The First Leaves Are Not Leaves. They Are the Lunchbox

The structure is part of the embryo inside a seed, formed before the seed ever left the parent plant. Its function is storage or absorption, holding the reserves the seedling lives on until it can photosynthesise, or taking up reserves stored elsewhere in the seed. In many species it emerges above ground on germination, turns green and photosynthesises briefly before withering away. In others it stays underground entirely and is simply consumed. Either way, the true leaves appear afterwards and look quite different.

Counting them separates flowering plants into two ancient groups:

• •One of them, in grasses, palms, lilies, orchids and cereals
• •Two of them, in most trees, beans, sunflowers and herbs
• •Leaf veins parallel in the first group, branching in the second
• •Flower parts in threes in the first, fours or fives in the second
• •Scattered vascular bundles in the stem against a ring
• •Fibrous roots against a main taproot

The split is far older and deeper than the seedling detail suggests, and it organises an enormous amount of practical knowledge. The single-structure group includes every cereal crop, which means it feeds most of the world, and it also includes almost all the grasses that support grazing animals. The two-structure group includes most broadleaved trees, most vegetables and most garden flowers. Herbicides exploit the difference directly, since compounds that disrupt growth in one group leave the other largely unaffected, which is how a lawn treatment kills weeds without killing grass.

Species differ in whether these structures come up or stay down, and the difference is not arbitrary. Pushing them above ground lets them photosynthesise and contribute energy immediately, at the cost of exposing the seedling's entire food store to grazing and frost, and it requires a stem strong enough to lift them. Leaving them below ground keeps the reserves safe and means the seedling must reach light on stored energy alone. Beans illustrate both strategies in closely related species, which is why some beans must be sown shallow and others tolerate depth.

The first thing above soil tells you several useful things immediately. Two rounded structures unlike the eventual leaves indicate a broadleaved plant, and a single narrow blade indicates a grass or its relatives, which distinguishes a seedling weed from a wanted crop before either is recognisable. Whether they emerged above ground or stayed below predicts how well a seedling tolerates being buried, since a species that must push them up cannot be sown deeply. And damage to them matters, because a seedling that loses its food store before its true leaves work usually dies.

The first structures above soil are part of the seed embryo, holding or absorbing the reserves a seedling lives on before it can feed itself, and the true leaves come later. Having one or two of them divides flowering plants into two groups differing in veins, flower parts, stems and roots, which is why a lawn treatment can kill broadleaved weeds and spare the grass.