CHAPTER III 



ORGANIZATIONAL UNITS OF PLANTS 

 AND ANIMALS 



Over the structure of the chemical molecule rises the struc- 

 ture of the living substance as a broader and higher kind of 

 organization. Over the structure of the cell rises again the 

 structure of plants and animals, which exhibit the yet more 

 complicated, elaborate combinations of millions and billions 

 of cells coordinated and differentiated in the most extremely 

 different ways. Hertwig. 



SINCE living matter is only known to us in the form of 

 individual animals and plants, individuals are the only 



FIG. 4. Cells, highly magnified, from the surface of the Frog's skin (A), 

 and a plant leaf (B). 



realities in living nature, and we turn now to a consider- 

 ation of the organization of the individual. 



A thin slice of material from the surface of the skin of a Frog 

 or the leaf of a Buttercup shows under the microscope the 

 same general structure. Each appears to be a mosaic of in- 

 numerable small bodies, no two of which are exactly alike even 

 in the same piece, though all are similar enough to be one and 



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