1 90 STRUCTURE AND LIFE HISTORIES 
its roots deeper, if light is entirely cut off no chlorophyll 
will form. A leaf, or the prothallus of ferns, is bilaterally 
symmetrical because the environment is uniform on all 
sides; the same organs have dorso- ventral differentiation 
largely because the environment is unlike above and be- 
low. The motility of sperms is an adjustment to water in 
the environment. Thus variations in the environment 
may result in different expressions of inheritance, just as 
variations in inheritance would be followed by differences 
in expression, even in an unchanging environment. In 
order correctly to understand a plant nothing is more 
necessary than to remember that its characteristics are 
the result, not of its inheritance alone, nor of its environ- 
ment only, but of the interaction between the two. 
173. Struggle for Existence. In Chapter XII atten- 
tion was called to the fact that a moderate-sized fern pro- 
duces each year about 50,000,000 spores. If each one of 
these spores ultimately produced a mature fern-plant, and 
if we allowed only i square foot of "elbow room" for each 
plant, the progeny of one parent only, in one season would 
require at least 50,000,000 square feet, or nearly i% 
square miles. If each of these plants in turn, produced 
50,000,000 offspring the next season, the descendants of 
only one fern plant would, in only two years, cover the 
stupendous area of over 83,000,000 square miles, or an 
area equal to that of the North American Continent. 
It has been calculated that a single plant of hedge mustard 
may produce as many as 730,000 seeds. If each seed 
developed another full-grown plant, and if the plants were 
distributed 73 to each square meter, there would be enough 
mustard plants to cover an area equal to 2,000 times 
the dry surface of the earth. One may easily imagine 
