PHotoPlasM and plant- cells. 5 



no markings of any kind, but when otherwise it shows dots, 

 pits, rings, spirals, reticulations, etc. etc. (Fig. 3). This 

 thickening gives strength to the cell-wall, and serves either 

 to protect the protoplasm, as in many spores and pollen- 

 grains, or to help in building up the framework of the 

 plant. 



10. In some part of the protoplasm of each cell (often in 

 the centre) there may generally be seen a rounded body 

 composed of denser protoplasm (Fig. 1). This has been 

 named the nucleus. It has been shown not to differ in any 

 essential particular except in density from ordinary proto- 

 plasm. Its function is not certainly known. 



11. Cells in plants are of various sizes and shapes. The 

 largest (with a few exceptions) are scarcely visible to the 

 naked eye, while the smallest tax the highest powers of the 

 best microscopes. Cells which exist by themselves, as in 

 many microscopic water-plants, are more or less spherical; 

 so, too, are many spores and pollen-cells, and the cells of 

 many ripe fruits where, in the process of ripening, the cells 

 have separated from each other. Ordinarily, however, the 

 cells are of irregular shapes, on account of their mutual 

 pressure. Occasionally they are cubical, rarely they are 

 regular twelve-sided figures (dodecahedra), but more com- 

 monly they are irregular polyhedra. 



12. In a few. plants, as the Slime-Moulds, the protoplasm 

 has no definite size or shape; it may be of microscopic size, 

 or it may form irregular masses as large as one's hand. 

 Such plants are not composed of cells. They are nothing 

 more than masses of shapeless protoplasm, and are among 

 the lowest of all living organisms. In all other cases, how- 

 ever, the cell is the unit out of which the plant is composed, 

 and in the study of different plants, no matter how much 



