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1911 Encyclopedia Britannica
THORN (0.(0. Eng. porn, cf. Du. doom, Ger. Dorn, &c), in botany, a hard pointed structure, also termed a "spine," generally representing a small branch, as in hawthorn, where a normal branch arising in the axil of a leaf is replaced by a sharply pointed thorn; accessory buds on each side of the thorn and developed in the same leaf-axil will grow in the next season into ordinary branches. The similarly developed thorns of the honey-locust ( Gleditschia ) are branched. In other cases, as the sloe or the wild pear, branches become spiny at the apex tapering into a stiff leafless point. On a cultivated tree these branches disappear owing to their more vigorous growth. Leaves may be modified into spines, as in barberry, the leaves of which show every gradation between a leaf with a spiny-toothed edge and those which have been reduced to simple or multiple spines. In some species of Astragalus the petiole of the pinnately compound leaf persists after the fall of the leaflets as a sharp spine. In the false acacia ( Robinia ) the stipules are represented by spines.
The reduction of the leaf-surface, of which the spinous habit is often an expression, is associated with growth in dry or exposed windy places. Thus, in the gorse, a characteristic plant of exposed localities such as open commons, the smaller branches, instead of being leaf-bearing shoots, are reduced to slender green spines, while the leaves on the main shoots are also more or less spinous in character. As the giving off of water from its surface is one of the chief functions of a leaf, this process is thus reduced to a minimum in situations where water is scarce or would be liable to be given off too rapidly. An extreme case is afforded by the cacti and cactus-like euphorbias, which are a characteristic type of desert vegetation where water is extremely scarce. The whole plant is reduced to a simple or branching succulent, leafless, columnar or flattened stem, the branches of which are represented by small clusters of thorns. Incidentally the thorns protect the plant which bears them from the attacks of animals seeking food.
Prickles are structures of less importance from the morphological point of view, being mere superficial outgrowths which may occur anywhere on stem or leaf, or even fruit.
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Chisholm, Hugh, General Editor. Entry for 'Thorn (Botany)'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/t/thorn-botany.html. 1910.