130 THE LIFE OF THE PLANT 



exhibited by the plant: it gives off bubbles of oxygen 

 in place of carbonic acid it has decomposed.^ 



In order that this may happen we must have a strong 

 light, and the water must contain carbonic acid ; in the 

 absence of either of these two conditions the giving off 

 of bubbles will not be observed ; but, on the other hand, 

 if the sunlight or the electric light is strong enough the 

 bubbles rise in a continuous stream, like a string of beads. 

 We have now to prove that this gas is oxygen, or rather 

 that it is rich in oxygen, since it generally contains 

 an admixture of other gases that are present in solu- 

 tion in the water. For this purpose we pass the ends 

 of several branches under the widened end of the 

 graduated tube (a), which is full of water like that in 

 the basin, and collect the gas which comes off. This 

 tube is tightly closed at its narrow end with a stopper 

 in the form of a glass rod (c) which passes right through 

 its wider part (b). When a sufficient quantity of gas 

 is thus collected we proceed to test it.^ It might be 

 oxygen given off by the plant, or atmospheric air, or 

 carbonic acid dissolved in the water and therefore 

 capable of penetrating into the cavities of the plant. We 

 pour some solution of caustic alkali into the wide funnel- 

 like upper part of the tube and gently raise the glass 

 rod so that the alkali may pass into the lower part which 

 is marked with divisions. Alkali, as we already know, 

 absorbs carbonic acid.^ At the beginning the tube 

 showed, say, fifty divisions of gas ; there will be only 

 forty-eight left after the absorption of the carbonic acid. 

 Then we pour another substance into the funnel, a 

 solution of so-called pyrogallic acid, which has the 



1 Water plants have no stomata on their submerged parts but are 

 provided instead with internal air cavities, into which oxygen difiuses, 

 coming ofiE later in bubbles through any chance apertures. 



2 The circles on Fig. 37 represent the circles of light thrown on the 

 screen by the magic lantern ; in order to get a larger image we look first at 

 one part of the apparatus and then at the other. 



^ See the third chapter on the respiration of the seed. 



