KEEPING WARM 
them. Most of the insects seemed con- 
tent to bask in the warmth of the sun as 
magnified by the collection plates of the 
petals and temporarily stored in the 
fuzzy center structures. If you are small 
enough, I thought. Palm Beach is as 
close as your nearest pasqueflower. 
These observations were sufficient 
stimulus to push me into searching out 
the published research on flower tem- 
peratures, which while most interesting, 
was very limited. In 1965, Brian Hock- 
ing and C D. Sharplin of the University 
of Alberta, Canada, had reported on the 
basking behavior of insects in dryas 
(Dry as integrifolia ) and arctic poppy 
(Papaver radicatum) flowers on Elles- 
mere Island in the Arctic Archipelago. 
They had measured temperatures of 1 8 
degrees or more above air temperature 
in the centers of the poppies and sug- 
gested that both insect and plant germ- 
cell development would be accelerated 
by this solar energy bonus in a climate 
where small energy advantages might 
be critical for survival. Following up on 
Hocking and Sharplin’s observations ten 
years later, Peter Kevan, now of the 
University of Colorado at Colorado 
Springs, carefully quantified aspects of 
flower shape and energy accumulation. 
He found that both spherical and para- 
bolic shapes would focus heat and light 
directly on the reproductive parts of the 
flower. Arctic poppies, Kevan calcu- 
lated, increase the degree-days available 
for growth by as much as 25 percent 
because of their passive solar heating. 
This means that during the six-week- 
long growing season in the High Arctic, 
poppies accumulate the equivalent of 
seven and a half weeks of heat. To test 
the hypothesis that insects that stayed in 
those flowers longer than strictly neces- 
sary for feeding would reap similar 
benefits, he measured the heat that in- 
sects accumulated by inserting tiny 
thermocouples in their thorax. On dryas 
flowers, insects developed temperatures 
as much as 32 degrees in excess of air 
temperature. 
If insects can accumulate energy 
from the heat available in flowers, they 
may seek out the warm flowers during 
colder periods. The plants could thus 
conserve energy by accomplishing polli- 
nation while providing less food. More- 
over, in cold climates, where heat is 
more abundant near the ground owing 
to reradiation of solar input and where 
selection favors small, low-blooming 
plants, the evolution of efficient para- 
bolic or spherical solar collectors may 
make the flowers easier for flying, heat- 
sensitive insects to locate. 
Whether selection in arctic and other 
cool-to-cold climates has produced col- 
lectors or whether those plants already 
bearing suitable flowers have been more 
successful in such climates is difficult to 
determine. The heating of flower cen- 
ters by radiant energy from the sun may 
merely be an incidental property of 
flowers that are bowl shaped for other 
reasons. If the heating is as important as 
Kevan suggests, however, then we might 
expect that selection would operate to 
produce flowers with deep centers and 
large, highly reflective petals that are 
retained for a long time. Petals often 
have unusual epidermal cells and cutin 
layers, but whether the reflectivity of 
arctic or alpine flowers is a result of 
these characteristics remains to be ex- 
amined. Meanwhile, as scientists study 
this little-understood aspect of floral 
evolution, many flowers of arctic, al- 
pine, and early-spring environments will 
continue to provide warming ovens that 
accelerate the development of pollen 
and seeds and probably also aid the 
survival and speed the reproduction of 
visiting insects. □ 
Left: Like many plants that flower 
in the early spring, these rue 
anemones begin to develop their 
fruit while temperatures are still 
cool. Right: On Ellesmere Island 
in the Arctic Ocean, arctic poppies 
bloom next to an ice-strewn fiord. 
Passive solar heating enables these 
High Arctic blossoms to accumulate 
the equivalent of seven and a half 
weeks of heat in a six-week-long 
growing season. 
Martha Cooper 
James N Skeen 
80 
