68 SELECTION AND PREPARATION OF ORGANISMS 



examples and certainly cannot describe in detail the methods that are 

 in use. Often the investigator's ingenuity is required to prepare his own 

 organism for his own experiment. 



One very important consideration in the handling of experimental 

 organisms is the treatment before the experiment. Failure to consider the 

 previous treatment can lead to disastrous results. A favorite examination 

 question of some professors describes two sets of organisms that show 

 quite different behavior in apparently identical circumstances. The de- 

 sired answer or explanation is that the previous history has influenced 

 the responses. In many laboratories, plants are raised in special insulated 

 chambers where temperature is controlled, humidity may be controlled, 

 illumination is provided by a color-balanced set of fluorescent tubes, 

 lights are turned off and on by a clock, and the plants are watered with 

 a mineral solution of known composition. Plants raised under these 

 conditions show much less natural variation than those raised in a green- 

 house or out of doors. 



Animals deserve and require elaborate handling. Certain minimum 

 standards must be met in providing comfortable quarters, adequate sani- 

 tation, food and water, and cages that are large enough in order to main- 

 tain the animals in good health. Certainly, you would expect that one 

 of the assumptions in animal experiments is that the animals are in good 

 health. Diseased animals necessarily produce abnormal results. Sev- 

 eral useful manuals and pamphlets on the general care of animals are 

 included in the references at the end of this chapter. 



Preparation of Plant Parts: The preparation of plant parts often is easier 

 than comparable preparations of animal parts. Many of the processes of 

 plants proceed in each cell, and the degree of coordination is lower than 

 in animals. In dissecting out a muscle we must worry about the coordi- 

 nation with the circulatory system and the nervous system. If a leaf is 

 removed from a plant, however, there may be relatively little effect on 

 either the plant or the leaf. A number of investigations have shown some 

 differences, usually stimulations, in the respiration of isolated plant parts, 

 but these are differences in degree rather than in quality. Young leaves 

 and old leaves are never identical, but it is not difficult to select leaves of 

 about the same morphological age. 



Leaves or pieces of leaves are well adapted to measurements of meta- 

 bolic activities. If we wish to express the rate of ox}'gen production 

 per milligram of green tissue, we merely weigh the pieces of leaf tissue. 

 The shape need not concern us. The measurements sometimes are more 

 meaningful when related to leaf area. If we must know the area, some 



