PHOTOSYNTHESIS 107 



bon dioxide is present in the atmosphere in an amount impracticably 

 small for ordinary analysis, advantage may be taken of the fact that 

 plants will thrive for a time in an atmosphere containing a much 

 larger percentage, even up to 10 per cent of that gas. 



Experiment. In the chamber of a photosynthometer place a 

 green shoot of appropriate size of a vigorous small-leaved plant (e.g., 

 Heliotrope, Ficus repens). Supply to the enclosed atmosphere enough 

 carbon dioxide to make a 10 per cent mixture, and place the whole 

 in strong diffused light for some three or four hours; then close 

 the communication with the gas-tube, and at leisure determine the 

 composition of the gas as to oxygen and carbon dioxide. 



Precaution. It is best in this, as in all experiments where plant tissues 

 are exposed behind glass, not to permit full sunlight to fall upon the tissues, 

 since this is likely to cause abnormal heating and injury in the absence of 

 ventilation; and besides, in the present case, the resultant gas expansion 

 would be likely to force off the stopper of the instrument. 



Photos ynthometers. Of these (though not under this name) several 

 forms have been invented, of which the most exact is Pfeffer's (Arbeiten 

 des botanischen Instituts in Wtirzburg, 1, 1871, 9; in synopsis by Detmer, 

 41, and by Darwin and Acton, 41). Though accurate, the apparatus is 

 not readily obtainable, and the manipulation is rather cumbersome. I de- 

 scribed a simplified modification of it in the first edition of this book, 93, 

 while Stone has described yet another way in Torreya, 4, 1904, 1. Much 

 more satisfactory in every respect, however, is the instrument which I have 

 described in the Botanical Gazette, 41, 1906, 209, and which is among my 

 normal apparatus (page 46), and figured herewith (Fig. 26). 



The instrument consists essentially of a pear-shaped plant chamber set 

 in a firm iron base, a graduated measuring-tube with a small stop-cock at 

 the upper end, and a connecting stopper furnished with a stop-cock of con- 

 siderable bore. The total capacity of the apparatus when closed is exactly 

 102 cc, of which the 2 cc. is for a shoot and 100 cc. for the gases concerned. 

 The proper amount of shoot is provided by selecting a small-leaved plant 

 and pushing a branch down into a measuring-glass until it displaces exactly 

 2 cc. of water; the water-level is then noted on the stem, which is cut at 

 this point under water, the shoot being later, when shaken free from water, 

 placed upright in the chamber. We now add some selected percentage 

 of carbon dioxide to the apparatus in the following way. The measuring- 

 tube, with stop-cock closed, and handled always by the top only to prevent 

 volume changes of its gas by heat of the hand, is inverted and filled with 

 water of room temperature up to a figure of the graduation expressing the 

 selected percentage, for the tube is graduated in cubic centimeters, which 

 are, of course, percentages of the total gas capacity of the apparatus. The 

 stopper is then placed on the tube and its stop-cock closed; its hollow is 

 filled with water and the whole is inverted in a pneumatic trough (or equiva- 

 lent dish of water) which has been standing in the laboratory long enough 

 to take the temperature of the air. The lower stop-cock is then opened 



