Testae

Long hours, social isolation, and tons of uncertainty can make any of us testy. Testae have nothing to do with any of that. This post is about seeds.

If you pick up a seed, the first thing you’ll see is its testa. Testae are the outer coverings of seeds. Tucked safely inside are the embryonic plant and its food supply.

Parts of a seed (Kate Russell)

Also known as husks, hulls, and seed coats, most testae look pretty boring at first glance. They tend to be smooth, hard, and simple. This is not always the case. There are winged seed coats, hairy seed coats, and even seed coats with prickles!

A pair of maple samaras with winged seed coats (Steve Hurst, USDA) Public Domain

Testae development

Seed coats start out as the maternal material that surrounds ovules. Put simply, ovules are a plant’s eggs. So, I suppose we could say a testa starts out as the thin film found between the shell and the hardboiled egg you peeled not too long ago. In both cases, that material is called an integument. Integuments are neutral outer coverings, such as skin and rinds.

Anatomy of an egg (Vkkodali) CC BY-SA 3.0

Did you know that hen’s eggs have two layers of protective membranes under the shell? There’s a third membrane around the yolk, but we can skip that one for now. It’s those membranes that make it easy (or difficult) to peel your snack. [Note: fresh eggs are harder to peel. If you start them cooking in hot water, rather than cold, they will be a little easier to peel.]

Types of testae

Seed hulls can be hard and rugged, as in the case of coconuts, or they can be extremely thin and delicate, as with the filmy covering over each pomegranate seed (sarcotesta). Botanically, there are two types of seed testae: bitegmic and unitegmic. [From the Latin tegō for "I cover"). Bitegmic seeds have testae made from two layers, while unitegmic testae are made from one.

Parts of a seed coat

It takes several layers of integuments to make a seed coat. These layers are first classified as inner and outer. If you want to dive deep down this rabbit hole, I suggest taking a look at Radchuk and Borisjuks’ Physical, metabolic and developmental functions of the seed coat. For the sake of today’s discussion, suffice it to say that these layers do everything from producing pigments, protective cuticles, and even toxic chemicals, all to improve a seed’s chance at survival.

Queen Anne’s lace fruit cluster with spiky seed coats (Greg Hume) CC BY-SA 3.0

Testae tasks

We all know that the primary function of a seed coat is protection. Hulls prevent mechanical injury. They keep seed contents from drying out and they prevent pests and pathogens from getting inside. But testae also prevent germination.

If a seed germinates too early or too late in the growing season, it usually dies. The seed coat prevents this from happening by blocking air and water from getting in, creating a forced dormancy. But how does a seed know when to germinate with all this protection?

Seed coat dormancy

It ends up that seed coats pass on information about environmental conditions to the embryo. Until conditions of humidity and temperature, and sometimes light, are right, the seed coat battens down all the hatches, creating a self-imposed dormancy. During seed coat dormancy, not even air and water can get through. Of course, insects, microbes, erosion, and passing through a digestive system can wear a seed coat down. When we damage the seed coat on purpose, we break that barrier. This is called scarification. Also, testae have holes in them, called micropyles. These holes are how pollen tubes get inside for pollination.

Either by choice or by outside influences, eventually, the seed coat stops protecting its charge, allowing water and air to start seeping in. This is when germination begins and the life of a seed coat ends.