O TVR 



THE FOOD OF PLANTS. 



the green cells of plants only when Carbon dioxide is present shall 

 be furnished later, but we will here perform an experiment which 

 clearly shows that chlorophyll-containing cells only produce 

 Oxygen when Carbon dioxide is at their disposal. Into a glass 

 Hask of about 400 c.c. capacity is poured 300 c.c. of spring water.* 

 We now place in the water a considerable quantity of Elodea, and 

 fit the flask with a perforated rubber stopper, through which passes 

 one end of a glass tube, whose other end is connected with a 

 U-tube containing fragments of pumice soaked in potash solution, 

 together with fragments of caustic potash (see Fig. 15). The 

 plants are not deprived of air because the water is in communi- 

 cation with the atmosphere through the U-tube. Access of Carbon 

 dioxide from the atmosphere on the other hand is excluded. If 

 we expose our apparatus to direct sunlight, we see that the plants 

 keep up a rapid evolution of Oxygen at the expense of the Carbon 

 dioxide dissolved in the water. We fix our attention 011 parti- 

 cular plants, and determine from time to time, say every half 

 hour, the number of bubbles of gas which they liberate in the 

 course of one minute. It is found that the evolution of Oxygen 

 gradually becomes weaker and weaker, and finally (in my experi- 

 ments after six hours) the production of Oxygen entirely ceases, 

 since the supply of Carbon dioxide in the water is exhausted. If 

 we now open the apparatus, and lead some Carbon dioxide into 

 the water, the evolution of Oxygen by the plants under the in- 

 fluence of the light recommences. 1 



1 See Frank Schwarz, in Unters. aus d. bot. Inst. zu Tubingen, Bd. 1, 

 p. 97. 



13. Volumetric Relations of the Gas Exchange in 

 Assimilation. 



We conduct our experiments according to a method which has 

 been accurately described by Pfeffer, 1 and which also Holle 2 

 employed. The apparatus is represented in Fig. 16. The most 

 essential part of it is a glass tube about 360 mm. in length, 

 expanded into a bulb towards its upper end. Of this tube the 

 calibrated portion c occupies about 260 mm., and the bulb 

 70-75 mm., above which the ^tube ends in the narrow portion a 



* A glass cylinder of small diameter is still more suitable. 



