72 PLANT PHYSIOLOGY 



circuit closer, send through it the charge of successively one, two, and three 

 dry cells, applying the current instantaneously (as nearly as possible), one, 

 two, three, five, and ten seconds, and noting whether the making and break- 

 ing of the circuit or the continuous action of the current produces the result. 

 It is well also to use an interrupted current through an induction coil, of 

 which the DuBois Raymond form, permitting a large range with every 

 intermediate gradation, is best. 



Mechanical Impacts. These may be of several sorts, including sudden 

 shock, pressure, alterations in atmospheric pressure, and variations of osmotic 

 pressure. As simplest and most typical, we may take the first. For its study 

 some method is needed by which the intensity of the shock may be measured. 

 This is well accomplished by Ewart's method (73), in which a solid object, 

 such as a knitting-needle, is dropped through a glass tube upon the cover- 

 glass from successively greater measured distances. For study of the effects 

 of pressure, a simple arrangement perrritting the application of a screw to 

 the cover-glass is possible. For variations in atmospheric pressure, the gas- 

 chamber described below can be adapted, the material being placed in a 

 drop of water hanging beneath the cover-glass, which is sealed (by sealing- 

 wax) to the chamber. The pressure is lessened by a Chapman exhaust. 

 Variations in osmotic pressure are easily observed by use of solutions of 

 sugar or salt applied precisely as for the plasmolytic studies given later in 

 this book under Absorption. 



Chemical Action. Of the chemical substances exerting a tonic effect, 

 three of the most important are oxygen, carbon dioxide, and anaesthetics. 

 For the application of these it is needful to use a gas-chamber (described 

 below), into which the gas can be drawn at will and thus applied to the object 

 which hangs in a drop of water beneath the cover-glass. For the practical 

 directions in detail the student should turn to the works of Detmer, of Dar- 

 win and Acton, or of Davenport. 



Electric Slides. These are very simple in principle, consisting of an 

 ordinary glass (and therefore self-insulating) slide, near the ends of which 

 are attached small binding-screws in contact with metal clips extending 

 near the cover-glass. From the clips tin-foil strips lead to the two ends of 

 the object, through which a current may thus be sent. Such slides are sold 

 by most dealers. The electrodes are obviously not unpolarizable, as they 

 must be for exact work, on which the works of Detmer or of Ewart should 

 be consulted. An efficient slide can be made up as shown by the accom- 

 panying figure (Fig. 12), in which simple binding-screws are attached to the 

 slide by sealing-wax or by asphalt vamish. Or a perfectly good slide can 

 be improvised from the ordinary clips of the rricroscope stage, which need 

 only to be insulated in their sockets by rubber sheeting and then swung near 

 the cover-glass, the wires being attached to the screws of the clips. 



Gas-stages. These consist of flat chambers, partly or wholly of glass, 

 made to lie on the microscope stage, and having inlet and outlet tubes through 

 which the desired gases may be drawn by an aspirator. The object usually 

 hangs in a water-drop on the under side of a cover-glass, which forms the 

 roof of the chamber and is sealed thereto by soft wax; the water-drop is 



