Pollination is by wind, with the female catkins lengthening considerably between pollination and maturity. Flies may have been among the evolutionarily earliest pollinators responsible for early plant pollination. Many plants are dependent on pollination from a pollinator. Pollinators may be excluded through the use of pollination bags. The females depend upon pollen from hermaphrodites for pollination. Some pollen grains will land on a female cone for pollination. Some species are important in the pollination of crops. Many rainforest plants depend on bat pollination. This, of course, results in fewer honeybees for pollination. This promotes flower pollination and outcrossing. Pollination and fruit formation depend on meiosis. The impact of pollination varies by crop. However, alkaloids in the nectar function as a deterrent for species unsuited to pollination. Two compatible varieties are needed for cross pollination and fruit set. This is beneficial to them in terms of pollination and feeding.