18 HORTICULTURE FOR SCHOOLS 



29. The cell-wall (Fig. 3) is non-living, but was built 

 up by the living part of the cell. It varies in composition 

 and may be modified in a number of ways to serve different 

 purposes. The cell-wall acts as a protective covering for the 

 living protoplasm. Solutions usually pass readily through 

 the cell-wall inwardly and outwardly. 



30. The ectoplasm. The living part within the cell-wall 

 is surrounded by a very thin membrane called the ectoplasm, 

 which is pressed against the cell-wall so that it is not easily 

 seen. If, however, the cell is placed in a salt solution, some 

 of the water will be taken out, with the result that the part 

 within will shrink, and the ectoplasm will become separated 

 from the cell- wall (Fig. 3). The process of shrinkage in t v his 

 manner is called plasmolysis. The ectoplasm serves to give 

 the young cell support before the cell-wall is formed, and 

 together with the nucleus controls the building of the cell- 

 wall from the material furnished by the living part of the cell. 

 The ectoplasm also regulates the passage of solutions into 

 and out of the cell by its power of selective absorption; that 

 is, it allows needed materials to pass into the cell, but keeps 

 out those which might be injurious or undesirable, and allows 

 the passage of some materials outward, but retains others. 

 In this way, the living cell is allowed to select the materials it 

 needs. 



31. Protoplasm. Most of the portion within the cell- wall 

 consists of a complex living substance called protoplasm, 

 which is necessary to all forms of life. As has been seen, 

 it enters into the composition of the nucleus and cyto- 

 plasm. 



32. The nucleus may be spherical, oval, or irregular in 

 shape. It may be in the center of the cell as shown in the 

 illustration (Fig. 3) or at the side near the cell-wall, or in any 

 other part. It is usually found near the portion of the cell 

 which is growing most actively. The nucleus is necessary to 

 the life of the cell and plays a very important part in its 



