THE WORK OF STEMS 285 



a funnel passing through the cork : a tube passing 

 through the cork serves to conduct the carbon dioxide 

 into the jar. A few moments must be allowed for the 

 carbon dioxide to drive the air out of the bottle and 

 tube; the tube should then be introduced into the jar 

 and the evolution of gas continued until the jar is 

 nearly filled with gas. 



Another method is to 

 grow the algse in small tubs 

 (of wood fiber) filled with 

 water, keeping a huge bub- 

 ble of carbon dioxide or air ^^^^.t^Tn^^St^^iSTb^^^^^^^^^ 



just below, them, as shown carbon dioxide or air. 



in Fig. 160. This bubble is kept in place by means 

 of a net (made of cheese-cloth or mosquito netting) at- 

 tached to a cii'cular hoop of small flexible lead-pipe (^), 

 which rests on the bottom of the tub. The bubble may 

 consist of air, or air blown from the lungs, or of carbon 

 dioxide generated in the apparatus- shown in Fig. 159. 

 The common method of placing aquatic animals and 

 insects and aquaria in order to supply carbon dioxide 

 is excluded for our purpose, for the reason that they 

 feed on the algae. We may, however, place a well- 

 developed cutting of Willow or Wandering Jew (Fig. 

 84) in the jar ; the carbon dioxide excreted from its 

 roots will be a great stimulus to the algsB. 



