KEEPING WARM 
environments and the creatures in them. 
During daylight hours, about one calorie 
of energy per square centimeter per 
minute reaches an exposed leaf at the 
earth’s surface, most of it as short-wave- 
length 1R from the sun. That amount of 
energy is sufficient to heat objects to a 
high temperature in a short time. If you 
have ever touched a piece of metal that 
has been in the sun or sat down on a car 
seat that has been illuminated for a 
while, you are aware of just how much 
heat can be accumulated. 
Leaves and petals are not cooked on 
the stem because of surface and ana- 
tomical properties developed long ago 
by the earliest land plants. Ninety per- 
cent or more of the short-wavelength IR 
striking the surfaces of leaves and petals 
is reflected, and if water is available and 
the microscopic stomatal pores of the 
leaf are open, nearly all the heat a leaf 
does absorb is carried away by evapora- 
tion, much as a sweating body is cooled 
by water loss. When evaporation is 
limited or reduced, a leaf in sunlight can 
rapidly heat to 1 8 to 36 degrees Fahren- 
heit above the air around it. According 
to Gates, leaves would rapidly heat to 
fatal temperatures if they did not lose 
heat by evaporation. 
In warm climates or habitats with 
abundant sunlight the problem faced by 
land plants was excess heat, but as 
plants moved into cooler climates, ad- 
vantages would have accrued to those 
plants able to warm critical parts with 
heat-giving IR rays from the sun and to 
direct this heat to the benefit of repro- 
ductive processes. Thinking about my 
student’s question and about Gates’s 
work, I wondered what adaptations for 
the accumulation of heat might have 
evolved in arctic, alpine, or cool-season 
temperate plants. 
The earliest plants to bloom in north- 
east Iowa are the snow trilliums ( Tril- 
lium nivale), which follow the sun well 
enough to make a brilliant display in 
late afternoon when the viewer’s back is 
to the sun. Although not particularly 
circular or dish shaped, the snow tril- 
lium does have a deep center and three 
spreading, bright white petals pointed 
