Page:An introduction to physiological and systematical botany (1st edition).djvu/253

 Rh in Atractylis cancellata. Linnæus observes that no bracteas are to be found in the class Tetradynamia.

The ochrea of Rottboll, Willdenow's Principles of Botany, 50, which enfolds the flower-stalks in Cyperus, see ''Engl. Bot. t.'' 1309, seems to me a species of bractea.

3. Spina. A Thorn. This proceeds from the wood itself, and is either terminal like Hippophae rhamnoides, ''Engl. Bot. t. 425, Rhamnus catharticus t. 1629; or lateral as Cratægus (or Mespilus) Crus-galli, tomentosa, parvifolia'', &c.

Linnæus observes that this sometimes disappears by culture, as in the Pear-tree, Pyrus sativa, which when wild has strong thorns; hence he denominates such cultivated plants tamed, or deprived of their natural ferocity. Professor Willdenow, Principles of Bot. 270, considers thorns as abortive buds, and thence very ingeniously and satisfactorily accounts for their disappearance whenever the tree receives more nourishment.