PROTOPLASM 31 



reactions which result in the formation of compounds not present during 

 life. In spite of their shortcomings, however, chemical analyses do yield 

 valuable information concerning the types of substance present in proto- 

 plasm and the relative proportions in which they may occur. Such 

 information, inadequate as it may be, is obviously prerequisite to any 

 understanding of protoplasmic activity. 



The chief point to be borne in mind is that chemical analyses do not 

 reveal a most significant characteristic of protoplasm, namely, its peculiar 

 organization. Just as in the case of any other operating system, such as a 

 watch, the activities of protoplasm can be understood only if the struc- 

 tural relations of the materials composing it are known. It is now 

 abundantly clear that the essential synthetic chemical reactions of proto- 

 plasm depend upon an essential type of structure present only during life 

 (see R. S. Lillie, 1923, 1924). 



The Physical Nature of Protoplasm. ^^ — Under the ordinary micro- 

 scope living cytoplasm appears as a colorless, optically homogeneous 

 fluid, called hyaloplasm, in which there are usually imbedded granules and 

 globules of varying size, shape, and number. It is commonly observed 

 to be in a state of active streaming, notably in highly vacuolate cells 

 of plants. In the leaf cells of Elodea, for instance, the cytoplasm is in 

 almost continuous rotation, while the stamen hairs of Tradescantia afford 

 a beautiful example of a more complex type of circulation. Various 

 theories, involving electricity, contractility, surface tension, imbibition, 

 and other phenomena have been propounded to account for such proto- 

 plasmic movement. ^^ The specific gravity of protoplasm is very slightly 

 above that of water. 



Noteworthy among the general physical characteristics of living 

 protoplasm are its elasticity and viscosity. These are properties which 

 it is difficult to estimate quantitatively. Much has been learned about 

 them through the use of microdissection apparatus, ^"^ the centrifuge, ^'^ and 

 electromagnets acting on inserted iron or nickel particles. ^^ The viscosity 

 varies greatly in different tissues, in different portions or organs of a 

 cell, and at different stages of cell-division and differentiation. In 

 general, it seems that animal protoplasm is on the average more viscous 

 than that of plants. In nerve cells and non-dividing epithelial cells no 

 Brownian movement is visible, even with dark-field illumination, but in 

 plant cells, as well as in eggs and many tissue-culture cells of animals, 



1^ See Chambers (1924), Lundegardh (1922, Pt. I, Chap. XI; Pt. II, Chaps. I-III), 

 Meyer (1921), Schaeffer (1920), Harper (1919), Wilson (1923, 1926), Heilbrunn 

 (1927, 1928), Seifriz (1929a). Weber (1926a) and Spek (1926) give bibliographies. 



>s Meyer (1921), Beikirch (1925), S. Nichols (1925), Fitting (1927), Spek (1926), 

 Umrath (1930). 



" Chambers, Seifriz, C. V. Taylor. 



" Lyon, Heilbrunn, Weber, W. Zimmermann. 



'* Heilbronn, Freundhch, Seifriz. 



