46 



HEEEDITY 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. 

 Thi^ 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 eUiptical, 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 Hgnin or chitin? 



The short answer is, because its 

 ancestors, for ages and ages, have 

 Fig. 33. -Individual gggd tj^e characteristics which 



plants of green slime '^ 



[Pleurococcus vulgaris) now characterize Pleurococcus 

 showing the tendency of plants. But that Only puts the 



the cells to remain ^iQj^|3g^j,]^g^j^jj^^lg£^jjgj^yjj^ber 



attached after cell-divi- . . _, , 



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

 tions from a one-celled to because the P/eMrocc»ccM5 protoplasm 

 a multi-cellular plant, possesses a physical and chemical 

 constitution — or in other words a 

 under normal external 



(C/. Fig. 34.) 



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 



