92 THE GEOGRAPHY OF PLANTS. 



indeed, including many more species than those 

 we have briefly noticed, is one of the most 

 marked peculiarities of a tropical scene. 



The Canary Islands are celebrated for a very 

 remarkable tree, the dragon's blood, (Draccena 

 draco.) One of these trees, at the Villa Ora- 

 tava, in Teneriffe, has been stated to be between 

 seventy and seventy-five feet high, and forty-six 

 and a half feet in circumference at the base ; it 

 was a very ancient tree in the year 1402, and 

 is still alive and productive. It yields a deep 

 red resin, called dragon's blood ; but the sub- 

 stance sold in our shops under that name 

 is the produce of some palms, of the genus 

 Calamus. 



In the north of Africa, we find the great 

 Desert of Sahara, which occupies a large por- 

 tion of the sub-tropical part of that continent. 

 Of course, on "the desert itself we do not expect 

 to discover any trace of vegetation ; except that 

 here and there, where a spring arises, a spot of 

 verdure is seen, called an oasis ; here palms, 

 acacias, and other prickly plants, are found. 

 On the borders of the desert, at the base of the 

 Atlas range, the date palm forms large forests, 

 nourished by the moisture which descends from 

 the mountains, and affords food to the inha- 

 bitants, and shelter to crops which could not 

 otherwise grow ; for the drought on the borders 

 of the desert is so excessive, that no other trees 

 can resist it, rain hardly ever falls, and the 

 scorching blasts from the south speedily dry up 

 any moisture that may exist. The desert has 



