ZONE OF GRASSES AND LICHENE. 
1]9 
sea. In the Cordilleras of IN ew Spain, under the torrid zone, 
the Mexican pines extend to the height of two thousand 
toises. Notwithstanding the similarity of structure existing 
between the different species of the same genus of plants, 
each of them requires a certain degree of temperature and 
rarity in the ambient air to attain its due growth. If in 
temperate climates, and wherever snow falls, the uniform 
heat of the soil he somewhat above the mean heat of the 
atmosphere, it is probable that at the height of Portillo the 
roots of the pines draw their nourishment from a soil, in 
which, at a certain depth, the thermometer rises at most to 
nine or ten degrees. 
The fourth and fifth zones, the regions of the retama and 
the gramina, occupy heights equal to the most inaccessible 
summits of the Pyrenees. It is the sterile part of the island 
where heaps of pumice-stone, obsidian, and broken lava, form 
impediments to vegetation. We have already spoken of 
those flowery tufts of alpine broom (Spartium nubigermm), 
which form oases amidst a vast desert of ashes. Two her- 
baceous plants, the Scrophularia glabrata and the Viola cliei- 
ranthifolia, advance even to the Malpays. Above a turf 
scorched by the heat of an African sun, an arid soil is over- 
spread by the Cladonia paschalis. Towards the summit of 
(he Peak the TJreeolarea and other plants of the family of 
the lichens, help to work the decomposition of the scorified 
matter. By this unceasing action of organic force the empire 
of Flora is extended over islands ravaged by volcanoes. 
f , On surveying the different zones of the vegetation o> 
Tenerife, we perceive that the whole island may he consi- 
dered as a forest of laurels, arbutus, and pines, containing 
in its centre a naked and rocky soil, unfit either for pastur- 
age or cultivation. M. Broussonnet observes, that the archi- 
pelago of the Canaries may he divided into two groups of 
islands ; the first comprising Lancerota and Forteventura, 
t he second Teneriffe, Canary Goniera, Ferro, and Palma, 
the appearance of the vegetation essentially differs in these 
" 0 groups. The eastern islands, Lancerota and Forteven- 
tura, consist of extensive plains and mountains of little 
<ie\ ation ; they have very few springs, and bear the appear- 
an ce, still more than the other islands, of having been sepa- 
rated from the continent. The winds blow in the same 
direction, and at the same periods : the Euphorbia maun- 
