v 

} 
. 
. 

PLANT GEOGRAPHY 447 
to lose water in the form of rain, it retains less and less mois- 
ture. The large rainfall at low and medium altitudes on moun- 
tains frequently results in a more luxuriant vegetation in these 
situations than in the neighboring and drier lowlands. The 
greater luxuriance of the vegetation at low and medium alti- 
tudes on mountains is particularly striking when the mountains 

Fic. 502. Two-storied forest on Mount Maquiling, Philippine Islands, at 
an elevation of seven hundred and forty meters 
rise from a dry or desert region. This is seen in Arizona, where 
the country around the mountains, and even the lower slopes 
of the mountains themselves, produce only desert vegetation, 
while at higher altitudes there occur rather luxuriant coniferous 
forests. 
On mountains the height of the forests decreases with rising 
elevation. At low and medium altitudes a greater luxuriance 
sometimes results from increased rainfall, but with this excep- 
tion the height of the forests decreases until, frequently, the 
