100 PHOTOSYNTHESIS AND RESPIRATION. 



* (6) Repeat the experiment, this time keeping the jars exposed to the 

 light on a bright day, or for two days. 



* (c) Repeat the experiment, but instead of using the taper pour into 

 each jar (at the end of the experiment) an equal volume of baryta- water ; 

 replace the cork, shake the jars well, and compare the degree of milki- 

 ness produced in each case. Make the experiment twice in darkness 

 and in light. 



(d) Suspend three healthy laurel leaves by threads from the well- 

 fitting cork of a large bottle containing lime-water, and expose them to 

 bright light. After several hours the lime-water is still comparatively 

 clear. Cover the bottle with black cloth, and in a few hours the lime- 

 water will become quite milky, owing to the respiration being no longer 

 masked by the re-assimilation of the carbon dioxide it produces. 



(e) Place some green leaves in a glass jar (Fig. 37), through which a 

 slow current of air is passed. This air is deprived of its carl.on dioxide 

 by the potash contained in the U-tube, so that the lime-water or 

 baryta-water in both bottles remains clear so long as the leaves are 

 exposed to sunlight or very bright daylight, whereas if the bell-jar is 

 covered with a black cloth, the liquid in the left-hand bottle soon 

 becomes turbid and milky. 



In Fig. 37, the plant used is covered with a bell-jar standing on a 

 glass plate, its rim being smeared with plasticine and vaseline to make 

 the junction air-tight. 



(/) Using the same apparatus, we can make rough quantitative 

 experiments. The amount of carbon dioxide produced is determined 

 by weighing the left-hand bottle before and after putting in the baryta- 

 water and again at the end of the experiment. The gain in weight of 

 the baryta-water represents the weight ot barium carbonate (BaC0 3 ) 

 produced, and from the atomic weights of barium, carbon, and oxygen 

 we calculate the weights of the carbon dioxide and from that its volume. 



A simpler plan, if the necessary apparatus and chemicals are avail- 

 able, is to "titrate" samples of (1) the baryta-water put in the bottles 

 before the experiment begins, and (2) the clear baryta-water above the 

 precipitate at the end of the experiment, using standard solutions of 

 oxalic acid and of phenolphthalein (see books on Volumetric Analysis). 



135. Comparison of Photosynthesis and Respiration. 



These are, to a certain extent, antagonistic processes, the 

 first involving a production of organic material, a consumption 

 of carbon dioxide, and a liberation of oxygen ; the second, a 

 consumption of organic material, a liberation of carbon dioxide, 

 and a consumption of oxygen. Photosynthesis is twenty or 

 thirty times more active than respiration in most healthy green 

 organs exposed to bright light and supplied with sufficient 

 carbon dioxide, so that these parts do not appear to respire 

 during the day-time, or at least do not evolve any carbon 



