All over the Northern Hemisphere, gardeners are eagerly watching for their vegetable seeds’ first sprouts. Why do the first leaves that break through the soil look so different from the mature leaves on our vegetable plants? These chunky leaves are called “cotyledons” aka “seed leaves”. These are the leaves that develop as part of the plant embryo inside the seed. In fact, the edible part of many seeds we eat, such as beans, peanuts, and peas, is the cotyledon inside the seed. The cotyledons have evolved to give the embryonic plant a running start by storing carbohydrates and lipids to fuel the development of the roots and shoots as the seed wakes from dormancy and begins growing.
The cotyledons of these pumpkins (Cucurbita pepo) and assorted winter squash (Curcubita maxima) are green, showing that they are capable of doing photosynthesis while transferring their stored nutrition to the growing root and shoot system. If you look closely, you can see the tightly curled up mature leaves with their wrinkled texture between the smooth cotyledons. I have included some examples of fully grown pumpkin leaves, which look very different from the cotyledons.
One of the first plant classification methods developed by European naturalists like John Ray and Antoine Laurent de Jussieu in the 17th century was to divide the plant world into “dicots” and “monocots”. This refers to plants that sprout with either 2 cotyledons (dicots) or 1 initial leaf (monocots). Although plant classification has become more detailed and sophisticated since that time, dicots and monocot still roughly represent one of the earliest divides in the evolutionary history of the plant kingdom.