1899] VEGETATION OF TROPICAL AMERICA Lp 
which in my opinion are of prime importance for the physi- 
ognomy of vegetation. : 
As to the “ight, I shall’remind you of the fact, that only in the 
light the plant becomes green, and only by aid of it and of 
green coloring-matter—the chlorophyll—can organic matter be 
formed. Without light and chlorophyll no organic matter would 
come into existence. Thus in the household of nature the 
importance of light and chlorophyll is enormous. But on the 
other hand chlorophyll may be destroyed by light, and the 
destructive action may be greater than the constructive. Cer- 
tainly for every species of plant an optimum light exists ; if 
this be exceeded, many, perhaps all, green plants are in pos- 
session of one or another means by which they protect them- 
selves against too intense light. 
Closely connected with the light is transpiration, which is 
dependent not only on the heat and moisture of the air, together 
with several other factors, among them those internal to the 
plant itself, but even upon the intensity of light, since the light 
is converted into heat. By a too profuse transpiration the plant 
wilts. Against this the plant must have means of defense, and 
these are partly the same as those that protect against too intense 
light. 
As before mentioned, the water in the soil is of prime impor- 
tance for building up the plant and for the production of dif- 
ferent types of vegetation, and for their distribution in the 
landscape. 
Taking into consideration these facts, besides the high 
degrees of heat and the intense light of the tropics, we may 
easily understand the differences of the tropical plants from 
those of our northern climates. Against too intense light and 
too profuse transpiration the tropical plants have to struggle, 
and that struggle imprints its stampupon them. As in physical 
respects there are only differences of degree, not of kind, 
between the two regions we are comparing, it is but natural that 
the plants in their structure also show only differences of degree. 
The same characteristic structures we find in the tropical vege- 
