Seeds come in different sizes and shapes across plant species. They range from dust-sized seeds in orchids to double coconut Lodoicea (yes, coconuts are seeds!). Depending on geographical location and the prevailing biotic and abiotic factors, seed size and shape can vary even within a species. Even within same species, for example corn, the size, shape, and shininess of the seeds vary. Why?
Seeds usually have three main parts:
- seed coat that protects the inner parts from outside hazards,
- embryo that grows to a plant, and
- endosperm (grasses) or cotyledon (non-grass) that stores and supplies food to the growing embryo.
Mother nature has fixed the balance among these parts across the plant species to ensure that the embryo is protected during the sleeping phase and thereafter, it gets sufficient food supply for germination in field.
Seed, which holds the embryo within it, is the connector between two consecutive generations. After they are produced in mother plant, seeds generally need to wait for the next or a few more growing season(s) to grow. This is so they get similar growing conditions to the mother plant – in terms of temperature, humidity, soil moisture, day length, soil microbes etc. This sleeping phase is called as seed dormancy.
Scientists have determined that small seeds have shorter dormancy lifespans than larger seeds, because they contain less food inside. Smaller seeds usually need to stay near to the soil surface so their smaller embryo can pierce only a thin soil layer to come out. Of course, insects, bird and other predators may seed and eat these smaller seeds planted close to the surface. But, if placed too deep, their small embryo cannot push out of the soil.
Larger seeds can stay in greater soil depth and still their embryo is capable of emerging out of the soil because of more food supply and bigger embryo size. Larger seed cannot stay and germinate well from a shallow depth because of low soil moisture availability. In addition, they would get consumed by predators on the top layer, too.
Scientists report that smaller and elongated seeds show greater percentage and faster germination than larger and compact ones. They are expected to be dormant for their ease of burial, whereas large, elongated, or flattened seeds are expected to germinate more for their pressure of predation.
These reasons are why your garden seed packet come with directions for planting at certain seed depths!
Depending on the plant species, seeds often travel to different geographical locations for finding new habitat, usually where there might be less competition for resources like sun, water, and nutrients. This phenomenon is called seed dispersal. Wind, water, insects, ants, birds, small and big pasture-animals, wild animals and so on, act as the carriers for this dispersal.
The size, shape, texture, and presence of special structures on the seed surface such as spines, glues, fluffs, claws, etc. that help adhering to the animal bodies or fly in the air or float in the water are critical to aid the seed in this process. You may have inadvertently carried a seed burr along on a hike – or on the bottom of your shoe. Many nature preserves have shoe scraping stations to remove seeds and prevent contamination of protected land. Research has shown that ancient cotton seed, while dormant, traveled across oceans in their protective seed coating!
Big seeds do not tend to travel, and light seeds can travel long distances. Round seeds tend to roll down when dropped from the plant on uneven soil surfaces or hilly regions, thus travel longer distances. Flattened, pyramidal or cone shaped seeds usually stabilize where they fall.
In nature, seeds are one of the main sources of food for any life forms – microbes, insects, squirrels, other animals, or humans. To deter the predators, seeds have evolved different mechanisms. While small seeds can easily be consumed by microbes, large seeds have the disadvantage of greater immobility hence greater risk of being consumed by the animals. Besides size and shape, the shininess, hardness, and color of the seed play a crucial role to repel and escape those natural enemies. Glossy seeds such as soybean can easily repel microbes and small predators, whereas dark color seeds such as black beans contain some helpful pigments that provide protection from different biotic enemies and weather hazards.
Although the seed size often indicates what the plant will look like, it is not always true. For example, the large coconut seed is very large proportionate with the large plant size, and small rice seed matches the smaller rice plant. But guava and peanut seeds are both big plants, with smaller seeds. Most famously, a huge, leafy plant like mustard comes from a tiny seed.
Why seeds are of different sizes and shapes has always been a relevant question in plant biology. The simplest answer to it is mother nature selects these traits for a species depending on their need during evolution and adaptation in newer areas. The purpose may be one or multiple, but there is always a reason behind why it is so.
Aniruddha Maity, Texas A&M University
About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.
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Read the other blogs in our seed series!