72 SELECTION AND PREPARATION OF ORGANISMS 



growth of isolated plant roots would supply sugar, mineral salts, vitamins 

 which normally are made in the leaves, and perhaps other compounds. 



The cultivation of animal tissues requires carefully prepared media, 

 attention to supplies of oxygen, and controlled temperature. Most cells 

 in higher animals have become so highly specialized that they have lost 

 the power to divide and therefore are unsuitable for tissue culture. 

 Embryonic cells or cells from malignant tumors are the most commonly 

 used animal materials. Certain advantages are offered by animal cells 

 raised in an artificial medium. The cells usually do not become special- 

 ized, but remain "young" and metabolically active. Tissue culture cells 

 can provide a convenient medium in which to raise viruses, which can 

 be cultivated only in living cells. The exact causes for the differentiation 

 and specialization of cells in animals are not understood. Tissue culture 

 cells, therefore, are a potent field for investigation in their own right. 



Plants normally grow at the tips of roots and stems as long as they 

 are alive. These growing tips can be cut off and will continue to grow 

 in the proper medium. Stem tips will produce new shoots and roots and 

 become new plants, but roots continue to grow as roots almost indefi- 

 nitely. P. R. White maintained a culture of tomato roots for several years, 

 using them in a variety of experiments. 



Plant or animal tissue culture is not easy and requires the same aseptic 

 handling as the culture of microorganisms. Mutation is always possible 

 in either microorganisms or tissue culture. Occasionally an experiment 

 calls for special living materials, however, and tissue culture may be 

 worth the effort. 



Preparation of parts of cells 



In present-day research, many of the important problems are those 

 involving the processes associated with all cells. More and more experi- 

 ments have been performed on isolated parts of cells. Until relatively 

 recently, it was believed that cells died immediately, completely, and 

 irreversibly upon being broken. It is now possible to isolate almost any 

 of the parts of cells and keep them functional at least for a short time. 

 Nuclei have been removed from Amoeha cells, leaving only the cyto- 

 plasm. The cytoplasm alone cannot grow or divide, but many of the vital 

 activities continue. Nuclei and even chromosomes have been separated 

 in fairly large quantities from a variety of plant and animal tissues. 

 Chloroplasts separated from green plant cells carry on an amazingly 



