94 PLANT PHYSIOLOGY, 



drawn and the burette is raised until the zero-mark is at the outside 

 level (it will fall somewhat inside) ; enough CO 2 must now be allowed to 

 enter the burette through a slender bent glass tube held in the mercury 

 beneath the burette, to depress the mercury to the zero-mark. If prop- 

 erly carried out, the level of the mercury will be exactly the same (and 

 at the zero-mark) inside and out, while the burette will contain 8% of 

 CO 2 , an amount which most plants can readily use. One of the bu- 

 rettes is now to be darkened by a covering of tin-foil (better than black 

 paper because it reflects heat), the exact temperature is to be noted from 

 the attached thermometer, and the whole is to be set in bright light for 

 a day. Towards sunset (in winter, a shorter time is ample in summer) 

 the plants are to be carefully withdrawn by the wires and the resultant 

 change in volume, marked by the rise of mercury, carefully noted. The 

 gas analysis can be made at leisure. For this prepare two sticks of 

 caustic potash of less length than the diameter of the burette ; attach 

 them to fine wires, and insert them into the burettes ; after three or four 

 hours they will have absorbed all the CO 2 in both tubes, when they may 

 be withdrawn. The tubes are now to be depressed until the mercury 

 is at the same level inside and out, and the levels read on the burette 

 graduation. The difference of level in the two tubes will show the total 

 disappearance of the S of CO 2 in the tube in light and its presence in 

 the tube in the dark. To test now whether the CO 2 shown to have 

 disappeared from the light tube has been replaced by oxygen, one either 

 transfers the burettes with their reservoirs into a large dish of water 

 (dropping off the reservoirs allows the water to enter the tube) and 

 applies the phosphorus method of removing oxygen, or one transfers 

 them to a dish, such as a large tumbler, containing a concentrated mix- 

 ture of pyrogallic acid and caustic potash, and drops away the reservoirs 

 as before, allowing the mixture to enter the tubes. As this is an oxy- 

 gen absorber, the height to which it will rise in the two tubes will 

 demonstrate whether or not the CO 2 of the light tube was replaced by 

 oxygen. The final readings for levels should always be made at the 

 same temperature at which the experiment was started, and theoreti- 

 cally also at the same barometric pressure, but this is hardly practically 

 important. Theoretically it would be better also to make the final read- 

 ings with the burettes depressed until inside and outside liquid-levels 

 are the same, but this also is hardly practically necessary. Also changes 

 of temperature from local warmth of the hands on the tubes just before 

 leadings are made must be avoided. (See Note 8 of the Addenda.) 



57. What are the substances, conditions, and processes con- 

 cerned in Photosynthesis? 



Tabulate these fully. Answer from your various sources 

 of information. 



