42 HEREDITY AND EVOLUTION IN PLANTS 



environment may result in different expressions of in- 

 heritance, just as variations in inheritance would be 

 followed by differences in expression, even in an unchang- 

 ing 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 environment only, but of the interaction 

 between the two. 



33. Struggle for Existence. — In paragraph 7 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 

 the result if all the seeds produced by one of our large 

 forest trees were able to mature. And yet the total 

 number of any given kind of fern, of hedge mustard, or 

 of forest tree does not appreciably change from year to 

 year. The reason, of course, is that not all of the spores 

 and seeds produced are allowed to come to maturity. 



