4 6 



HEREDITY AND EVOLUTION IN PLANTS 



37. Heredity Reduced to Its Lowest Terms. We may 



study heredity under the very simplest conditions in the 

 descent of one-celled organisms, such as Pleurococcus. 

 This plant, a unicellular green alga, is a globule of proto- 

 plasm, containing chlorophyll, and surrounded by a 

 cellulose cell- wall (Fig. 33). But; why is it globular, why 

 does it contain chlorophyll, why has 

 it a cell-wall of cellulose? Why is 

 it not elliptical, why is it not red in- 

 stead of green, why does it have a 

 cell-wall, instead of existing naked 

 like the plasmodium of a slime- 

 mold, why is its cell-wall of cellulose, 

 rather than of lignin or chitin? 



The short answer is, because its 

 ancestors, for ages and ages, have 

 . G * ? 3 '~ m ,. ua possessed the characteristics which 



plants of green slime r 



(Pleurococcus vulgar is) now characterize Pleurococcus 



showing the tendency of plants. But that only puts the 

 the cells to rema in question back an indefinite number 

 attached after cell-divi- 

 sion, thus causing transi- of generations. The real reason is, 

 tions from a one-celled to because the Pleurococcus protoplasm 

 a multi-cellular plant, possesses a physical and chemical 

 constitution or in other words a 



mechanism that, under normal external conditions, 

 manufactures green pigment instead of red, cellulose in- 

 stead of lignin, or any other substance, at the surface, 

 and makes the cell-wall of even resistance to the osmotic 

 pressure within, thus producing a sphere and not an ellip- 

 soid, or filament, or any other shape. 



38. What is Inheritance. When the Pleurococcus cell 

 divides, this wonderful, invisible mechanism the certain 



