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The macro structure of the seed highlighting the primary tissues is presented in Figure 3.4. The seeds of leguminous plants differ greatly in general appearance (color, size, shape) and in several less apparent attributes such as seed coat thickness and permeability of the hilum. However, all possess a similar seed structure comprising a seed coat, cotyledons, and embryonic parts. The seed coat or testa is the outermost layer of the seed and serves as a protector of the embryonic structure. The cotyledons comprise the largest mass of the seed and serve primarily as energy storage (starch and protein) tissue. The embryonic structures are varied in size, relatively high in lipid content and provide the germinating loci for the seed (Moïse et al. 2005; Bassett et al. 2020).

Two external anatomical features include the hilum and micropyle, both of which have a role in water absorption and seed gas exchange. The hilum, commonly referred to as the “stem scar,” is a large oval scar where the seed and stalk were previously joined within the pod (Kigel et al. 2015). The micropyle is a minute opening in the seed coat that served as a junction where the pollen tube entered the valve. The remaining portion of the seed is the embryonic structure that includes two cotyledons, the epicotyl or embryonic stem tip, the hypocotyl or embryonic stem, and the radicle or embryonic root. This portion of the seed is responsible for germination and is extremely vulnerable to damage during adverse handling and storage.

Fig. 3.4. Schematic of a dry bean seed.

Source: Adapted from Georges (1982).