INTRODUCTION 



pig in a litter very often grows slower than any of the 

 others, i.e., is more feeble and less vigorous than any of 

 the others. Feebleness is the opposite of vigor. The most 

 vigorous plant or animal usually 

 attains the largest size, and as 

 a rule is most satisfactory to its 

 owner. Vigor is promoted by a 

 favorable environment. It is 

 usually greatest in rather young 

 plants and animals, and declines 

 with advancing age. It may 

 be reduced by disease or im- 

 proper treatment, and when thus 

 reduced is often difficult to re- 

 store. Reduced vigor tends 

 to early maturity and shortened 

 life, and sometimes to increased prolificacy. 



10. Hardiness and tenderness are terms used to express 

 the relative power possessed by different plants or animals 

 to endure extremes in their environment. The Oldenburg 

 apple endures with little harm vicissitudes of temperature 



FIG. 1. Four individual 

 plants of a species of Pro- 

 tococcus. A shows a plant 

 before commencing to di- 

 vide into other plants. B, 

 C and D show how the 

 cells divide to form other 

 plants. Highly magnified. 



FIG. 2. Part of a filament of a species of Spirogyra, a plant consisting 

 of a single row of cells united at their ends. The places where the cells 

 join are indicated by the vertical lines. Highly magnified. 



that are fatal to many other varieties ; in other words, it 

 is hardier as regards temperature than many other varieties. 

 The reindeer is hardier as regards cold than the horse, 

 but tenderer as regards heat. The melon plant is hardier 

 as regards heat and drought than the lettuce, but tenderer 

 as regards wet or cold. 



