36 - The Cell 



these reactions require a complex structural 

 organization even in the single cell. 



The unicellular condition has limitations, 

 however, particularly as regards the maxi- 

 mum size to which the organism can grow 

 (p. 17). Larger organisms — up to a limit, 

 which is quite large — possess many ohvious 

 biological advantages. Large organisms are 

 less vulnerable to small environmental forces 

 and they are less at the mercy of smaller 

 enemies. Bacteria, because of their smallness, 

 are continually buffeted from side to side by 

 the random bombardment of the molecules 

 of (he water in which they live; but the 

 greater bulk of the larger aquatic forms 

 makes them immune to such small forces. 



When a small insect, or other animal, falls 

 upon still water, it may be caught and torn 

 by surface tension; but larger animals may 

 clamber in and out of water without any fear 

 of such a puny force. A salmon breasts the 

 strongest stream, impossible for even the fast- 

 est swimming protozoan; while an elephant 

 may easily uproot a tree that previously was 

 the home of many smaller creatures. 



The advantages of greater size were gained 

 by plants and animals mainly through the 

 evolution of cell aggregates. In simplest form 

 such aggregates are mere colonies, with the 

 individual cells independent of each other. 

 But colonial forms were followed in evolu- 

 tion by multicellular organisms, in which 



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Fig. 2-22. Rare phenomenon, found only in a few slime molds. In this case a multi- 

 cellular "organism" originates by the coming together of many separate cells. Most of 

 the time this mold (Dicfyosfe/ium) takes the form of a multitude of separate amoebalike 

 cells that move and multiply in the soil, among the rotting leaves. Sometimes, however, 

 the cells all come together (stage I) forming a multicellular mass that displays integrated 

 locomotion while it migrates for several hours (stages l-lll). Then all the cells cooperate 

 in building a complex structure with a spore capsule at the top (stages IV-VI). Finally 

 a multitude of unicellular spores are discharged from the capsule. Each spore gives rise 

 to another individual "amoeba." (Redrawn after J. T. Bonner, Princeton University.) 



