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Day 0. The beans planted in perlite. Perlite is used because it supports the seed, allows aeration and makes it easy to view the roots.
Figure 1. Beans planted in perlite. Red kidney beans have a hard seed coat. Image: M. E. Conley.
Bean seeds have an interesting morphology. The hilum is the scar where the bean was attached to the seed pod (Fig. 2). Inside the seed itself looks like a big white mass (Fig. 3). This big white mass (dehydrated storage cells) is actually the 2 cotyledons (Fig. 4). At the tip of the bean seed is the embryo (Fig. 5). Protecting the cotyledons and the embryo is a hard seed coat (Fig. 6). This seed coat is hard because it includes elongated macrosclereids (thick-walled dead cells) which limit water transport and protect from pests (Fig. 6).
Figure 2. The hilum. A picture of the whole bean seed showing the hilum. Image: E. T. Paparozzi.
Figure 3. Inside the bean. A picture of a cracked open bean seed showing the seed coat and one cotyledon. The embryo is hidden by the seed coat (top right). Magnified 12 times. Image: M. E. Conley.
Figure 4. Two cotyledons inside the bean seed. A dry bean seed that has been cracked in half to show two cotyledons (arrows). Magnified 24 times. Image: E. T. Paparozzi.
Figure 5. Magnified embryo inside the bean seed. Embryo at tip of a bean seed – magnified 12 times. Each division is 1 mm. Note: unifoliate leaves (star); root (radicle) (two stars) and hilum (arrow). Image: E. T. Paparozzi.
Figure 6. Scanning electron microscope view of the macrosclereids. A section through the dry seed coat of a red kidney bean showing the elongated macrosclereids (star). Think of the macrosclereids as a sort of porous concrete. Photo taken with a scanning electron microscope. Image: E. T. Paparozzi.