at sea. As a general pattern, however, 
pollen grains become widely separated 
as the wind carries them along, their 
concentration falling by a factor of 30 
million in just 800 feet. This means that 
beyond several feet from their source, 
wind-transported pollen grains will be 
relatively sparse. 
If insects are the more dependable 
means of pollen transport, the issue 
then becomes one of enticing them to 
provide this valuable service. The early 
angiosperms apparently solved the 
problem by offering insect visitors a 
meal of pollen, a persuasive induce- 
ment since pollen is an extremely rich 
source of protein. The presentation of a 
pollen meal was not without problems, 
however. William Crepet of the Uni- 
versity of Connecticut has pointed out 
that ovules, if they were to receive the 
gametes of the insect-transported pol- 
len, had to be close to the insect visitors, 
and ovules are every bit as nutritious as 
pollen. They are not, however, nearly 
as abundant. Natural selection appar- 
ently favored plants in which ovules 
were afforded some protection from in- 
sect predation, ultimately resulting in 
the angiosperm ovary, a vessel that 
completely encloses the ovules. (The 
word angiosperm means “vessel seed”; 
gymnosperm means “naked seed.”) 
At this point it is possible to recon- 
struct an interesting (and universal) 
phenomenon. A problem, in this case 
the unreliability of the wind as a means 
of pollen transport, elicits a solution, 
for example, insect pollination. The so- 
lution produces its own set of problems, 
but unless the solution is no solution at 
all, these are less severe than the origi- 
nal problem. The secondary problems 
do, however, necessitate further adjust- 
ments in the system: in the evolving an- 
giosperms, ovules had to be protected. 
Theoretically, this sequence of prob- 
lems, solutions, and still more problems 
and solutions continues forever, but in 
reality, the adjustments probably soon 
become attenuated to the point of evo- 
lutionary insignificance. The important 
part of this phenomenon is that for 
many problems, there are several pos- 
sible solutions. Some of these merely 
solve the problem at hand, but others 
may provide capabilities not directly 
selected for. 
The Chinese incorporate an aware- 
ness of this duality into their language, 
using the single word ji to mean both 
“crisis” and “opportunity.” In the case 
of the evolving angiosperms, the unex- 
pected occurred in the third cycle of 
crisis and opportunity. Enclosing the 
ovules in protective structures did pro- 
tect them from insect predation, but it 
also isolated them from the pollen 
grains. The solution to this third-level 
problem lay in the evolution of pollen 
tubes that could grow from the pollen 
grains through the intervening protec- 
tive structures and into the ovules. This 
solution may have presented the angio- 
sperms with a great opportunity. Con- 
sider the fate of the pollen grains of 
gymnosperms. Carried by the wind, 
most are lost, regardless of their quality 
or genetic content. For those that do 
reach an ovule, the worst is over; devel- 
oping eggs lie just a few cells away. 
Even fairly weak genetic types are 
probably able to complete this final 
part of the journey. The challenge of 
this last stage is further reduced by the 
small number of pollen grains likely to 
reach any particular ovule, so competi- 
tion among pollen tubes should be 
modest. 
In contrast, all available evidence 
suggests that although many of the 
pollen grains of an insect-pollinated 
angiosperm are sacrificed to insect 
nourishment, enormous populations 
still reach their target, the angiosperm 
stigma. There they germinate and rap- 
idly produce pollen tubes, which must 
penetrate the great length of tissue sep- 
arating them from the ovules. In five to 
ten hours, for example, the pollen grain 
of Petunia produces a pollen tube 
whose length is about 900 times the di- 
ameter of the original pollen grain. This 
rate is comparable to a basketball-sized 
sphere producing a tube about one-fifth 
of a mile long. Since only a well-bal- 
anced and vigorous physiology can sus- 
tain such growth, this marathon of 
pollen-tube development presents a 
challenge to the metabolic system of the 
pollen. Furthermore, with insect polli- 
nation the number of pollen grains 
reaching the stigma probably greatly 
exceeds the number of ovules available 
for fertilization. This creates an intense 
competition among the pollen tubes, 
and only the fastest growing, only the 
most vigorous will pass their gametes 
on to the next generation. Therein lies 
the surprising opportunity for the an- 
giosperms. 
Pollen competition apparently pro- 
vides a mechanism whereby poor ge- 
netic types are eliminated and vigorous 
types preserved. This idea is not new 
but neither is it universally accepted. 
Some evolutionists view the extraordi- 
narily intense competition that occurs 
among pollen tubes as a real danger 
point in the life cycle of a flowering 
plant. In 1932, for example, the geneti- 
cist J.B.S. Haldane suggested that a 
gene that greatly accelerates rates of 
pollen-tube growth will spread in a 
population even if it has mildly deleteri- 
ous effects in other parts of the life cy- 
cle. “A higher plant,” in Haldane’s 
words, “is clearly at the mercy of its 
pollen grains.” Experimental evidence, 
however, supports a much more san- 
guine view. Several investigators, be- 
ginning with the Russian geneticist 
D.V. Ter-Avanesian in 1949, have 
found that in cotton, wheat, com, to- 
matoes, petunias, and carnations, ga- 
metes from rapidly growing pollen 
tubes give rise to vigorous plants. Thus, 
with apologies to Robert Frost, one 
might say that “good pollen tubes make 
good plants.” It therefore becomes sig- 
nificant that only the angiosperms have 
a highly effective method of selecting 
for rapidly growing pollen tubes. 
One of the most powerful aspects of 
pollen tube selection is that vast num- 
bers of individuals are involved, but the 
system has a qualitative advantage as 
well. Remember that within plant em- 
bryos, seedlings, and mature individ- 
uals, every cell (except the gametes) 
carries two sets of genes, one inherited 
from each parent. This diploid state has 
some advantages, chief among them be- 
ing the ability to store genetic variation, 
but it has disadvantages as well. Reces- 
sive genes, for example, are expressed 
only when they are carried in all (here 
both) sets of genes within a cell. Sup- 
pose that, by chance, a rare but highly 
beneficial recessive gene appears within 
a population. Because it is so rare, there 
is little chance that it will occur within 
both gene sets of a cell. The gene, with 
its beneficial characteristics, will re- 
main unexpressed. Within pollen, how- 
ever, each cell is haploid and thus 
contains only one set of genes. Conse- 
quently, any gene beneficial to the pol- 
len, even a very rare one, will be 
expressed within the pollen grain and 
favored by natural selection. This bene- 
ficial gene will increase within the pop- 
ulation and eventually reach levels at 
which both gene sets in some seedlings 
will carry it. In these respects, pollen 
grains are similar to bacteria, organ- 
isms that exhibit a truly astonishing 
ability to respond to selection. We have 
only to consider the ease with which 
bacteria have developed resistance to 
many antibiotics in order to realize how 
adaptable they are. Microorganisms 
can produce a new generation as quick- 
ly as larger organisms can produce a 
new cell. Thus they can invade any new 
34 
