SELECTION AND PREPARATION OF ORGANISMS 69 



shapes are more desirable than others. A cork borer easily punches out 

 uniform discs of leaf tissue. By carefully selecting ^ cork borer of 1.12 

 cm diameter, we get discs with an area of 1 cm". Determination of the 

 total area of leaf tissue thus requires no calculation, only counting. 



Sections of other tissues can be cut in the same way. Storage roots or 

 tubers are often quite uniform in cellular composition and have been 

 used to advantage in many experiments. A cork borer conveniendy 

 punches out cylindrical plugs of such tissues. The plugs may then be 

 sliced to any desired thickness so that the volume of tissue is calculated 

 easily. Alternatively the cork borer will cut through a stack of tissue 

 sHces to yield the same result. No extra effort is required, and the number 

 of calculations is reduced. This, in turn, decreases the chance of compu- 

 tational errors. 



If the process under investigation demands it, whole leaves, stems, 

 roots, flowers, or fruits may be used. Stems, leaves, or roots of germinat- 

 ing seeds are useful because at this stage the plant is more actively per- 

 forming a variety of functions than at any other time. Often there is a 

 choice of several kinds of plants. If one species will provide the desired 

 organs more conveniently than another species without any attendant 

 disadvantage, then by all means select the convenient species. 



Preparation of Animal Tissues: Usually more care must be used in 

 the preparation of animal tissues because of the more highly coordinated 

 organization of the animal. Changes in the acidity, in the concentration 

 of oxygen or other gases, or in the osmotic balance in the vicinity of a 

 tissue may alter its behavior markedly. With proper attention to such 

 details, however, many different animal tissues will continue to function 

 for some time after being removed from the animal. 



Before any such dissections are attempted, it is imperative to immo- 

 bilize the animal somehow. Rats, rabbits, and other mammals can be 

 anesthetized with barbiturates or similar drugs. Frogs are usually immo- 

 bilized by destroying the brain and spinal cord. The manner in which 

 animals are handled is important because they can become emotionally 

 excited. 



Of all the various animal tissues, perhaps muscle has been used as 

 commonly as any other for physiological experiments. Pigeon breast 

 muscle was exceedingly important in unraveling the sequence of reac- 

 tions in cellular respiration. Much of what we know of movement and 

 reflex action was learned from experiments on isolated muscles. Liver 

 has also been used very extensively, partly because this easily accessible 

 organ provides a large mass of fairly uniform tissue, and partly because 



