906 TRANSACTIONS OF SECTION JC. . 



several cases among the work of otlierg. When this explanation is valid a further 

 analysis of the conditions is iiectjssary, cspocially in regard to the relative efficiency 

 of the water-supply. 



3. The Factors influencing Photosynthesis in Water Plants 

 By A. M, Smith, M.A. 



Description of Apparati/s.— The apparatus has as its object the maintenance 

 of a current of water containing dissolved carbon dioxide, so tbat it passes at a 

 constant rate over a plant enclosed in an oval chamber. This chamber is placed 

 in a copper bath which can be maintained at a desired constant temperature. 

 Through a window in the bath the plant-chamber is illuminated for assimilation 

 experiments by Keith high-pressure incandescent burners, wliich can be put 

 ut varying distances away. Thus the conditions of assimilation (1) amount of 

 CO.i supply, (2) temper.ature, (3) light, can all be regulated to any desired 

 intensity. The assimilation or respiration is measured by analysing the CO^ 

 content (1) of the supply to the plant, (2) of the water which ,has passed over 

 the plant. This analysis is performed by precipitating the CO,, in a known 

 volume of the solution as insoluble carbonate by addition o"f baryta and 

 titrating the excess of baryta by standard hydrochloric acid. The apparatus 

 includes arrangements for collecting and analysing the gas given off' during the 

 experiment, so that a complete account is obtained of the CO., changes due to the 

 plant. ~ • 



Experiments. — Blank tests with no plant in the chamber show that a high 

 degree of accuracy can be attained in analysing the stream of water and compar- 

 ing samples drawn oif near its source with samples taken after passing throuo-h 

 the whole of the apparatus. 



The general course of the earlier experiments made in 1905 — and these form 

 the greater bulk of the work— showed the effect on assimilation of a CO., 

 supply increasing in amount from that present in ordinary tap-water 0-0007 

 grms. per 50 cc. to 00G20 grms. per 50 cc, which represents a solution about 

 two-thirds saturated at 15° C. 



Light and temperature being constant the general form of the curve obtained is 



2 



A COj supply. 



Where the part A b represents increase of assimilation with increase of CO. 

 f'^'PPly) while the part B c represents a constant assimilation due to some limit- 

 ing factor. That this limiting factor is the intensity of the light was proved by 

 increasing this intensity when an increase of assimilation above that represented 

 by B c was obtained. No change was observed on raising the tem))erature. 

 It was shown by experiments in 1908 with temperatures of 7°C. and 13° C. that 

 in all the earlier experiments the temperature was above the limiting intensity for 

 this amount of assimilation. The later experiments fixed the limiting temperature 

 for this assimilation at 16° C; and all the temperatures used in the earlier part 

 of the work were higher than this. 



That CO., supply, light, and temperature were actually limiting under 

 different conditions was shown, not only by the form of the general curve, but in 

 each case by changing the intensity of that factor during the course of a single 

 experiment, when, if the factor was limiting, a corresponding change of assimila- 

 tion followed. 



Besides this demonstration of the conditions under which each factor in turn 

 becomes limiting, other interesting general results are : — 



(1) That when COo is limiting, plants with an internal atmosphere (flowering 



