VITALITY OF SEEDS. 161 



influence of moisture, oxygen and warmth the first seed has 

 been thus changing into a young willow, the other though 

 appearing unaltered to the eye has been rapidly losing its ca- 

 pacity for germination and if at the end of a fortnight it be 

 planted in the most favorable circumstances for growlh, it will 

 merely absorb water and then speedily decay. All that the 

 wisest botanist can tell us in explanation of these phenomena 

 is that the first seed was alive and that the other was dead, 

 though no violence had been done to it and no appreciable 

 chemical change had afiected it. Why one seed dies in twelve 

 days and another retains its vitality unimpaired for many 

 years, are equally mysteries yet to be solved. 



The young willow under kindly influences will increase in 

 size and weight daily until the chilling winds of autumn 

 breathe upon it. Then the leaves fall, growth ceases and the 

 plant enters upon a period of repose similar in many respects 

 to the hibernation of animals. This annual cessation of the 

 vital action in plants appears to be essential to the health of 

 most species, though a few, like the orange and lemon, do not 

 require it. In cold climates, the absence of heat in winter, 

 and in warm latitudes, the want of v^ater during the periodical 

 dry season, are the principal causes which operate upon vege- 

 tation to enforce this law of nature. 



The distinction between deciduous and evergreen species 

 is, that the former lose their foliaije at the end of the ijrowinoj 

 season, while the latter retain each perfected leaf one, two or 

 three years. Nevertheless, evergreens usually have their time 

 of rest no less than deciduous plants, and those which are. 

 deciduous in one climate may become evergreens in another. 

 Thus the apple and the plane-tree have become evergreens in 

 Madeira. 



Professor Hofmann made a series of experiments from 

 1863 to 1870 to determine whether this annual period of rest 

 was really necessary for ordinary plants. He found that when 

 the lilac and other similar species were forced under glass to 

 grow continuously by the constant presence of heat, light, 

 moisture and proper soil, they ceased to blossom after the 

 first year, and died on the second or third. Hence the im- 

 portance so well known to skilful gardeners of giving alter- 

 nations of heat and cold, moisture and drought to plants 



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