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 



