PRELIMINARY DESCRIPTION OF THE CELL 



25 



nucleus, which at least in many respects is the most important of these 

 organs. 1 



The cytoplasm, a more or less transparent, viscous, granular fluid, 

 may, with its inclusions, occupy the whole volume of the cell. This is 

 generally true of animal cells and the younger cells of plants. If the 

 cell is vacuolate, as is usually the case in the mature plant cell, the 

 cytoplasm may constitute only a thin layer lining the wall, the central 

 vacuole with its cell sap often far exceeding it in volume (Fig. 2, C). 



FIG. 2. 



A-C, diagram of a plant cell in three stages of development: the vacuoles increase in 

 volume and the protoplasm becomes limited to the parietal region. D, cell of stamen hair 

 of Tradescantia, showing streaming movements in the cytoplasmic strands. E, paren- 

 chyma cell from cortex of Polygonella, showing nucleus, plastids, and scanty cytoplasm. 



In many cases the cytoplasm forms a system of anastomosing strands that 

 often show active streaming (Fig. 2, D}. Externally the cytoplasm is 

 limited by a layer of different consistency, the plasma membrane, or 

 ectoplast. Where it comes in contact with the enclosed vacuole it is also 

 limited by a membrane, the vacuole membrane, or tonoplast. 



The nucleus is bounded by a nuclear membrane and contains an 

 extremely clear fluid, the nuclear sap, or karyolymph. In the karyo- 

 lymph is imbedded the nuclear reticulum, composed usually of linin, an 

 achromatic supporting material, and chromatin, the " nuclear substance 

 par excellence." One or more true nucleoli, or plasmosomes, are commonly 

 present in the nucleus, and may or may not be closely associated with 

 the reticulum. There are often present also chromatin nucleoli, or karyo- 

 somes, which represent accumulations of chromatin at certain points on 

 the reticulum, and should not be confused with the true nucleoli. 



1 According to the older usage the extra-nuclear portion of the protoplast was 

 called "protoplasm," which was unfortunate because we now know that the nucleus 

 also is composed of protoplasm, or living substance in its broader sense. It is now 

 the general custom to avoid this ambiguity by employing Strasburger's terms cytoplasm 

 and nucleoplasm (karyoplasm, Flemming). The older usage, however, has not been 

 entirely superseded. 



