STRUCTURE AND PROPERTIES OF PROTOPLASM 71 



tion. Above the maximum or below the minimum, for an inter- 

 val of a few degrees, as the student should confirm for himself, 

 the Protoplasm remains quiet, but can have its motion restored 

 by dropping or raising, respectively, the temperature. This 

 condition of quiescence is known as rigor, and is also character- 

 istic of differential relations. Above or below the rigor tempera- 

 tures disorganization of the Protoplasm ensues, with death, the 

 heat then acting upon it directly in the first of the ways above 

 given. 



The cardinal points have been found, experimentally, to 

 be not fixed for each organism, but alterable by gradual accom- 

 modation to higher or lower temperatures. This is the principal 

 physiological basis of acclimatization, a subject on which the 

 student should here inform himself. 



The foregoing case of a thermotonic relation is the clearest 

 and most complete example of this kind of relation which can 

 actually, be seen in operation. The action of most others can 

 in practice best be inferred from the results upon entire plants, 

 as the student will find during his later studies. Yet there are 

 some other cases which the student may observe if his time per- 

 mits. 



Suggested Experiments. In all cases streaming Protoplasm mounted 

 for the microscope, as in the earlier experiments, is to be used. 



Light. For this it is needful so to arrange that light in various degrees, 

 from darkness to full sunlight, can be thrown at will on the streaming Proto- 

 plasm, the rate of movement being determined at the various intensities. It 

 is essential that the influence of heat be eliminated, which can be done by 

 avoiding concave mirrors and condensers, and by interposition of an alum 

 bath or a stream of running water, for which purpose the gas-chambers, 

 described below, may be employed. A control should be provided by plac- 

 ing a thermometer bulb in place of the object on a neighboring and similarly 

 treated microscope. 



Electricity. For the exact study, involving measurement, of this rela- 

 tion, somewhat elaborate and special appliances, including batteries, induc- 

 tion coils, non-polarizable electrodes, electrometers, etc., are needed, which, 

 with the appropriate methods, are described in Davenport's, in Detmer's, 

 and especially in Ewart's works. But for qualitative results, sufficient for 

 the present purpose, it may be simply studied as follows. On an electrical 

 slide (described below), in contact with tin-foil terminals, place a Nitella 

 or Tfadescantia filament in active streaming, and then, with aid of a simple 



