THE CELLULAR STRUCTURE OF PLANTS 



47 



Cell WCL ll 



Cytoplasm 



-Nucleus 



" 'Vacuole 



with a semifluid, viscid substance, not unlike the honey in a 

 honeycomb in consistency and in general appearance. This sub- 

 stance, which fills the cell cavities of all living plant cells, is the 

 living substance of the plant body, to which Von Mohl first gave 

 the name protoplasm. If sections of roots or of other living 

 parts of plants are properly stained, this living substance, proto- 

 plasm, within the cell cavities will be found to be composed of 

 a darker central body, called 

 the nucleus, and a less dense 

 part surrounding the nucleus, 

 called the cytoplasm. For 

 convenience in designating 

 the parts of a living cell the 

 entire mass of living proto- 

 plasm within one cell cavity 

 is termed the protoplast, 

 which, as we have already 

 seen, is composed of two 

 distinct parts, the cytoplasm 

 and the nucleus. 



The protoplast in young 



f * / The figures are designed to illustrate the 



plant Cells (a) usually bears a C ell parts and the gradual formation of 



vacuoles. a, a young cell with small vacu- 

 oles ; b-d, progressive vacuole formation as 

 it occurs during the enlargement of a cell 

 by growth. Consult the text for further 



'Nucleus 

 Nucleolus 



Cytoplasmic 

 c d 



FIG. 27. Camera drawings, greatly 



magnified, of root tip cells containing 



cytoplasm, nuclei, and vacuoles 



discussion 



somewhat different relation 

 to the other parts of the cell 

 from what it does in older 

 cells (b-d). In young cells 

 it fills the cell cavity with a dense mass of protoplasm, while in 

 the older cells it contains water in the form of water drops, 

 called vacuoles. As the cells grow, these water drops, or 

 vacuoles, enlarge and unite until they finally accumulate as a 

 large central water drop, vacuole, or sap cavity in the center 

 of the cell. In such instances the solid protoplast of the young 

 cell becomes gradually forced outward as the cell grows by the 

 accumulation of water in the large central vacuole. The cyto- 

 plasmic portion of the protoplast then forms a thin layer, lining 

 the cell wall and inclosing the central vacuole. It is now prop- 

 erly called the cytoplasmic sac (d). The nucleus in these older 



