THE ORGANISM AND THE CELL 13 



accompanied by an increase in size, although a cell or other mass of 

 protoplasm may at times become much larger because of the accumula- 

 tion of some nonprotoplasmic product, such as cell sap. Certain effects 

 of growth upon the form assumed by protoplasts will now be examined. 



For the sake of simplicity we shall begin with a typical cell consisting 

 of a spherical mass of cytoplasm enclosing a spherical nucleus, these two 

 portions being bounded by a plasma membrane and a nuclear membrane, 

 respectively. Synthesis of new protoplasm involves interchanges of 

 materials between cytoplasm and nucleus, the part that the nucleus can 

 play being determined quantitatively by the area of its membrane. 

 As the cell and nucleus increase in volume, the area of the membrane 

 through which the interchanges must occur does not increase at the sam(^ 

 rate ; hence a point may be reached beyond which the interaction between 

 nucleus and cytoplasm would be insufficient to support further growth 

 of the whole protoplast. This difficulty is usually overcome in nuclear 

 division, whereby the nuclear surface present in the protoplast is increased 

 without a corresponding increase in the volume of any individual nucleus. 

 Further growth can then proceed until the critical nucleocytoplasmic 

 ratio is again reached. Sometimes a nucleus may increase its surface- 

 volimie ratio by altering its shape, the relative amount of surface 

 being greater in nonspherical objects. 



A similar limitation on the growth of a spherical cell is imposed by the 

 area of plasma memljrane through which the protoplasm interacts with 

 the external environment. Here again the block to further growth is 

 overcome by a division of the protoplast, or often by the assumption of a 

 flattened or a filamentous form, as in so many algae; either of these 

 methods leads to an increase of surface per unit volume. In large organ- 

 isms like ourselves there are, of course, elaborate structural features, such 

 as the respiratory and circulatory systems, that make possible interactions 

 between the environment and the innermost regions of the body. 



As the protoplasm continues to grow and the nuclei to multiply, the 

 patterns assumed by the cytoplasm, nuclei, and membranes may come to 

 differ widely in various tissues and organisms. For convenience one may 

 refer to such patterns as protoplasmic growth patterns, remembering, how- 

 ever, that these are not always sharply distinct. In Fig. 5 there is 

 represented diagrammatically the development of six such patterns from a 

 typical protoplast. 



1. The growth of the protoplasm and the multiplication of its nuclei 

 may not be accompanied by the formation of partition membranes, the 

 result being a 'multinucleate mass. Such a coenocytic condition is seen 

 throughout the vegetative bodies of some algae and fungi and as a 

 temporary or permanent feature of certain tissues of other plants and 

 animals. 



