542 STRUCTURE AND LIFE HISTORIES 



descent of one-celled organisms, such as Pleurococcus. 

 This plant, as we learned in Chapter XVIII, is a globule of 

 protoplasm, containing chlorophyll, and surrounded by a 

 cellulose cell-wall. 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 instead 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 possessed the characteristics which now char- 

 acterize Pleurococcus plants. But that only puts the 

 question back an indefinite number of generations. The 

 real reason is, because the Pleurococcus protoplasm pos- 

 sesses a physical and chemical constitution — or in other 

 words a mechanism — that, under normal external condi- 

 tions, manufactures green pigment instead of red, cellulose 

 instead 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. 



463. What is Inheritance? — When the Pleurococcus cell 

 divides, this wonderful, invisible mechanism — the certain 

 definite physical and chemical constitution — is transmitted 

 to each of the daughter-cells; each, in other words, re- 

 ceives Pleurococcus protoplasm. This protoplasm, with 

 its definite organization, constitutes the inheritance. The 

 daughter-cells do not inherit a spherical shape (as is evident 

 from Fig. 183), but a definite kind of protoplasm, cell-sap 

 of certain osmotic properties, and surface cellulose of even 

 elasticity, so that, in surroundings uniform on all sides, 

 a spherical shape must finally result. The shape is an 



