In the momentous move from water to land, plants have had to overcome several barriers in order to survive. They’ve had to change in three major categories: reproductive strategies, physical structure, and combat dehydration.

Evolved reproductive Strategies:

Because the sperm can no longer freely float through the water, plants have had to adapt other methods for fertilization. Brightly-colored petals attract insects with promises of nectar, and the insects then cross-fertilize the plants. The sperm’s convalescence into pollen is another adaptation that makes the sperm stick better to the insects or float through the wind, increasing the chances of fertilization. The evolution of the seed—keeping a greater supply of food while protecting newly wedded sperm and egg—is another sexual advancement for plants that helped them deal with a non-aqueous environment. Seed dispersal, such as with dandelions where the seeds are transported by the wind, or with burrs where the seeds are transported by animals, is another evolved trait. It is important to note that not all land plants use seeds or pollen to propogate themselves: ferns send out runners that fertilize underground while other plants spawn off asexually when a piece of the root breaks off or simply grows in another direction, much like worms, in the popular conception, create two worms when cut in half (false, but a useful analogy).

Evolved Physical Structure:

Plants have developed several structures to help support them on land that they didn’t need,  as much anyways, in water. Stems are one... while before plants could float in water, stems allow land-bound plants to support themselves and reach for more light. And since they are no longer free-floating, plants have developed several diverse types of root systems to anchor them, support them by propping them up, and, of course, absorbing water. A firmer cell wall has also developed to keep what water they do have inside them, where as before this wasn’t an issue since they were in an aqueous environment (Note: it actually was still an issue in keeping the inside/outside environments pH balanced, ion gradient, etc. But it wasn't as difficult as in a dry environment.). Plants have also evolved to take advantage of Turgor pressure, where osmosis swells the cell and reinforces it. This allows the plant to remain upright.

Fighting Dehydration:

Plants also must now face dehydration on land. Plants have developed waxy cuticles, so that water is kept inside the leaves. Similarly, seeds are coated to protect against dehydration. Seeds also remain dormant until wet conditions are present, increasing the chance of survival. Roots also developed to absorb water—they twist around and have a high surface-area: volume ratio. They also have tiny hairs that aid in absorbing water. Stomata have developed to control water movement in and out of the plant.    

Cam and c4 plants have modified stomata to deal with their unique environments. Plants also lose leaves during cold weather so as not to waste, and lose, precious water resources.

Thanks to m_turner for some help on reproduction... err, the plant reproduction information.

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