Number of ;owers
Number of mature fruits
plant was able to choose,” Lankinen says.
She, like many other biologists, instead
starts with the basic attitude that choice
doesn’t have to be conscious; it’s not
clear whether even Homo sapiens truly
thinks when undressing.
In many species, a female organ can
reject pollen from other blooms on the
same plant. Among those species are the
Flanders poppy, flowering tobacco and
field mustard, as well as the Oxford ragwort analyzed by Alexandra Allen of the
University of Bristol in England and her
colleagues in the September 2011 Annals
of Botany. The female reproductive tissues in a bloom can recognize genetic
factors signaling that a pollen grain is
too closely related. And depending on
the species, those factors may block or
Rejecting pollen isn’t the only way a
plant could wield its influence, though.
Female parts may be able to affect mate
number and diversity too. Typically the
female part of a flower does not need
just one pollen grain, but a dusting of
grains to sire the multiple seeds in her
seedpod. That pollen may originate from
one plant or many. In the sometimes
alien green zone of plant sex, mating is
not about finding Mr. Right Pollen Grain,
but Mr. and Mr. and Mr. Right.
In the flowers that Lankinen studies,
Chinese houses, the female pistil takes
the usual form of a receptive surface,
the stigma, atop a stalk that leads down
to carefully wrapped and protected
ovules at the pistil base. Pollen lands on
the stigma and then sprouts tubes that
grow downward through the plant tissue
to reach the ovules. When a pollen tube
arrives at an ovule, fertilization occurs.
But Chinese houses delay the start
of the race, a timing twist proposed as a
form of female choice. Even though the
flower sits open and, it would appear,
ready to go, pollen that lands on the
stigma for the first day or so is just stuck
sitting there. The female structure keeps
elongating and bending until at last, the
pollen grains are off. By instituting the
delay, the stigma collects a variety of
pollen, increasing the diversity rushing toward the ovules. In plant studies,
Lankinen has found, a batch of pollen
from multiple sires starting pollen tubes
all at once led to more seeds from more
sires than did comparable pollen delivered over two days.
The finding joins other examples that
suggest that female plant parts may
manipulate the fertilization process. In a
classic study of wildflowers called anemones, the stigma’s surface begins turning
receptive at the outer edges and matures
in an inward wave. The final patch to
allow pollen to start tubing toward the
prize is the center, where pollen has the
shortest distance down to the ovules.
The pattern evens the odds for pollen
that landed near the outer edge.
Once pollen tubes have reached and
fertilized the ovules, some fertilized eggs
wither instead of growing into seeds.
Botanists have proposed that flowers may be selectively aborting certain
embryos — possibly because they come
from a loser sire.
Barrett cites two classic studies he
finds convincing: Small legumes called
bird’s-foot trefoil (Lotus corniculatus)
end up producing fewer seeds than the
number of ovules fertilized. In an effort
to explore whether such abortions hap-
pen randomly or might be a mechanism
for weeding out unpromising offspring,
Andrew G. Stephenson of Penn State
University and his colleagues removed
a random selection of flowers containing
fertilized ovules so the ovules couldn’t
develop. The fruits that formed on the
plants allowed to abort their own way
held more seeds with higher quality than
did the fruits on plants that the research-
ers plucked. And with yellow-flowered
desert perennials in the borage family
(Cryptantha flava), natural processes of
seed elimination produced better seeds
than random laboratory processes did,
Brenda Casper of the University of
Pennsylvania has found.
That bar fight
If one plant sex can do some choosing,
members of the other sex, whatever that
means in a plant, may indeed compete or
outright fight among themselves.
One of the most recent tests of the
idea, with Varis’ Scots pines, pitted
clones of trees from southern Finland
against clones from the northern region.
(This contest was part of a long-term
project on tree breeding and adaptation to climate change.) In the lab, Varis