CHAPTER VII 

 CELLS AS BIOLOGICAL UNITS 



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The development of every science is invigorated and its aims are 

 redirected from time to time as the result of some important invention. 

 A striking example in the field of biology is the invention of the micro- 

 scope in the seventeenth century. Superficial observation and study of 

 plants had been going on for untold centuries. The origin of many of our 

 most important cultivated and medicinal plants antedates the oldest 

 archeological discoveries. In Europe and Asia several thousand plants 

 had been described by the time that the invention of the microscope 

 made it possible to examine their more minute structure and to discover 

 that many plants consist only of single cells. ^ 



Cells. The early microscopists were so fascinated with the world of 

 minute plants and animals previouslv unseen and unsuspected that they 

 studied and described them in preference to the finer structures of the 

 larger plants. Cells were seen and recognized as structural parts of 

 plants, but for a hundred years observations were limited mainly to cell 

 walls. In the latter part of the 18th century various microscopists began 

 to study "cell contents." By the middle of the 19th century it became 

 evident to a number of eminent biologists- that the properties which we 

 associate with life are the properties of that part of the cell contents 

 which had come to be called protoplasm. Moreover, its organization into 

 cytoplasm and nucleus, and the enclosed vacuole had been recognized. 

 Many biologists had by this time accepted three general principles: ( 1 ) 

 that the bodies of all living organisms are composed of cells, or products 

 of cells; (2) that in certain features the cells of plants and animals are 

 essentially alike; and (3) that protoplasm is the physical basis of life 

 phenomena. To these principles we may add (4) that when plants are 



^ Perhaps the oldest book of plant descriptions extant is Shen Nung's Tree and Herb 

 Book, written during the 28th century b.c. In it 252 species of plants are classified accord- 

 ing to their alleged medicinal values. 



- Attention may be called especially to Robert Brown, \on Mohl, Schleiden, Schwann, 

 Dujardin, Nageli, Payen, Cohn, and Schultze, whose contributions are admirably discussed 

 in L. W. Sharp's Introduction to Cytology, 3rd ed., McGraw-Hill Book Company, Inc., 

 1934, pp. 422-447. 



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