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Plant Sex Ed

Posted: April 9, 2013 8:05 a.m.
Updated: April 8, 2013 5:00 a.m.

Do all plants have flowers? It is a timely topic in spring, with flowers abounding and air that is laden with pesky pollen. Like any good love story, plant sex has no paucity of details and drama. To make our investigation easier, we can divide plants into four groups. They all do it differently, in ways that make a lot of sexual sense depending on the respective environment.

Mosses offer us a picture of primitive plant reproduction, using tiny spores. At the right time of year, brushing the tops of mosses generates an expulsion of spores, forming a cloud that drifts into the distance. When a spore lands, it grows into a new plant, one that will eventually make a sperm and egg. The sperm must then swim to the egg in order for reproduction to occur. Because of this limitation, mosses exist on wet ground, close to one another, and in a shortened life form to assure proximity to the wet forest floor. After fertilization, the moss grows a stem that supports a spore capsule, which releases the spores and begins the cycle once again.

Ferns are similar. They too manufacture spores. These grow into heart-shaped structures that live against the ground and generate the sperm and egg. Once the sperm swims to the egg successfully, this structure turns into what we typically call a fern. Within the growing season, fern fronds produce tissues that harbor -- you guessed it -- more spores. While ferns hold some of the physical ingredients to grow taller than mosses, they still share some limitations, like the reliance on water as a conduit for sperm.

Conifers, like our pine trees, have elevated their status, quite literally. Their technique took the sperm and packaged it into a pollen grain born on a male cone. Instead of swimming, the sperm can travel by air to the female cone, no longer dependent on a moist forest floor. Historically, this enabled plants to move away from water and into unoccupied, upland habitats. With the accompaniment of a few physical modifications, conifers grew taller than their predecessors, sending pollen long distances in the wind. Of course there are tradeoffs. Wind is not accurate in its delivery, but conifers compensate by making extra pollen. They release it before many trees expand the leaves that would otherwise block the wind-thrown pollen. Conifers were also among the first plants to develop seeds, a nifty addition since seeds can remain dormant for many years before germinating, unlike many spores.

Finally -- and to answer the question -- we come to the fourth plant group, the flowering plants. In contrast to the aforementioned, these produce petals, stamens, pistils, and the like, all structures that qualify for the definition of a “flower.” They are the most widespread and diverse of all, representing ninety percent of all plant species. Why did flowering plants become dominant over the mosses, ferns, and conifers? Some, like the tiny flowers of our oaks, still use the wind for pollination. Yet others found a new partner, the insects, and this relationship has blossomed like no other.

Unlike wind, insects are more targeted in pollen deposition. The relationship is often one of admirable devotion, using specific insects for specific flowers. Magnolias and Dogwoods use beetles. Sassafras, Spicebush, and Pawpaw depend on flies. Of course, bees do their part too, with an estimated five hundred species existing in our state. Another advantage for the flower is that if fertilized, the ovary expands, forming a fruit. This fruit is true temptation for the animals that devour its contents and consequently disperse the seeds within.

Mosses and ferns are primitive but powerful. The lights in our homes are a direct testament to their timeless achievement, as their ancestors comprise a large portion of the fossil fuels we burn for energy. Conifers donate nearly eighty percent of our lumber products, and flowering plants give us the fruits and vegetables we need for our nutrition. And of course, they give us flowers, which have been known to help humans in their own sexual pursuits! So while you sneeze a few times this spring from pollen that misses its mark, remember it’s a small price to pay given the benefits of flowering plants and their close friends, all products of reproductive processes as diverse as they are profound.

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