1IKRKDITY AND INVOLUTION 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 



FIG. 33. Individual 

 plants of green slime P OSSessed the characteristics which 



(Pleurococcus vulgar is) now characterize Pleurococcus 



showing the tendency of plants. But that only puts the 



the cells to remain ques tion 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. posse sses a physical and chemical 



(Cf. Fig. 34.) 



constitution or m 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 



