264 



THE CELL 



of protoplasm forming definite structures, usually scattered 

 in the layer of cytoplasm. They are the plastids. The ones 

 most familiar are the chloroplasts, in which the green pig- 

 ment chlorophyll is imbedded. They are found in cells of 

 leaves and stems exposed to the light. Plastids are not found 

 in all cells. The dead part of the cell is the cell-wall, the cell- 

 sap stored in chambers or pockets in the protoplasm called 

 vacuoles (Fig. 438), and various inclusions. The cell-sap 

 contains mineral nutrients in solution or suspension, as well 

 as organic foods, as sugar and other sub- 

 stances. Imbedded in the cytoplasm or in 

 the plastids may be starch grains, oil 

 droplets and other substances. In the 

 nucleus is a densely granular body called 

 the nucleolus. 



440. Study of Cell. — Examine with the 

 aid of the microscope the cells in the sta- 

 men hair of tradescantia or spider-wort. 

 (Fig. 438.) If the flowers of this plant are 

 not available, use the young bristle hairs 

 of squash plants; a plant a few weeks old 

 will supply sufficient hairs. Note the shape 

 of the cell and the contents. The nucleus 

 will probably be located near the middle 

 of the cell, and to it run the strands of 

 cytoplasm. The protoplasm is not entirely 

 homogeneous. It is composed of a viscous, 

 colorless fluid in which are imbedded many 

 minute granules. In a young cell the pro- 

 toplasm fills almost the entire cell. In an 

 old cell the vacuoles are of increased size. 

 . Compare old and young cells in the stamen 

 topiasm in a cell of a hairs of tradescantia or in squash hairs for 



stamen hair of trade- . . . , 



scantia or spider- their protoplasmic content. Examine the 

 ^Magnified eoo cells of the ep idermis of the onion. Note 



