104 



A TEXT-BOOK OF BOTANY 



C' 



In the great majority of cells there is a single nucleus, and 

 all about it, filling the general cavity within the cell- wall, 

 is a mass of much less dense protoplasm, known as cytoplasm. 

 The cytoplasm seems to form the general background or 

 matrix of the cell, and the nucleus lies embedded within it. 



Another protoplasmic organ of 

 the cell is the plastid. Plastids 

 are relatively compact bodies, 

 and variable in form and num- 

 ber. The most common kind of 

 plastid is the one that contains 

 chlorophyll, and hence is known 

 as the chloroplastid or chloroplast. 

 An ordinary cell of an alga, 

 therefore, consists of a cell-wall, 

 within which the protoplasm is 

 organized into cytoplasm, nu- 

 cleus, and chloroplasts. With 

 proper staining the nucleus and 

 the chloroplasts of Pleurococcus 

 can be seen; but these structures 



may be seen more distinctly and with much less trouble 

 in the cells of a moss leaf (Fig. 96). 



The cell-wall is elastic, so that the cell can be compressed 

 or inflated. The single cell of Pleurococcus, unless pressed 

 upon by neighboring cells, retains a spherical form as long 

 as it is alive, a fact which shows that there is constant and 

 uniform pressure on the wall from, within the cell. It is 

 found that this pressure is due to the absorption of water 

 in sufficient amount to stretch the wall, this distended con- 

 dition of the cell being called turgor, a name indicating 

 that the cell is turgid. Pleurococcus retains its spherical 

 form, 4'heref ore, because it is turgid; and the bulging of 

 free walls of Oscillatoria ( 60) is due to the turgor of 

 the cells. 



FIG. 96. Cells of a moss leaf, show- 

 ing chloroplasts (a), nucleus (6), 

 and cytoplasm (c). 



