EVOLUTION OF PLANTS 



mals the so-called " Monera," which were formerly 

 supposed to be composed of structureless protoplasm, 

 are now known to possess a nucleus, so that we cannot 

 assert positively that any known forms consist of un- 

 differentiated protoplasm as was once supposed to be 



the case. As a rule the 

 protoplasm is segregated in 

 masses of definite form, usu- 

 ally furnished with a more 

 or less evident envelope and 

 provided with a special 

 structure, the nucleus. 

 These nucleated masses of 

 protoplasm are generally 

 called cells, although the 

 name is occasionally re- 



w 



pr 



n - 

 nu 



served for such as are pro- 

 vided with a single nucleus 

 and a definite membrane, 

 each nucleus with its ac- 

 companying protoplasm be- 

 ing designated an "energid." 

 We shall, however, for con- 

 venience' sake use the term 

 cell in its ordinarily accepted 

 sense. 



In all but the lowest forms of life the cells always show 

 at least two parts, the cell-plasm or cytoplasm, and the 

 nucleus. The latter is usually of definite form, globular 

 or lenticular (Fig. 1, A, w), and bounded by a definite 

 membrane, which, however, is apparently not chemically 

 different from the cytoplasm in which it is imbedded. 



FIG. 1. A, a cell from a hair at the 

 base of the stamen of a spider- 

 wort (Tradescantia), showing 

 the parts of a typical plant cell ; 

 iv, the cell-wall; pr, the proto- 

 plasm in which is imbedded the 

 nucleus, n, with the nucleolus, 

 nu; v, vacuoles, spaces filled 

 with watery cell-sap. B, a Des- 

 mid (Cosmarium), a plant con- 

 sisting of a single cell ; pi, one 

 of the chloroplastids ; n, the 

 nucleus. 



