Further experiments with anucleate "cells," 

 especially those in which radioactive tracer 

 methods (p. 141) have been used as a means of 

 following constructive metabolism, have 

 yielded some basic conclusions, which will be 

 discussed more fully later (Chap. 4). Suffice 

 it to say here that the nucleus, with its con- 

 tent of specific DNA proteins (genie mate- 

 rials), provides for the continued production 

 of complementary RNA proteins, which are 

 passed on to the cytoplasm. In the cytoplasm, 

 these RNA compounds provide templates for 

 the synthesis of enzymes and other proteins, 

 which are essential for continued growth and 

 activity. In the absence of a nucleus to re- 

 plenish RNA, the cytoplasm gradually loses 

 its powers of synthesis and begins to waste 

 away. Moreover, since only the DNA pro- 

 teins of the nucleus display any capacity for 

 self-replication, no reproductive potential 

 remains in the anucleate "cell." In any event, 

 these experiments give firm support to the 

 conclusion that each cell, nucleus and cyto- 

 plasm together, represents a protoplasmic 



Protoplasm, the Cell, and the Organism - 35 



unit from the viewpoint not only of form, 

 but also of function. 



The importance of the nucleus in construc- 

 tive metabolism is emphasized by other ob- 

 servations. The nucleus usually migrates to 

 any part of a cell where active synthesis is 

 occurring (Fig. 2-21); and gland cells — in 

 which constructive metabolism goes on very 

 rapidly — tend to possess exceptionally large 

 or even lobulated nuclei, which expose a 

 maximum of surface to the surrounding cy- 

 toplasm (Fig. 2-2 ID). 



THE CELL AND THE ORGANISM 



In the simplest organisms the whole body 

 consists of a single cell, and within the limits 

 of such a cell there may be a fairly complex 

 differentiation of structures, especially in the 

 case of animal cells. The nutrition of a typi- 

 cal animal (Chap. 7), even in unicellular 

 forms, presupposes a capacity to sense the 

 location of food, to move toward food, and 

 to ingest, digest, and absorb the food; and 



CELL WALL 



A 



k 



NUCLEUS. 

 CYTOPLASM. 



CELL WALL 



Fig. 2-21. The nucleus is concerned with synthetic processes in cells. A, growth 

 of root hairs from epidermal cells of root of plant; note the location of the 

 nuclei close to the points of rapid growth. B, C, plant cells synthesizing local 

 thickenings of the cell walls, with the nuclei close to the point of special 

 thickening in each case. D, a cell of the silk gland of a caterpillar, with a large 

 lobulated nucleus. (A, B, and C after Haberlandt; D, after Korschelt.) 



