IX Leaves 165 



is dissolved in the water, while most plants absorb it from 

 the mixture of gases in the atmosphere, is an unimportant 

 detail. These experiments can be readily varied and 

 extended ; thus we might put roots in place of the leaves 

 in our last experiment, but no bubbles of oxygen would be 

 formed ; or we might place the pondweed in water from 

 which the carbonic acid had been expelled by boiling, and 

 allow no air to enter the vessel except through a tube of 

 caustic potash, which will absorb its carbonic acid, with the 

 result that the evolution of oxygen would entirely cease. 



More exact Experiment. — A rough analysis of the 

 evolved gases can easily be made by collecting *them in 

 a graduated tube, shaking up first with potash solution to 

 absorb all traces of carbonic acid, and then adding pyro- 

 gallic acid, which absorbs the oxygen and leaves only the 

 nitrogen (with a little water- vapour). This method be- 

 comes of the greatest ease, rapidity, and exactitude, as also 

 delicacy in dealing with the smallest quantities of gases 

 by help of the apparatus recently introduced by MM. 

 Bonnier and Mangin {Revue Geiterale de Bota?iique 1890), 

 which is figured below. It will be found of great use to 

 the teacher of practical botany, as also to the experi- 

 mentalist, especially in our uncertain climate. 



Experiment can show us many other things about leaves. 

 It is commonly known that a starch solution turns a deep 

 blue colour when a little iodine solution is added to it. We 

 may use this fact, as Sachs recommends, to demonstrate 

 the presence of starch in leaves. If some leaves, e.g. of 

 sunflower or of potato, are placed for a few minutes in 

 boiling water, and then for a short time in warm alcohol, 

 they become colourless. The green colouring matter or 

 chlorophyll is dissolved by the alcohol. If the discoloured 

 leaves are then placed for an hour or two in a dilute alco- 

 holic solution of iodine, and then removed to a saucer full 



