4 INTRODUCTION TO CYTOLOGY 



of granules, rods, or threads and constitute a group of materials of uncer- 

 tain function (see Figs. 43 to 46). 



Ergastic substances are accumulations of nutritive materials and other 

 products of metabolic activity. These non-protoplasmic substances, 

 which occur chiefly in the cytoplasm and vacuoles, may exist in the form 

 of visible granules, droplets, or crystals. Such substances may also occur 

 in the dissolved state. 



The cell wall of plants, as at present understood, is usually regarded 

 not as a part of the cell proper, or protoplast, but rather as a secretion of 

 the latter. It is often absent, as in motile spores and gametes, which 

 have only delicate limiting membranes. The intercellular substance of 

 many animal tissues also bears a somewhat problematic relation to the 

 protoplast. 



Even in such a brief description as this, one is struck by the general 

 similarity of plant and animal cells. This is obviously associated with 

 the fact that all organisms, be they plants or animals, must carry on 

 certain basic vital activities in common. Differences in cell structure 

 may be taken to indicate differences in the vital processes habitually 

 performed. The plastids, for instance, are to be viewed as structural 

 and functional modifications by the aid of which plants are able to 

 manufacture their own food out of simpler compounds derived from the 

 environment, an act which animals cannot accomplish. The prevalence 

 of large sap vacuoles in plant cells is also associated with peculiarities 

 in metabolism. The elaborate cell walls of plants furnish support for 

 large bodies, a problem which animals have solved in quite different 

 ways. These are probably the three most conspicuous structural differ- 

 ences between the cells of the two kingdoms. Other minor differences 

 will appear in subsequent chapters, but we shall continue to be impressed 

 by the remarkable cytological unity of both branches of the organic 

 world. 



It is scarcely necessary to point out that the cell should not be thought 

 of as a static thing with a permanent physical structure. It is rather a 

 dynamic system in a constantly changing state of molecular flux, its 

 constitution at any given moment being dependent upon antecedent 

 states and upon environmental conditions. In the words of Harper 

 (1919), it is a colloidal system in w^hich the various processes have become 

 progressively localized in certain regions, with the resulting formation of 

 organs, which, with the increasing constancy of the processes involved, 

 have come to possess a permanence and individuality of their own. The 

 characteristic organization of the cell is a result of the differentiation of 

 protoplasm, and this organization is of such a nature that it plays in turn 

 an important part in determining the development of a higher degree of 

 differentiation. Its efficiency in this respect is indicated by its almost 

 universal occurrence in organisms of so many types. 



