160 The Living Plant 



protoplasm, denser than the cytoplasm, and different, somewhat, 

 chemically. It varies comparatively little in appearance in differ- 

 ent cells, and ordinarily exhibits no particular structure; but when 

 the cells are dividing or reproducing, then a definite number of 

 rod-shaped structures become differentiated and perform re- 

 markable manoeuvres which we shall later consider in the suit- 

 able place along with reproduction (figure 101) . These rods, called 

 chromosomes, are the seat of the controlling power of heredity, 

 and thus guide the constructive work of the cytoplasm in growth. 

 The nucleus, therefore, bears to the cytoplasm a relation sug- 

 gestive of that of the brain to the body. Indeed, the resemblance 

 may extend pretty far, since there are those who maintain that 

 heredity in the chromosomes is substantially the same thing as 

 memory in the brain. But I hope the reader will not therefore 

 call the nucleus the brain of the cell, for it isn't. As to the nu- 

 cleolus, that is irregular hi its appearance, and probably repre- 

 sents a reserve of chemical substance for use in the growth of the 

 chromosomes. 



The plastids, likewise, are living protoplasm, and are present in 

 all cells of the typical plants, though sometimes they are incon- 

 spicuous. Thus, it is the plastids which hold the green color 

 in leaf-cells, where they are already well known to the reader as 

 chlorophyll grains, called also chloroplastids. In other cells, 

 of some fruits, such as the familiar Jerusalem Cherry, they con- 

 tain yellow or orange colors (chromoplastids), thus aiding to 

 make the fruits conspicuous. And in other cells yet, especially 

 in the storage parts of plants, they remain colorless and are called 

 leucoplastids, but perform the remarkable and indispensable 

 function of converting sugar to starch. It is, indeed, a fact of 

 the greatest interest about these leucoplastids, that they and the 

 homologous chloroplastids comprise the only places in nature, 

 either within the plant or outside of it, where starch is known to be 

 made. Starch is one of the substances which the chemist has not 

 yet been able to make in his laboratory. 



