30 PLANT BIOLOGY 



To this substance is given the name protoplasm. We know 

 now that the living substance or protoplasm is the essential 

 part, while the wall may be missing, so that in such a 

 case there is no resemblance to a cell or box. Biologists 

 now understand a cell to be a bit of protoplasm (cell-body) con- 

 taining a nucleus (which is a denser portion of the protoplasm) . 



Protoplasm, when examined with the highest powers of the 

 microscope, appears as a colorless, semifluid substance, in 

 which are often seen solid particles or granules, which are 

 probably little masses of food. The nucleus, as already 

 stated, is commonly found near the center of the cell, and is 

 composed of protoplasm denser than the protoplasm of the 

 body of the cell. The appearance and composition of the 

 protoplasm may be well represented by raw white of egg ; but 

 in making this comparison one should bear in mind that the 

 white of an egg is not living substance. 



Within the cell, too, and occupying some of the space out- 

 side the nucleus, especially in plant cells, is cell-sap, which is a 

 lifeless fluid composed of water in which are dissolved the food 

 substances (such as sugar and mineral matters) used by cells 

 in their growth and repair, and in the various kinds of work 

 which they carry on (Fig. 6). 



43. Assimilation, growth, and cell division. To make 

 protoplasm the plant must have proteins, water, and addi- 

 tional compounds containing iron, calcium, and several other 

 chemical elements. But only protoplasm has the power to 

 combine these compounds in such a way as to form living 

 matter. Bearing in mind the facts we learned in studying 

 food manufacture (33 and 34), we see that the plant begins 

 with simple substances, water and carbon dioxid, and manu- 

 factures a more complex substance, sugar. It uses this and 

 other substances to make a still more complex substance, 



