Seed is an important food for animals and humans. It carries the embryo of the plant and its food reserve inside, often protected by a coat or hull.
In angiosperms, seeds form after fertilization by sperm and egg in the ovule. They have a wide range of shapes and sizes, including bean-shaped (reniform), discoid, ellipsoid and ovate.
The Embryonic Plant
The seed embryo contains the earliest forms of the plant’s roots, stem and leaves. It is contained within a container, called the seed coat. A seed also provides nutrients to the embryo. In this way a seed is like a Russian doll with nested generations.
The development of a seed is a complex process. Scientists are beginning to deconstruct it and understand some of the innovations that allow seeds to work.
Following fertilization, the zygote undergoes asymmetric cell division to form an embryo and a seed coat. The embryo grows to the globular stage, during which time it introduces the ground tissue (procambium) and primary meristem.
Unlike the spores produced by non-seed plants, seed embryos can sense the conditions of their containers and grow if they have enough food reserves. This ability to grow is known as germination and the resulting new plant is a seedling. This is why seeds are more than just a single-celled organism, like a sperm, with an unprotected exterior.
The Food Reserves
The accumulation of fat, carbohydrate and (rarely) protein in tissues and cells serves as a storage compound for energy that can be used when needed by the organism. In animals, this occurs in adipose tissue and the liver and muscle. In plants, it is stored in starch within the colourless leucoplasts called amyloplasts, located in roots and underground stems, or in seeds, where it is mobilized at germination.
Low food reserves have exacerbated past global food crises and are a major factor in the current crisis. Modeling shows that higher reserves could reduce price spikes by 38-52%.
Public stockholding programs have a millennia-long history as hedges against bad weather, pests and other calamities. During times of plenty, governments build up reserves that can be tapped during periods of drought or conflict. In the United States, these include the Farmer Owned Reserves and the Food Security Wheat Reserve. Governments should consider building or reviving these programs to provide a price floor for farmers and to protect domestic and foreign consumers from volatile prices.
The Dispersal Mechanisms
There are a number of different ways seeds can be dispersed from their parent plants. They may be blown away by wind, carried away by water, or deposited far from their parent plant by animals or birds.
Many weeds and ruderal species, such as tumbleweed, dandelion and physalis, have evolved to use wind dispersal for their seeds. The classic example of this is a dandelion blowing its seedpods in the wind. This is a type of long-distance dispersal (LDD).
Another method of LDD is for the seeds to cling to the fur, skin or feathers of animals, and be carried away from their parent plant. This is called allochory. Many plants, such as burdock and sea holly, have fruits with hooks on them that catch on the fur of animals or birds, or they pass intact through animal digestive tracts, to be dispersed far from their parent plant. The same is true for seeds clinging to the shells of turtles or birds.