Plant seedlings emerging from the ground use the cotyledon’s engineered preparation for life. You may not be familiar with cotyledons, but you have undoubtedly seen them on newly emerged seedlings.
To get the idea, think about some other engineered devices that serve an essential preparatory function. When skydivers jump from a plane, they use carefully engineered equipment. The first thing they deploy to prepare for landing is a pilot chute. The pilot chute can’t land them safely on the ground. Its purpose is to deploy the main parachute. Perhaps more familiar to most people is the limited-use spare tire for automobiles. Those “donuts,” as many people call them, are not designed for high-speed driving or for driving long distances. They are engineered to get you to the nearest service station where the punctured tire can be repaired or replaced. The pilot chute and the limited-use spare tire are examples of engineered preparation.
Just as the pilot chute is packed into the jumper’s gear and the donut is packed into the vehicle, there is something packed into the seed called a cotyledon. Scientists classify flowering plants (angiosperms) as monocots or dicots depending whether they have one or two cotyledons folded into the seed. As soon as the seed has sent a taproot into the soil, it pulls in moisture and uses the hydrostatic pressure to push up a green shoot bearing the cotyledons. As those “donuts” break through the surface, they inflate to provide temporary, emergency photosynthesis. The seedling begins to drink up the water and nutrients from the taproot and use energy from sunlight to kickstart the photosynthesis process.
We live in a part of the world where there are many trees. We also experience heavy winds that frequently blow down human-made structures. It is interesting that healthy trees are almost never blown down. When you stop to think about it, you would expect trees to be major victims of high winds. That is not the case, and it is due to leaf designs to preserve trees.
To survive strong winds, trees need two things. The most obvious is structural support–strong, flexible branches, sturdy trunks, broad bases, and good root anchorage. A more subtle requirement is leaf designs to preserve trees. Leaves must have minimal wind drag. A fluid, such as air, flowing around an object generates drag. To minimize drag requires some streamlining to reduce the amount of friction between the fluid and the object. A highly streamlined object will usually be gently rounded upstream and elongated and pointed downstream.
For healthy trees, the leaves offer the most surface area and thus the most drag. Trees most commonly blow over when in full leaf, so leaf design is critical to the survival of the tree. Different trees have different design features, but all of them are designed to avoid destruction in a wind storm. American holly leaves have a method that involves the leaves being able to flatten themselves against each other. When the wind becomes strong, the leaves turn and lie flat significantly reducing the drag.
Tulip tree leaf design allows the leaves to roll up when the wind gets strong. The blade of the leaf points away from the stem. As the wind blows against the leaf, it forms a cone pointing upwind at the stem. The blade forms the broad area of the cone away from the wind direction. The higher the wind, the tighter the cone and the less the wind resistance. Black locust leaves similarly roll together to produce a cylinder.
Each of these designs depends on the properties of the leaf. If the leaves were too stiff, they could not assume the right geometry. The flexibility of their stems has to be high, and the surface of the leaf must be carefully designed and restricted. You can argue that natural selection does all designing and that given enough time it will select the proper shape. But remember that changes in climate mean you don’t have infinite time to apply the process.