Pollen, a powdery substance, which is produced by flowering plants and contains male reproductive cells, is usually carried from plant to plant by insects or birds, but some plants rely on the wind to carry their pollen. Wind pollination is often seen as being primitive and wasteful in costly pollen and yet it is surprisingly common, especially in higher latitudes. Wind is very good at moving pollen a long way; pollen can be blown for hundreds of kilometers, and only birds can get pollen anywhere near as far. The drawback is that wind is obviously unspecific as to where it takes the pollen. It is like trying to get a letter to a friend at the other end of the village by climbing onto the roof and throwing an armful of letters into the air and hoping that one will end up in the friend's garden. For the relatively few dominant tree species that make up temperate forests, where there are many individuals of the same species within pollen range, this is quite a safe gamble. If a number of people in the village were throwing letters off roofs, your friend would be bound to get one. By contrast, in the tropics, where each tree species has few, widely scattered individuals, the chance of wind blowing pollen to another individual is sufficiently slim that animals are a safer bet as transporters of pollen. Even tall trees in the tropics are usually not wind pollinated despite being in windy conditions. In a similar way, trees in temperate forests that are insect pollinated tend to grow as solitary, widely spread individuals.

Since wind-pollinated flowers have no need to attract insects or other animals, they have dispensed with bright petals, nectar, and scent. These are at best a waste and at worst an impediment to the transfer of pollen in the air. The result is insignificant-looking flowers and catkins (dense cylindrical clusters of small, petalless flowers).

Wind pollination does, of course, require a lot of pollen. Birch and hazel trees can produce 5.5 and 4 million grains per catkin, respectively.There are various adaptations to help as much of the pollen go as far as possible. Most deciduous wind-pollinated trees (which shed their leaves every fall) produce their pollen in the spring while the branches are bare of leaves to reduce the surrounding surfaces that “compete" with the stigmas (the part of the flower that receives the pollen) for pollen. Evergreen conifers, which do not shed their leaves, have less to gain from spring flowering, and, indeed, some flower in the autumn or winter.

Pollen produced higher in the top branches is likely to go farther: it is windier (and gustier) and the pollen can be blown farther before hitting the ground. Moreover, dangling catkins like hazel hold the pollen in until the wind is strong enough to bend them, ensuring that pollen is only shed into the air when the wind is blowing hard. Weather is also important. Pollen is shed primarily when the air is dry to prevent too much sticking to wet surfaces or being knocked out of the air by rain. Despite these adaptations, much of the pollen fails to leave the top branches, and only between 0.5 percent and 40 percent gets more than 100 meters away from the parent. But once this far, significant quantities can go a kilometer or more. Indeed, pollen can travel many thousands of kilometers at high altitudes. Since all this pollen is floating around in the air, it is no wonder that wind-pollinated trees are a major source of allergies.

Once the pollen has been snatched by the wind, the fate of the pollen is obviously up to the vagaries of the wind, but not everything is left to chance. Windborne pollen is dry, rounded, smooth, and generally smaller than that of insect-pollinated plants. But size is a two-edged sword. Small grains may be blown farther but they are also more prone to be whisked past the waiting stigma because smaller particles tend to stay trapped in the fast-moving air that flows around the stigma. But stigmas create turbulence, which slows the air speed around them and may help pollen stick to them.

花粉，一种由开花植物产生且含有雄性生殖细胞的粉状物质通常是由昆虫或鸟类在植物间传播的，但有些植物也靠风来携带花粉。风媒传粉常被视为是原始和低效率的方式，但这种传粉手段极其常见，特别是在高纬度地区。风可以把花粉带到很远的地方，远至几百公里，这点上，只有鸟类传播才能勉强达到这个距离。但风媒传播有个缺点，那就是传播的不定向性。这就好比爬上屋顶，把一大堆信扔到空中，希望其中一封能落在村子另一端的朋友家的花园里。如果是在只有相对较少的几种树种占主导地位的温带森林，花粉的传播范围内存有众多相同的树种，风媒传播就相对比较安全。这就像是村子里有许多人都把信从屋顶扔下来，你的朋友终归会收到一封一样。相比之下，在热带地区，每种树的数量很少，而且散布在各处，靠风把花粉从一棵吹到另一棵的几率太小，这时靠动物来传播花粉就会更保险。因此，即便是热带的高大树木也不会靠风来传播花粉，即使它们处于大风环境中。类似地，在温带森林中靠昆虫授粉的树木通常会长成孤立的大树。

由于风媒传粉的花不需要吸引昆虫或其它动物，它们无需鲜艳的花瓣，诱人的花蜜与花香。因为这些东西对于风媒传粉的花来说充其量只是多余的，搞不好还会变成花粉在空中传播的阻碍。所以这些花只有不起眼的外观和花序（密集的圆柱状、无花瓣小花团）。

当然，风媒传粉需要大量的花粉。桦树和榛子树的每个花序可分别能产生550万和400万粒花粉。要使得更多的花粉传得更远，植物们各有各的适应方式。大多数风媒传粉的落叶植物（每年秋天落叶的植物）在春天产出花粉，这时候树枝光秃秃的，花粉就有更大几率落到柱头（接收花粉的部分）上，落到树的表面上的部分就不会太多。常绿针叶树由于不会落叶，春天开花对它们来说并没有什么好处，因此，确实会有部分树在秋天或冬天开花。

越高的树枝上产生的花粉，就越有可能飘得越远，因为高处风更多（也更大），花粉在落地前也就可以吹得更远。此外，悬挂的榛序直到被风吹弯了才会把花粉释放出来，确保花粉只在风力够强时脱落进入空气中。天气也很重要。只有当空气干燥的时候，花粉才会被释放出来，以防止花粉被潮湿的表面粘住或者被雨打落到地面。尽管有这些适应方式，大部分花粉还是无法离开顶端的树枝，只有千分之五到百分之四十的花粉能够飘到离树木100米以外的地方。不过飘过100米后，这些花粉相当一大部分就能飘到一公里或者之外的地方。事实上，花粉可以在高海拔地区飘至数千公里外。有这些花粉在空气中漂浮，难怪风媒传粉的树是过敏的主要来源。

一旦花粉被风带走，花粉的命运就交给喜怒无常的风了，但也并非说一切都取决于运气。风传花粉是干燥的，圆形的，光滑的，比虫媒传粉的植物的花粉小。但是花粉的大小是一把双刃剑。小花粉可能被吹得更远，但它们也更容易拂过花的柱头，因为较小的颗粒会被困在在柱头周围快速流动的空气中。但与此同时柱头处也会有动荡，从而降低风速，有助于花粉附着在柱头上。