Torus

There are two types of torus in the plant world. One is related to flowers, and the other is a donut-shaped plug that protects plants when they cavitate.

Raspberry torus (Kate Russell)

Xylem torus

The donut-shaped plug variety occurs within the xylem. The xylem is part of the vascular bundle that pulls water from the ground. The torus found within the xylem protects plants from cavitation. Cavitation occurs when the water pressure within the plant is greater than the atmospheric pressure outside the plant. You can hear it happening to trees in summer. It sounds like a large crack. Cavitation causes sap to vaporize, creating embolisms. The torus, held within a pit called the torus-margo structure, acts as a plug. This plug reduces the spread of the embolism.

Diagram of a torus-margo structure, top open and bottom closed Note how the margo flexes under stress. (Pagliaccious) CC BY 4.0

Flowering plant torus

The place where a stem meets a flower is called the torus, or receptacle. The torus is the thickened part of a stem from which flowers emerge. Accessory fruits, such as apples and strawberries, grow from the torus. And it is the way you can tell blackberries from raspberries! That mammoth sunflower head? It’s a torus, too.

Types of flowers

The torus is found at, below, or surrounding a plant’s reproductive organs, depending on the species. Flowers can be hypogynous, perigynous, or epigynous. Don’t let the words scare you off. The -gynous part of these words means it refers to the Lady Parts of your plant. Stamens and petals are below the ovaries (or gynoecium) of hypogynous flowers. They are at the same level in perigynous flowers. The ovary is enclosed in the torus of epigynous flowers, with the petals and other parts above. You can use this information to help identify plants and win at Scrabble.

Gray torus in relation to red ovary in hypogynous (I), perigynous (II), and epigynous (III) flowers (Ulf Mehlig) CC BY-SA 2.5

Raspberry or blackberry?

Do you know how to tell blackberries and raspberries apart? It’s not the color. There are black raspberries and red blackberries. The only way to tell the difference between blackberries and raspberries is to look at the way the fruit comes away from the torus. 

Halved blackberry with present torus (Ivar Leidus) CC BY-SA 4.0

Halved raspberry with absent torus (Ivar Leidus) CC BY-SA 4.0

If the torus comes with the fruit, it is a blackberry. If the torus breaks away from the fruit, it is a raspberry.

Now you know.