Thorns in Plants: Their Role and Origin

Experts assert that thorns in different plants can form from various organs and tissues.

Thorns should be distinguished from spines—hard, massive, pointed outgrowths of the epidermis and underlying tissues that form on stems and petioles (less
often on leaves) of many plants, such as species of the rose family from the genera Rubus (raspberry) and Rosa (rose). Although they are morphologically similar
to thorns, they do not have a strict order in their arrangement, meaning they are not tied to nodes, leaf axils, etc. Thorns, on the other hand, have a strict
 correspondence in their presence and location to other parts of the plant, which spines do not.

It is believed that thorns are the result of evolutionary changes in plant parts. In different plants, shoots (hawthorn), leaves (barberry), parts of leaves, bud
scales (cactus), and even roots (in some palm species) have gradually transformed into thorns.

Plants with thorns are most commonly found in regions with hot or arid climates.

Classification of Thorns

By structure:

• Simple
• Branched

By location:

• Radial
• Central

Functions of Thorns

The following functions of thorns are usually highlighted:

1. Water Supply. In this case, thorns serve to condense water vapor. Water flows down them to the stems and is absorbed by the plant.
2. Plant Protection. This can be protection from being eaten by animals, from too bright sunlight, from damage, etc.
3. Reproduction. In some plants, thorns are intended for seed dispersal. Such seeds have a hooked end that clings to the fur of animals, which then
   spread the seeds. This type of seed dispersal is called zoochory. Some species of cacti have thorns that secrete nectar and attract insects for
   pollination (Coryphantha, Ferocactus).

Examples of Thorns in Plants

• In sea buckthorn, thorns are short branches.
• In cacti, thorns are modified leaves.
• In thistle, thorns are derivatives of the leaf edge.
• In roses, thorns are outgrowths of the epidermis.

The easiest evidence can be provided for sea buckthorn. During the growing season, the thorn is covered with leaves, and after leaf fall, buds can be seen
on the thorns, which were laid in the axils of the fallen leaves. Leaves can only attach to stems, so the thorns of sea buckthorn are short sharp stems, i.e.,
branches.

It is somewhat more difficult to prove the leaf nature of cactus thorns. Firstly, there are never buds, leaves, or side shoots on the thorns. This indicates that
the stem nature of the thorns is unlikely. Secondly, if you look closely at where the side shoots or flowers of cacti are laid, you can see that they are always
located in the axils of the thorns. In some species of cacti, early falling leaves (some Opuntia) or well-developed leaves (genus Pereskia) develop in place
of the thorns. All this confirms the leaf nature of cactus thorns.

Examining the thorny edge of a thistle leaf, it is easy to see that the thorns form a single whole with the conducting veins of the leaf—the veins seem to grow
beyond the edge of the leaf.

Difference Between Spines and Thorns Using the Example of a Rose

Rose spines are scattered along the stem and leaves, and sometimes on the false fruits of the plant. If spines were leaves, they would not be able to grow on
leaf petioles. The fact that side shoots never develop in the axils of spines further argues against the leaf nature of spines. They are also not side branches,
which can only grow in the axils of leaves, while in roses, spines are scattered along the stem.

Anatomical sections show that rose spines are not connected by conducting elements to the conducting system of the stem. This indicates that rose spines
are outgrowths of the epidermis.