Carbon Assimilation. 
119 
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. F. 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 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 
