An up-close look at pollen (achoo!) By Kathy Ikeda

For many people, pollen is the “p-word”. . . as in, “PLEASE, don’t say that word!” The mere mention of pollen can conjure up runny noses, watery and itchy eyes, and looks of desperation from those with hay fever. Here in the fertile, crop-rich San Joaquin Valley, it’s often said that if you don’t already have allergies, you’ll develop them.

Trees such as oaks, birches, conifers (pines, firs, spruces), and nut crops (walnuts, almonds, pecans) are especially prolific producers of pollen, and they’re ubiquitous in our area. They’re joined by other common pollen-producing plants, including grasses and many of our favorite garden flowers. It’s an allergy sufferer’s nightmare.

Pollen literally means “fine flour” or “mill dust” in Latin. While it’s a health nuisance, it’s also a valuable and necessary evil.

We’re familiar with pollen as that yellowish, powdery-looking stuff that drifts away from trees and other plants in spring and summer, making us miserable as it floats through the air, collects on our cars and homes, and settles in our nostrils. But without pollen, we literally couldn’t survive. The vast majority of plants on Earth depend on pollen for their reproduction, and we in turn depend on plants for food and oxygen.

Pollen is unique to seed-producing plants, which are divided into the flowering plants (Angiosperms) and the cone-bearing plants (Gymnosperms). The pollen-bearing structures of these plants are contained either in the flowers or the cones. More primitive plants such as ferns, fungi, mosses, and horsetails don’t make pollen; instead, they produce spores.

So, what exactly is pollen, anyway?

A clue to pollen’s specific purpose lies in the aforementioned scientific classifications of pollen-producing plants: the suffix –sperm means “seed” in Greek. Pollen is the male vehicle for a seed-forming plant’s sexual reproduction. Each microscopic pollen grain has a hard outer coating that protects the inner contents—two sperm cells and a tube cell—from damage and dehydration.

Plants fall into two categories when it comes to the type of pollination: (1) self-pollinated, or (2) cross-pollinated, where pollen from one plant must transfer to another plant of the same species.

Pollination begins when grains of pollen move from the male part of a plant to the female part of a plant. Pollen transfer can occur in one of two ways: (1) abiotic pollination, where pollen is carried by wind or water (most common in grasses and trees), or (2) the far-more-typical biotic pollination, where a living organism such as a bee, butterfly, moth, wasp, fly, bird, bat, or other animal moves the pollen (most common in non-tree flowering plants).

Once a pollen grain comes in contact with the female part of a plant, it germinates. A pollen tube develops, emerging through a specialized opening in the outer covering then extending toward the structure that holds the egg. The sperm cells then leave the pollen grain and travel through the pollen tube, ending their journey when they reach the egg. Fertilization occurs once the plant’s egg/ovule and sperm cells unite, and that initiates seed development.

Some amazing pollen-related facts about corn: Just one stalk can produce about 18 million pollen grains! The pollen grains land on the ends of the female corn silk; there is one strand of silk attached to each developing kernel. The single-celled pollen tubes that grow from the pollen grains through the silk can be up to a foot long!

Many small members of the animal kingdom depend upon pollen. Honeybees and native bees use protein-rich pollen, either eating it themselves or mixing it with sugary nectar before feeding it to their developing larvae. (Pollen is not used to make honey; honey is regurgitated nectar.) Some beetles—including ladybugs—and many common types of web-weaving spiders also consume pollen as part of their diet.

When seen under a microscope, pollen grains are exquisitely beautiful natural works of art. Their outer coatings are intricately patterned, spiked, or pitted, and different plant species produce their own unique pollen designs. Pollen can be sticky-surfaced or spine-studded so that it’s easily carried on the hair, feathers, or fur of pollinators, or it can be lightweight and aerodynamically crafted to float in a breeze.

False-color scanning electron microscope image of pollen grains from a variety of common plants: sunflower, morning glory, prairie hollyhock, oriental lily, evening primrose and castor bean. (Public domain image from the Dartmouth Electron Microscope Facility)

For more information and a visual treat, read the book Pollen: The Hidden Sexuality of Flowers, or search the Internet for “pollen electron microscopy” to see extraordinary images. And try very hard to appreciate pollen, even as it makes you sneeze.

For gardening related questions, call the UC Master Gardener office at 209-953-6112, or use our website: http://sjmastergardeners.ucanr.edu/CONTACT_US/.

 

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  • Blog Author

    Marcy Sousa

    Marcy Sousa is the San Joaquin County UC Master Gardener Program Coordinator. She is a Stockton native and enjoys teaching others about gardening. She has her bachelors from Stanislaus State in Permaculture. She has been with the program since 2007. Read Full
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