The Pollination Process
In my last blog post I talked about some of the basics of flower structure and function. Flowers produce fruits which house the seeds. Seeds are the structures produced through sexual reproduction to ensure continuation of the plant in subsequent generations. The seeds are formed from the ovules (that should be ovulous to you) which are located inside of the ovary. The ovary is typically a swollen, basal part of the carpel or pistil. Pollen is transferred from the anther of one flower to the pistil of another, a process called pollination. If a pollen grain lands on an appropriate stigma, it then must germinate and produce a pollen tube that grows down the style of the pistil. The pollen tube eventually reaches the ovule where it releases sperm cells that effect fertilization of the egg cell present in the embryo sac. Once the egg is fertilized (that is, forms the zygote) it then develops into a seed.
Angiosperms are plants that produce flowers and are the group of plants most familiar to us. One other group of plants that produces pollen and seed but not fruits is the gymnosperms. Gymnospermous plants are familiar to you because this group contains the pines and cedars, plants that produce cones. Regardless of whether the plant is a flowering plant (angiosperm) or a cone-bearing plant (gymnosperm), it is essential for pollen to be transported to an appropriate and compatible plant if seeds are to be produced which can serve as propagules for the next generation.
Many plants produce large, colorful flowers which often have distinct floral scents as well, for example, a rose. Such flowers are designed to attract pollinators to assist in the process of pollination and thus ensure successful seed production. Other plants have their flowers grouped together to form inflorescences. These groups of flowers are more likely to be noticed by pollinators such as insects and birds due to the enhanced floral display (visual as well as olfactory). A sunflower for example is an inflorescence made up of hundreds of individual flowers that are grouped together to give the appearance of one large flower. Each flower in the head of the sunflower produces one sunflower seed (technically a fruit which bears a single seed inside). A gladiolus plant produces a spike (type of inflorescence) of large, showy flowers to attract pollinators such as hummingbirds and hawkmoths.
Flowers of wind-pollinated plants on the other hand are not showy or large and don’t have floral scents. They typically have reduced flowers that lack a lot of “extras” such as petals and sepals and they don’t waste energy producing nectar or floral scents. They are normally part of floral inflorescences, some of which may be quite large; for example, think of corn tassels, or pussy willows, or oak catkins. By grouping the individual flowers together into inflorescences a given plant increases the amount of pollen that it can produce and thus helps ensure successful pollination.
One large group of plants that uses wind to transport its pollen is the grass family. The grasses produce flowers in inflorescences called spikes. Their flowers are small and lack petals and sepals but have anthers that produce large amounts of pollen and pistils that have feathery stigmas that enhance the flowers ability to trap pollen in the air. (See figures of grass flowers below.) Grass pollen triggers allergic responses in many individuals and is most likely to be a problem for allergy sufferers in late spring and summer.
The next time you go outside be sure to look for flowers on the plants around you. Look for both big and small flowers, and those which are single as well as those found in inflorescences. Learn to identify those plants that cause you allergy problems and hence bear the flowers or cones that produce the pollen that acts as an allergen for you.
H. Wayne Shew, Ph.D.
NAB certified counter
BSC/AAAC Collection Station—Birmingham, Alabama