CHAPTER 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 para- 

 site 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 dispense 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 upon ice, and to 

 send out roots into it. Barley and oats will also send out 

 roots with the thermometer down to freezing point, but 

 they cannot develop seed-leaves without a few degrees of 



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