384 Dr. F. F. Blackman and Mr. A. M. Smith. [Dec. 19, 
solution by collecting 200 c.c. slowly (in two minutes) in the pipette W, and 
estimating 1ts COs-content as described in Section I, f. 
As in theory every particle of liquid takes about one hour to pass right 
through the apparatus, so any sample of effluent should strictly be compared 
with the sample of affluent taken an hour previously. When the composition 
of the affluent keeps fairly constant, as in this experiment, this point is not 
important, but if the affluent is altered in CO2-content, either intentionally 
or by failure in the CQ2-generating tower, then it is important to bear this 
relation in mind. 
For this particular experiment, only the average assimilation during six 
hours of the selected conditions is required, and the averages of the affluent 
and of the effluent liquids are taken, neglecting the first reading in either 
column of the table. 
(b) Data Required for an Exact Evaluation of the Assimilation.—The magni- 
tude of the following eight factors must be known numerically before a 
precise statement can be put forward about the assimilatory activity of the 
plant: (1) the weight of plant; (2) the intensity of illumination : (3) the 
temperature in the chamber ; (4) the percentage of CO, in the circulating 
fluid ; (5) the difference between the COz in the affluent and effluent ; (6) the 
rate of the current; (7) the CO2 lost as gas-bubbles in the chamber; and 
(8) the magnitude of the plant’s respiration. 
We must say yet a little about the determination of each of these magni- 
tudes in this particular experiment of June 7, 1907, before we pass to the 
calculation of the final result. 
The unit of comparison for all cases and all plants in this method is the 
quantity of leaf that will cover an arlitrary surface, here 137 sq. em., which is 
the area of the silver grid. In such a plant as Elodea the ideal unit, a unit 
area of the leaf-tissue itself, cannot be employed: one can only pack the 
shoots so closely (there is no objection to overlaying) that there are no gaps 
left where light is not being arrested, and then, however much leaf there is 
shaded and wasted as regards assimilation, there is yet the unit area of chloro- 
phyllous cells exposed to the full illumination on the surface of the matted mass. 
(1) The weight of plant required to cover the grid varies with its habit. It 
must in each case be ascertained in order to make the necessary allowance for 
the respiration, which varies with the total weight employed. As water- 
plants cannot be surface-dried satisfactorily, we have not taken the fresh 
weight, but have air-dried the material after each experiment, and have based 
the allowance for respiration on this dry weight. 
(2) The standard light, at 13:4 cm., was used, having the arbitrary intensity 
5"7 (see Section I, d). 
