PHOTOSYNTHESIS 103 



cited (page 23, note), describes a method of projecting the plant upon a 

 screen in a way to make the rise of the gas visible to an entire class. A very 

 valuable critical study of this method is given by Pantanelli in a paper 

 cited below under Literature. 



A method very different in principle is the invaluable one of Engelmann, 

 based upon the collecting of certain bacteria in places where free oxygen is 

 being released. It is applicable only microscopically, however, though it 

 is invaluable for many special purposes. Directions for its use are in Det- 

 mer, 38, and in Darwin and Acton, 48. And there are other methods of 

 minor importance (compare Jost, 105). 



Still another method, applicable to land plants, is offered by the fuming 

 of phosphorus as oxygen is released in an atmosphere which lacks it. This 

 is arranged by placing a potted plant with the exposed phosphorus in an 

 atmosphere containing only hydrogen and carbon dioxide. 



Erroneous Experiments. Two erroneous experiments upon this sub- 

 ject are contained in current text-books. The first, against which warning 

 has already been given under Precautions above, is very wide-spread and 

 embodied in recent figures, namely, the use of a funnel, containing water- 

 plants, which either rests upon the bottom of the containing vessel, or even 

 just fits within the walls of the latter. Under such an arrangement, as is 

 easily proven by control experiments, the release of gas presently stops, for 

 the carbon dioxide under the funnel is soon used up, and no more is obtain- 

 able. To secure a good result it is indispensable to allow ample room for 

 diffusion of carbon dioxide from the remainder of the vessel into which it 

 is absorbed from the atmosphere, and the larger the vessel the better. The 

 other erroneous experiment, given in several books accompanied by a false 

 illustration, is the one in which leaves of land plants, placed under water, 

 are represented as giving off bubbles of oxygen which rise through the water. 

 It is true that leaves which are enveloped in a film of air do carry on some 

 photosynthesis under water (compare Pfefeer, i, 179), but the amount 

 is so small that it is doubtful if any visible bubbles of oxygen are released, 

 the tiny quantities being taken directly into solution. The bubbles which 

 do collect abundantly upon the leaves of land plants placed in cool fresh 

 water and then stood in the light do not come from the leaves, but from 

 the water itself, for they consist of the dissolved air which is always freed 

 from the water when its temperature is raised, whether this be through 

 standing in the sun or by heating in the dark. It is the same air which col- 

 lects upon the side of any vessel under these circumstances or upon other 

 solid objects in it; and the bubbles will collect as readily upon dead leaves 

 as upon those alive, and as abundantly in darkness as in light if the tem- 

 perature be raised as high. 



Oxygen Absorbents and Tests. Before attempting to use these 

 absorbents, the student should, to make himself familiar with their action, 

 apply them to the removal of known quantities of oxygen (e.g., as in air) 

 from closed graduated tubes. 



(1) Potassium pyrogallate, in alkaline solution, is for most purposes 

 the best means for removing oxygen from a confined space, and thus, inci- 



