Seeds and Seedlings. 17 



the year. But the dry seeds of these plants can withstand 

 great extremes of heat and cold, and do not need water to 

 keep them alive ; indeed, the ability of seeds to survive 

 adverse seasons is due in large measure to the small amount 

 of water which they contain. Since seeds as a rule retain 

 their vitality for several years, in some cases, indeed, for 

 twenty-five or even fifty years, they can, if necessary, tide 

 a species over one or more years which are unfavorable to 

 growth. 



6. Multiplication by Seeds. A single plant of Indian 

 corn produces on the average about 130 grains of corn, 

 which, under the favorable conditions resulting from culti- 

 vation, might in the succeeding season give rise to 16,900 

 grains. An example of this kind will serve to demon- 

 strate the immense capacity of multiplication by means 

 of seeds ; although under natural conditions only a small 

 portion of the seeds produced ever result in mature 

 plants. 



7. Migration by Seeds. Since land plants must draw 

 their water and some other raw food materials from the 

 soil, it is of great advantage, and even necessary, for them 

 to be fixed in the soil by means of their roots. While the 

 individual is thus anchored, the species is still able to move 

 from place to place by means of the seeds. In this way, 

 species have migrated through the long geological periods 

 from regions which were becoming unhabitable to others 

 which were more favorable ; and by this means the borders 

 of continents which are rising from the oceans, and newly 

 formed volcanic or coral islands, become colonized by 

 plants from greater or less distances. It is a matter of 

 common observation that tracts of land which have been 

 protected from grazing animals become inhabited in the 

 course of a few years by plants which were never seen 



