Yucca L. (1753),in the family Agavaceae, contains around 30 species in North America. Description Trees to 15m, or shrubs, sometimes stemless and suckering. Leaves sword-shaped, usually stiff and ending in a sharp spine. Flowers in tall, upright sprays; individual flowers large, white or purplish, bisexual, nodding. Sepals and petals similar, 6, waxy. Stamens 6, with small, short anthers. Ovary superior, with 3 cells and 1 thick style. Pollination is by moths. The fruits are capsules, sometimes fleshy, with numerous flat, black seeds.
Key Recognition Features The stiff, sword-shaped leaves on a thick trunk, and the white, hanging flowers.
Evolution and Relationships Yucca is related to Agave L., which has yellow, green, or brownish flowers and an inferior ovary, and to the large succulents Furcraea Vent. and Beschorneria Kunth. The pollination of Yucca is unusually specialised; the flowers are visited by the female of the yucca moth, Tegeticula, which collects pollen from the reduced anthers and carries it in special tentacles under her head. She then visits another flower, and if it is at a suitable stage, lays an egg among the young ovules, before stuffing the pollen into a cavity in the stigma of the flower; 1 egg is generally laid in each cell, and the stigma is pollinated each time. She then collects more pollen before moving on to the next flower. The developing larvae eat some of the seeds at the apical end of the capsule, which develop abnormally, but leave the lower seeds to develop normally. One species of Tegeticula pollinates yuccas east of the Rockies; another species pollinates Y. whipplei Torr., which has glutinous pollen, and another, which looks more like a sawfly, pollinates the tall, desert species Y. brevifolia Englm.
Ecology and Geography In deserts, on dunes, and in other open, sandy places in western North America northwards as far as Monterey County in California, and in eastern North America, on the coast northwards as far as Maryland.
Comment Several yuccas are grown for ornament, both for their spectacular , branching spikes of flowers and, in cultivars of some species, for their striped leaves. Photograph: Yucca brevifolia |
