Chapter 7 



LIVING PLANTS AND ANIMALS CONTRASTED 



Living plants and animals possess many characteristics which are com- 

 mon to both groups, but in many ways they are dissimilar or opposite. 

 While the distinctive features between higher plants and higher animals 

 are obvious, discrimination between them becomes quite difficult in some 

 of the lower forms. In fact, there are certain organisms which are 

 claimed by the botanists as being plants and by the zoologists as being 

 animals because of the presence of both plantlike and animal-like traits 

 in the same individual. Such individuals might be considered as plant- 

 animals and may be illustrated by such forms as Euglena, Volvox (Figs. 

 173, 174), etc. One might consider the Tree of Life as consisting of two 

 main trunks (one plant, the other animal) with numerous branches and 

 subdivisions to represent the various types in each kingdom. In this case 

 the roots are hidden from our view (origin in the past), and new 

 branches are being constantly formed while others are dying. The two 

 main trunks arise from a common ancestral trunk, which suggests the 

 close relationships which exist between the two groups, in spite of the 

 fact that certain individuals on either side may at times be quite dis- 

 similar. Since it is theorized that plants and animals may have had a 

 common ancestry, there is no single difference which absolutely sepa- 

 rates all plants from all animals. 



In spite of the fact that no absolute criteria can be established, what 

 bases, even though they may be somewhat unsatisfactory, can be used 

 to separate the two groups or to identify individuals in each group? 

 The following are stated briefly so that contrasts and comparisons can 

 be made more easily. 



CELLULAR STRUCTURE 



Outside the plasma memhrane of plant cells there usually is a semi- 

 rigid cell wall composed of cellulose which gives some rigidity and sup- 

 port but at the same time prevents excessive movement. Animal cells 

 also possess a plasma membrane but usually without a semirigid cell wall; 

 thus support is lacking but certain movements are permitted. Cellulose, 



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