9 8 PLANT PHYSIOLOGY. 



shown in Fig. 22. When these, graduated and of known capacity,* are 

 used, and the seeds (about thirty in number) are put in through a smaller 

 inner tube so they cannot wet the sides of the tubes, they give the best of 

 results. Indeed this particular experiment, using the graduated bulb tubes 

 and the three liquids recommended (Fig. 22), seems to me to give an ideal 

 method for demonstrating the gas-exchange in respiration, for it can be 

 measured quantitatively and the demonstration is logically complete. Of 

 course there should be some proportion between the size of the tubes and 

 the number of seeds used, and I have found by experiments with oats 

 that r cc. of seeds to some 20 cc. of space is very good. Of course 

 seeds, such as peas, with pronounced intramolecular respiration (see 

 page 99) cannot well be used for this purpose, but oats have very little of 

 this power. (See Note 9 of the Addenda.) 



The elimination of CO 2 in respiration maybe demonstrated strikingly 

 by the method given in Darwin and Acton, 2 ; but, as figured by them, 

 the experiment may be simplified by having the bent tube pass through 

 the rubber cork. An exact method using a Respirometer is described by 

 Arthur (Laboratory Exercises, 14). One of the principal physical phe- 

 nomena of respiration, the release of heat, is considered later under 

 Growth (page 110). 



What does the above experiment show as to the quantitative 

 relations of the two gases in the exchange ? 



EXPERIMENT 31. Prepare two bottles with shoots as for Experi- 

 ment 29. Ascertain how long a candle will burn in such a vessel ; 

 start both with pure air on a bright morning, and keep one in light as 

 much as possible for three days and the other in darkness for that 

 time, and test the air with the candle at the close of the third day. 

 Use a large quantity of leaves. 



It seems to be shown by Experiment 30 that seeds will 

 not grow at all without free oxygen. This is, however, not 

 universally true. There are other kinds (peas are an example) 

 which are said to respire a certain amount without free oxygen. 



53. Can peas respire without free oxygen? 

 Answer by Experiment 32. 



Those shown in the cut are graduated for the length of a part of the tube only ; 

 if specially made, it would be better to have them graduated for the capacity of the 

 entire bulb and tube. Of course ungraduated tubes, which are much cheaper, can 

 be used, marks being made for the different levels of the liquids, and the capacities 

 being subsequently determined by gradually filling the tube with water from a meas- 

 uring-glass. 



