XIII. 



CLIMATE 



The plant's whole life depends, as we have seen, upon 

 the sun, for without sunlight it cannot obtain carbon 

 from the air and must starve. And this is equally true 

 whether the plant manufactures its own food or 

 whether it feeds upon what has been manufactured by 

 others. The parasite sucks the juices of plants growing 

 in the light ; the fungus feeds upon dead vegetable 

 matter which has grown in the light; the seedling 

 lives at first upon food stored in the seed, by the leaves 

 which waved in the sunlight, and so on. 



But plants require of the sun something more than 

 light ; they must have some degree of heat as well — a 

 very small degree in some cases, but this small degree 

 is essential to bring them to maturity. Even those 

 lowly plants which grow in snow and ice cannot dis- 

 pense with some amount of heat, and though they 

 contrive to exist in the lowest temperatures, they 

 remain dormant during the winter, and only wake up 

 when the summer sun begins to shine. 



Many seeds will even begin to grow while it is 

 freezing, though they cannot make much progress; 

 and wheat has been known to germinate when actually 



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