Growth. 



31 



twenty-four hours more, then measure again. Note the final 

 amount of difference of elongation. 



24. Relation of oxygen to growth.— Soak a number 

 of seeds of wheat in water for a few hours, and then place a 

 dozen in each of the bulbs of two respiration tubes or two small 

 retorts (Figs. 15, 16). If 

 respiration tubes are used, in- 

 vert and fill with water, then 

 invert in a dish of mercury, 

 taking care to exclude all air. 

 If a retort is used — and this 

 apparatus will be found most 

 suitable for this work — sup- 

 port with the end of the out- 

 let resting in a dish of 

 mercury. Fill with water, 

 and seal the stopper in place 

 with vaseline or wax. Raise 

 the end of one respiration 

 tube or one retort, and allow 

 the air to enter as the water 

 runs out, then replace in its 

 original position. The water 

 in the remaining tube or re- 

 tort is to be replaced with 

 carbon dioxide or hydrogen. 

 To prepare hydrogen, put a 

 handful of granulated zinc 

 in a wide-mouthed bottle for which a tightly fitting stopper 

 with two openings for tubing should be provided (Fig. 17). 

 Insert a thistle tube or funnel in one opening, with its tube 

 extending to the bottom of the flask. Insert a bent glass tube 

 in the second opening, and connect it with a section of tubing 

 bent at right angles, which extends through the stopper of a 



Fiff. 16. — Small retort used for testing 

 germination of seeds in an atmosphere 

 lacking oxygen. 



