The Tiny Tree Pollen Grain:
Tree pollen is a fine to coarse powder containing grains of genetic material of seed plants including trees. Pollen contains the male gametes, called sperm cells, and are grains with a hard coat that protects the sperm cells during transportation from the stamens (male plant part) to the pistil (female plant part) of flowering trees. Conifers have both male and female cones.
The Job of a Tree Pollen Grain:
Tree pollen has the ability to produce a male cell called a gamete that has the ability to fuse with a female gamete cell which results in pollination. The male stamen produces pollen which eventually lands on the female pistil (which contains the receptive ovary) of flowering trees. The male gamete generates a pollen tube that transfers the sperm to the ovule of a receptive ovary.
Profile of a Tree Pollen Grain:
Remember that tree pollen itself is not just the male reproductive material. Pollen grains are made up of both vegetative cell(s) and one cell that reproduces. That one reproductive cell contains two nuclei: a tube nucleus (that produces the pollen tube) and a generative nucleus (that divides to form the two sperm cells). The size of these tree pollen grains determines the method of reproductive transport - wind or insects/animals - and the shape aids in successful pollen grain germination and a successful introduction of sperm cells to the female ovary.
Tree Pollen and Pollination:
As I have mentioned, these tree pollen grains are distributed by wind and insects/animals. Small tree pollen is blown by wind to female parts while larger tree pollen grains are deposited by insects onto the female ovary where pollination takes place. Tree pollen grains germinate and develop a pollen tube which grows down to the ovary and deposits the sperm cell which causes the production of seed.
Tree Pollen and Allergy:
Tree flowers with large blooms usually produce heavy (large particle) pollen. Large blooming trees attract insects that transport pollen and do not depend on wind transportation. These trees are generally lower in their allergy potential and include female ash, female red maple, yellow poplar, dogwood, magnolia, double-flowered cherry, fir, spruce and flowering plum.
Trees with high allergy potential are often dioecious males, which will bear only pollen and no fruit or seed. Trees to avoid are male ash, pine, oak, sycamore, elm, male boxelder, alder, birch, male maples and hickory.