52 Carbon Assimilation. 



their results. It is enough to say that none of them have furnished 

 indisputable evidence on the matter, and their work becomes chiefly 

 of historical interest after the clearing up of the problem by 

 F. P. Blackman in whose papers on the subject (1895 a, 1895 b) a 

 summary and criticism of earlier work is to be found. 



The essence of Blackman's work, is the measurement of the 

 quantity of carbon dioxide passing in and out of the two surfaces 

 of living leaves on which the distribution of the stomata is known. 

 For this work it was necessary to devise a special apparatus by 

 which could be measured the small quantities of carbon dioxide 

 with which one has to deal in such experiments. This apparatus 

 is described in the first of Blackman's papers (1895 a). By its 

 means a current of air either free from carbon dioxide or containing 

 any desired concentration of this gas, iS passed over the surface of 

 a leaf in a closed chamber and the intake or evolution of carbon 

 dioxide by the leaf measured, This is effected by estimation of 

 the carbon dioxide in the gas leaving the leaf-chamber by passing 

 this through standard baryta solution which is subsequently titrated 

 against standard hydrochloric acid solution. 



For details of the apparatus we must refer to the description 

 in the paper cited above. It is especially noteworthy that although 

 the apparatus is complicated yet the manipulation is exceedingly 

 simple, consisting only in the turning of taps. The different parts 

 of the apparatus are in duplicate so that two different surfaces of 

 a leaf or two different parts of a plant, can be examined under 

 exactly similar conditions at the same time. 



Special mention should be made of the plant chamber by means 

 of which the two surfaces of a leaf can be examined simultaneously. 

 This chamber consists of two circular rims of brass, 5 millimetres 

 deep and 36 millimetres in diameter, to one face of each of 

 which is hermetically cemented a plate of thin glass. Through 

 the brass rim are drilled at opposite ends of a diameter two small 

 holes, into each of which a copper tube of 1 millimetre bore is 

 soldered. These form the channels by which the gas enters and 

 leaves the chamber. For convenience of handling one tube is curved 

 half way round the rim of the half chamber so that it lies parallel 

 with the other. The leaf to be examined is slipped between the 

 two half-chambers and hermetically sealed to them by means of 

 wax, and the leaf is then ready for experimentation. For leaves of 

 different forms, plant chambers of different shapes may be used. 



Blackman experimented with various kinds of leaves, including 



