CHARACTERISTICS OF CELLS 7 
enings are on the inner surface of the cell wall, but in 
many spores (e.g. pollen grains or spores of ferns or fungi) 
they are external. This is also the case in some of 
the lower, one-celled plants such as desmids. The 
thickenings have various functions, such as strengthen- 
ing the wall, providing means for transportation (in the 
case of spores and pollen grains which sometimes depend 
upon animals for their dispersal, the rough projections 
enabling them to cling to the animal), etc. 
13. After attaining their full differentiation most of 
the cells of the higher plants (at least of the woody 
plants) die, their cell walls remaining to make up the 
bulk of the plant body. We usually continue to speak 
of such dead, empty cell walls as cells, although the 
essential parts, the cytoplasm and nucleus, may have 
disappeared long ago. 
- 14. Cells vary greatly in size, those of some of the 
bacteria being less than half a micron (i.e. less than one- 
fifty-thousandth of an inch) in diameter, while the egg 
cell of Zamia may have a thickness of over a millimeter 
and a length of 3 mm. (i.e. a volume over twenty billion 
times as great), the egg cell of Dioon being even larger. 
Some fiber cells have a length of many centimeters, e.g. 
bast fibers of ramie (Boehmeria nivea). 
15. In some of the lower aquatic plants occur repro- 
ductive cells with no cell walls (e.g. zoospores, tetra- 
spores, etc.). These cells are frequently motile by means 
of protoplasmic processes called cilia or flagella. Such 
cells in many cases settle down and, becoming attached 
to something, form a cell wall before proceeding further 
in their development. Even in the higher plants the egg 
and sperm cells are naked. 
16. Typical cells have but a single nucleus. In certain 
stages of the life history of some groups of plants the 
