Question

Describe corn, peas, and beans type of seed embryo.

What are the differences and similarities of the relative levels of water uptake (imbibition) of monocot seeds and dicot seeds?

Answer 1

Pea seeds: The embryo of mature seeds of Pisum sativum consists of the embryonic axis and the cotyledons. FA4-type seed. The fleshy storage cotyledons make up most of the seed's volume and weight. The pea embryo is enclosed by the testa and the endosperm is obliterated during seed development, when it's nutrients are taken up by the embryo.

Corn seed

The single cotyledon is called a scutellum; the scutellum is connected directly to the embryo via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.

Bean seed

A bean seed that has been soaked in water has a soft outer covering. This outer covering is the seed coat. Inside of the seed would be a tiny plant called the embryo. The two large parts of the seed are called the cotyledons. The cotyledons are stored food that the young plant will use while it is growing.

Some monocotyledonous plant seed increase in size (and mass) as they take up water prior to germination, others change very little. In two of the largest monocot families, Poaceae (grasses) and Orchidaceae (orchids) the seeds don’t change much for most species. In the case of orchids, the seeds are almost microscopic to begin with, consisting almost entirely of the embryo. The seeds of rice (Oryza) can swell markedly, but the seeds of corn (Zea) don’t gain a lot of mass, owing to the tough cuticle that surrounds the seeds, limiting expansion. Many dicots do increase in size and mass with imbibition of water, notably legumes (Fabaceae). If you put a cup of beans in a pot of water and let it sit overnight, it will be at least two cups in the morning.

In all cases, the increase in mass corresponds to the water uptake capacity of the seed, which in turn is tied to tissue types that have a high osmotic potential. That is to say those tissues, mostly starches, can draw water in, increasing the mass of the seed.