74 THE PLANT: A GENERAL INTERNAL VIEW 



protoplasm and its contents after they have been stained. 

 Protoplasm has more fluid and less fluid parts. The more 

 fluid part is called cytoplasm. The cytoplasm streams 

 slowly about in active cells. Through exceedingly small 

 spaces in the wall it connects one protoplast with its 

 neighbors. A denser part of the protoplasm which has 

 rather definite shape is called the nucleus. The nucleus 

 is indicated by n in Figure' 25. So far as we know, every 

 living cell has a nucleus. The cytoplasm appears to be 

 concerned with the nutritive work of the cell, and the 

 nucleus with the reproductive work of the cell. Cells re- 

 produce by division, but the power to divide is not possessed 

 by all cells. 



Nucleus and cytoplasm are the only things which occur 

 in all living cells, but there are other things which occur 

 in many living cells. Some of these other things are of 

 great importance to plant life. Chloroplasts, for example, 

 occur in the green cells of leaves and stems, and they are 

 of great importance to plant life. They are the structures 

 which do the actual work of transforming inorganic sub- 

 stances into food. That is, they are the organs of photo- 

 synthesis. The numerous chloroplasts in Figure 25 are 

 indicated as ch. (See also Figure 26, in which the chloro- 

 plasts are scattered through the cell.) The chloroplasts 

 are principally composed of protoplasm. In density they 

 are similar to the nucleus. They contain chlorophyll, a 

 substance manufactured by the protoplasm. (Ckloro 

 means green, phyll means leaf.) It is the chlorophyll 

 which gives the green color to the chloroplast; without 

 chlorophyll photosynthesis does not occur. 



Another important thing usually present in cells is the 

 vacuole. (Indicated by v in Figure 25.) There may be one 



