CELL STRUCTURE AND PROTOPLASM 25 



that enter and leave the cell. For example, if an uninjured 

 beet is soaked in cold water, the color and the mineral salts 

 will be held in the beet by this layer, but if the cytoplasmic 

 layer is injured, as by heating the water, the color and min- 

 eral salts will diffuse from the beet into the water. The 

 green color of plants and most of the other colors are located 

 in special parts of the cytoplasm called plastids. A single 

 cell of a leaf may have several hundred of the green plastids 

 called chloroplasts, named from the pigment chlorophyll. 

 The cells in the leaf diagram of Figure 15 show small numbers 

 of chloroplasts located near the cell walls. 



The nucleus is often spoken of as the center of life be- 

 cause cells cannot live without the nucleus. It is believed 

 to control the activities of the cell by its secretions, but this 

 needs further study because of our rapidly growing knowl- 

 edge in the fields of hormones and of enzymes, and their 

 regulatory power. Since the size of the nucleus of a young 

 growing cell is about 10 per cent of the total cell but decreases 

 with the age and diminishing activity of the cell, it suggests 

 that the nucleus may secrete the above all-important sub- 

 stances as one of its functions. The highest phosphorus , 

 content of the cell is in the nucleus. The material of most 

 interest in the nucleus is a protein called chromatin, which 

 in very small granules is scattered, doubtless in an orderly 

 manner, through the nucleus. At the time of cell division 

 these chromatin granules are clearly seen to make larger 

 pieces in an orderly fashion called chromosomes (see 4, 5, 

 and 6 in Fig. 6) . Very recently a group of scientists claimed 

 that by proper techniques it may be demonstrated that the 

 chromatin retains its orderly identity at all times. 



The number of chromosomes in a cell is always the same 

 for all the cells throughout a plant and is generally uniform 

 for the same species or variety. Many plants, but not all, 

 have had their number of chromosomes determined by 

 actually counting them in highly magnified dividing cells. 



