8 Forms of Trees. [ZOE 
In the tropics the intense heat develops another tree form, the um- 
brella form. In this region the heat is always accompanied by moist- 
ure, and is thus never excessive or dangerous for trees which naturally 
seek the light. The moisture and heat combined produce a most 
vigorous and dense vegetation, the very opposite to what is found 
in the arid zones. The effort of the tree is therefore concentrated 
in its endeavor to reach the light and to push out from the dense 
shade nearer the ground. The most vigorous growing trees in this 
region send up straight and undivided trunks to a level with the top 
of the dense undergrowth, branch at this level and form immense 
umbrella-like crowns above less vigorous trees. This umbrella- 
form gives to the tropical landscape a distinct and characteristic ap- 
pearance. A tendency to assume such an umbrella-form can also 
be recognized among those trees of the temperate zone, which grow 
in moist places, such as river bottoms, cafions and other sheltered 
localities—trees in fact, which delight in moisture. But nowhere 
is the form so pronounced as in the tropics, where it is common 
with all large species of the denser forests. The uplands of the 
tropics, where the rainfall is less and where heat and drying winds 
are more powerful, and where accordingly the vegetation is less 
dense, the umbrella form is rare, or where it exists is caused by other 
agencies. : 
The origin of the tropical umbrella form is therefore not exactly 
identical with that of the umbrella form assumed by most pines in 
such districts as the Mediterranean or the gulf region of the United | 
States, and to a certain extent also by a few more northern pines. 
This umbrella form is caused by the falling off of the lower branches, 
which never possess the strength of the upper limbs. The um- 
brella form, however, greatly favors their struggle against wind and 
heat. 
In these drier places in the tropical districts the umbrella form 
gives place to the globular form, the conditions there being quite 
similar to what they are in the drier regions further north. Ob- 
serve, for instance, the form of the ceiba (Bombax Ceiba), which 
inhabits dryer localities in the Central American tropics. This tree 
is almost globular in shape, in order that its branches may give 
necessary shelter to the trunk and to keep away the reflected heat. 
An effort to change the form of this tree by pruning results fatally, — 
as the branches become sun scalded and a prey to borers which 
