Protoplasm, the Cell, and the Organism - 37 



the many cells are of different kinds, each kind of cell, taken as a group and including 



being fitted for some special function. Ac- any intercellular matrix that may be present, 



cordingly, the different cells of the multi- constitutes a tissue (for example, bone and 



cellular organism are dependent upon each muscle, in animals; xylem and epidermis, in 



other to a very significant degree. 



Except in rare cases (Figs. 2-22 and 2-23) 

 multicellular organisms arise, not by the com- 

 ing together of previously dissociated cells, 

 but by the staying together of cells derived 

 by division from a single parent cell. In fact, 

 virtually all multicellular plants and animals 



plants). The tissues in turn compose the 

 organs (such as the stomach of an animal, or 

 the leaf of a plant), each organ being fabri- 

 cated of several different tissues. Finally, the 

 organs themselves are grouped into organ 

 systems (such as the digestive system, or the 

 nervous system), each system subserving some 



return to the unicellular condition each time general function in the nutrition, responsive- 



a new individual is conceived. Among plants ness, or reproduction of the organism as a 



and animals, generally all the cells of each whole. 



offspring are derived by division from a fer- In higher organisms, the distribution of 



tilized egg cell. substances is effected by body fluids (such as 



In multicellular organisms each different blood, lymph, or sap) that flow throughout 



0M- If 



§ 7 ! 



** *■ - e ■ m. 

 ** / M Mj 



Fig. 2-23. Photographs showing the separate cells of the slime mold coming together to form a multicellular 

 amoeboid mass (such as is shown in Fig. 2-22, stage I). Left, aggregation commencing; right, multicellular mass 

 building up. (Courtesy of J. T. Bonner, Princeton University.) 



