In less than two months, this little seedling will be feeding me delicious spinach greens in a salad! And most of the solid material for the growth that will make that possible will come from the thin air surrounding the leaves. Carbon dioxide, a gas that makes up a growing proportion of our atmosphere, is the sole source for all of the carbon that will form the backbone of the proteins, carbohydrates, and fats contained in this plant. Water, of course, along with essential elements such as nitrogen, phosphorus, and potassium, will come up from the soil through the belowground roots.
All of that carbon dioxide will enter my spinach plant through many thousands of tiny pores spread all over the surface of the leaves, called stomates. Thus does most of the solid mass of any plant first pass through a microscopic hole in a leaf as a gas molecule - a gas molecule that currently makes up just under one-half of one percent of the air!
The energy to get this lettuce seedling to burst out of its seed capsule, still attached, and grow up through the soil, came from molecules stored in the first, specialized leaves, called cotyledons. The two cotyledons are at the top of the short stalk, already turning green with chlorophyll and switching from using the energy that they came with inside the seed to the energy from the light bathing them almost 12 hours every day. With the energy from that light, the chlorophyll molecules will work with the other cellular machinery in these leaves to split carbon from oxygen in the carbon dioxide coming into the leaf. That energy will also be used to split hydrogen from oxygen in water coming up from the roots, and then to attach the carbon atoms to each other and to other atoms to make more cells and grow this lettuce plant from a tiny seedling to a delicious meal.
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