The soybean plant, Glycine max, is a globally important legume whose appearance changes dramatically throughout its life cycle, moving through distinct vegetative (V) and reproductive (R) phases. The plant begins as a small seedling before developing into a bushy, leaf-covered structure that later transforms into a mature plant bearing dry pods.
Early Growth and Vegetative Features
The soybean plant begins its above-ground life when the hook-shaped hypocotyl pushes the two cotyledons, or seed leaves, out of the soil. These cotyledons are thick, fleshy, and initially serve as the plant’s main food source, appearing green as they begin to photosynthesize. The first true leaves emerge immediately above the cotyledons at the unifoliolate node, presenting as two simple, single leaves positioned opposite each other on the stem.
Following these initial leaves, all subsequent leaves are compound and distinctively trifoliolate, consisting of three oval to lance-shaped leaflets attached to a single petiole. The stem is generally green and rigid, covered by a fine, dense layer of soft hairs known as pubescence. This vegetative phase is characterized by increasing height and the sequential unfolding of new trifoliolate leaves on the main stem.
Flowers and Developing Pods
The shift to the reproductive phase begins with the appearance of small, self-pollinating flowers on the main stem and branches, typically initiating at the lower nodes. These blossoms are inconspicuous compared to many other flowering crops and are generally white or various shades of purple. A single plant will often have flowers of both colors depending on the variety.
The flowers cluster in small groups along the nodes of the stem, often hidden beneath the dense canopy of leaves. After pollination, the flower withers, and the ovary develops into the characteristic soybean pod. Young pods are soft, bright green, and covered in fine hairs. Each pod typically contains two to four seeds and grows rapidly, reaching full size before the seeds inside fully develop.
Mature Plant Appearance and Senescence
As the plant completes its seed-filling stage, it enters senescence, where the entire plant structure begins to dry down. The vibrant green foliage is the first to change, progressively turning yellow and then brown as the plant redirects its remaining nutrients into the developing seeds. The leaves eventually drop completely, leaving behind a brown, brittle stalk.
The plant’s height at full maturity varies significantly based on the variety and growing conditions, ranging from about two to over six feet tall. The pods also change color, shifting from green to a mature tan or brown color. The mature plant is stiff, dry, and often appears shrunken, with the hard, dry pods clinging to the stem, signaling readiness for harvest.
Key Features for Visual Identification
The soybean plant possesses several unique physical markers that make it visually distinguishable from other legumes or field weeds. The most telling characteristic is the pervasive pubescence, the dense covering of fine, velvety hairs that gives the stems and pods a soft, fuzzy texture. This “fuzziness” covers virtually every external part of the plant.
Another signature trait is the structure of the leaf: the mature trifoliolate leaf, consisting of three leaflets, distinguishes it from simple-leafed plants. The small, clustered flowers are almost always either white or purple. The subsequent development of short, hairy pods, which typically bear two to four seeds, further confirms the identity of Glycine max.