If nectar is the food reward for insect visitors this must be produced in
sufficient quantities and of suitable concentration to attract insect pollinators,
and one plant species may have to compete with coexisting species for the atten-
tion of visiting insects. More insect visits will generally lead to greater pollen
dispersal but not necessarily greater seed set (see below).The success of a plant as a female parent is easier to measure than its success as
a pollen donor, since the plant forms its seeds on the plant. Plants differ greatly
in the size and number of seeds and fruits produced and how many pollinations
they require for maximal seed formation. Plants with many seeds per fruit may
require few insect visits for full seed set if they are effective pollinators. The
most extreme example is the orchids which produce pollen in sticky pollinia
(Topic D1) and have thousands of microscopic seeds per fruit. They require just
one successful insect visit per fruit so need attract much smaller numbers of
pollinators than other plants. In contrast, members of several families, such as
the daisy family and grasses, have one-seeded fruits, so each flower or floret
must be pollinated separately. The amount of resources required from the
parent plant is highly variable. Small fruited species such as grasses and orchids
require few resources compared with those with large fruits such as coconuts
(here mainly the seed) or water melons that set only a few fruits, each with a
large amount of resources. On some plants with large fruits, such as many
fleshy-fruited trees, only a small proportion of flowers can mature into fruits
because of shortage of space.
In most flowers, seed set is limited by a combination of the amount of pollina-
tion and resources available for the developing seeds. In large-fruited plants, the
resources from the plant limit fruit set and it is well known that many plants
have a poor fruiting year after a good one, suggesting that there are insufficient
resources for two consecutive years of high yield. Pollination may also limit
fruit set, and in many studies in which the amount of pollination has been
increased experimentally, more fruit has been set. This has often led to poorer
growth or less fruit set the following year even in quite small-fruited plants, so
clearly these two factors interact.
In dioecious plants (Topic H2) female flowers are frequently produced in
smaller numbers than male flowers and there is also a smaller floral display
suggesting that limitation of resources is important.For pollen to spread to its maximum, many pollinator visits or effective spread
of the pollen by wind is required. Few resources are needed for the pollen
grains compared with fruit and seed set, and attraction of numerous insects to a
flower will mainly enhance pollen dissemination. This suggests that, in many
animal-pollinated species, much of the floral display is in relation to a flower’s
male function. In wind-pollinated flowers, wide dissemination of pollen is
assured by the plants producing an enormous quantity on fine days, as hay-
fever sufferers know only too well.
In animal-pollinated flowers the quantity and nature of the pollen depends
on the animal. The furry bodies of many bees, moths and bats, as well as the
feathers of birds can carry much more pollen than the smooth bodies of flies,
some bees or the thin proboscis of a butterfly. They are likely to be more
efficient as pollinators as a result, but their efficiency will also depend on their
behavior. For instance, bees forage systematically, visiting adjacent flowers,
often on the same plant and sometimes crawling, with occasional long flights.The
dissemination of
pollen
Seed and fruit
set
184 Section L – Reproductive ecology