82 PLANTS 



Temperature 



196. There are certain parts of the earth's surface which are 

 always destitute of vegetation because of the fact that the 



FIG. 40. A small fern, Polypodium, which grows as an epiphyte on the bark of 

 trees. See next figure. XL 



surface waters are always frozen, a condition which renders 

 vegetable life impossible. On the other hand, in the hottest 



