8 EVOLUTION OF PLANTS 



we are still far from understanding the ultimate struct- 

 ure of the cell, although many ingenious hypotheses 

 have been formed to explain the structure of the proto- 

 plasm. 



We have seen that cells multiply by division. In 

 the lowest organisms the cells thus produced usually 

 separate at once, resulting in the formation of two or 

 more individuals exactly like the parent. In case this 

 division is repeated at short intervals, as happens, for 

 example, in the Bacteria and many infusorians, the 

 result is the production of an enormous number of in- 

 dividuals in a surprisingly short time. In all but the 

 lower forms of life the cells do not usually separate 

 after division, the result being a multicellular organism. 

 The cell-aggregates, of which these higher plants and 

 animals are composed, are known as tissues, and these 

 may be combined to form special organs. The cells of 

 growing parts of the higher plants resemble the simple 

 unicellular forms in structure, but as they grow older 

 they may become extremely modified to fit them for 

 special functions. 



If we examine one of the lower vegetable forms, such 

 as a desmid (Fig. 1, B), we find that the single cell of 

 which the plant is composed is at once vegetative and 

 reproductive. Such a green cell is capable of perform- 

 ing all the life-functions. It can absorb water con- 

 taining certain food elements in solution, including the 

 oxygen necessary for respiration, and, by virtue of the 

 chromatophore containing chlorophyll, is able in the pres- 

 ence of light to decompose carbon dioxide and water, 

 and from the oxygen, hydrogen, and carbon so separated, 

 to manufacture the primitive carbo-hydrates necessary 



