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 
