What you need to know about spots and spines

Some plants, and many succulents in particular, have developed a variety of different protrusions on their stems. The most well known of these are the spines, generally found in cactus species. However, succulents native to southern Africa, particularly those in the plant subfamily Asphodeloideae, have developed a different kind of stem adornment: tubercles. Others have enlarged ‘cilia’ that give them a hairy appearance. But what are these structures, and what sets them apart from the spines of cacti?

What are spines?

Cacti and a variety of other succulent species have developed spines as a form of protection against herbivores. In cacti, spines grow from structures called areoles, which are a form of ‘branch’ that is highly reduced and specialized. The spines themselves are in turn specialized leaves, and can vary in size between different species and cultivars. Some Mammillaria, for example, have quite small spines in comparison to the elongated ones of the famous ‘golden barrel’ cactus (Echinocactus grusonii).

Spines are not reserved for cacti. There are a few plant families in southern Africa that have spines. Euphorbia, Pachypodium, and even some Aloe species have all evolved to have spines on either their stems. In the case of Aloe, some even have spines on their leaves.

Tough tubercles

The basic definition of a tubercle is merely a warty protrusion growing from the stem of a plant. These growths can vary greatly between different plant species. In contrast to spines or thorns, tubercles tend to only protrude slightly from the stem. In Haworthiopsis, tubercles can be fine white lines in some species, such as Haworthiopsis attenuata, or round nodules (bumps) on Haworthiopsis reinwardtii.

In the genus Tulista, plants have been extensively bred for decades by expert Japanese growers to exemplify their tubercles. This resulted in excellent cultivars like Tulista ‘Emperor’ and ‘Tears of Angels’.

Intriguing cilia

There is another form of stem protrusion that is much smaller than either a spine or a tubercle. These cilia are small, hair-like structures cover much of the leaf surface.

A valiant defense

All these structures on plants have a similar purpose, in that they are in defense against large herbivores from eating the plants, or keeping insect pests from doing the same. Spines and cilia in particular occur in many plant families that are not succulents either, and can be seen in many different parts of the world. If a plant can survive being eaten because it is a bit prickly, and its neighbour gets eaten because it is not, then that will mean more plants in the population will become prickly or spiny and avoided at all costs by herbivores. The ‘circle of life’, so to speak, though a circle that has some self-defense capabilities to it…

- Elita F

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