PROTOPLASM 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; 
80, 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 
