lead to a diminution in 

 weight. Loss of weight may result from transjDiration, from the 

 translocation of material from the leaf, or from respiration (p. 241). 

 The two latter causes of loss in weight can only be approximately 

 estimated, and the first does not enter into consideration if instead of 

 the variable weight of the fresh leaf its dry weight be taken. 



The method invented by Sachs is as follows : — lu the morning portions of 

 leaves, usually halves, are removed ; their superficial 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 CO., is that of 

 Kreuslek which has been used by Giltay and Brown. A leaf still attached to 

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

 passes ; the amount of CO., removed from the air by the leaf is determined. The 

 amount of sugar or starch which could be formed from this amount of CO2 can 

 then be easily calculated. 



The results obtained by Sachs' method do not completely agree 

 with those obtained by the method of Kreusler. Nevertheless it can be 

 said that a square metre of leaf of an actively assimilating plant under 

 optimal external conditions produces between O'o and 2 grammes of 

 dry substance per hour. No fundamental difference in formation of 

 material could be found between tropical plants and our native plants. 

 Considerable differences in the behaviour of distinct species were, how- 

 ever, found. 



If one considers how many square metres of leaf-surface are actively 

 assimilating day after day on the surface of the earth a conception of 

 the huge production of organic material in this largest of all chemical 

 works will be obtained. The German wheat harvest alone contained 

 in the year 1900 about 23,000 million kilogrammes of assimilates. 



2. The Gain in Carbon by Nitrifying Bacteria (^^) 



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 

 exactly determined for the Nitrite- and Nitrate-bacteria, and also 

 probably holds for the Sulphur-bacteria and for the Bacteria which 

 oxydise methane and hydrogen. 



Nothing is known at present of the products of carbon-assimilation 

 in the Nitrifying Bacteria. For the gain in organically combined 

 carbon is slight; in the best-growing culture of AVinogradski it 

 amounted to 26 mg. Only a quite minimal fraction of the organic 



