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Tiny but Complex Foliage Leaves Occur in Many “Leafless” Cacti (Cactaceae)
James D. Mauseth
International Journal of Plant Sciences
Vol. 168, No. 6 (July/August 2007), pp. 845-853
Published by: The University of Chicago Press
Stable URL: http://www.jstor.org/stable/10.1086/518273
Page Count: 9
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Most cacti in subfamily Cactoideae have tiny foliage leaves (most less than 1.5 mm long) hidden by enlarged axillary buds (the spine clusters). Leaf development similar to that of Arabidopsis and other model plants occurs in six species that have a rudimentary lamina, mesophyll, vascular tissue, stomata, and dorsiventral asymmetry. In the other 141 species, mature foliage leaves are progressively simpler, as if development stops at different points during otherwise ordinary morphogenesis. In 21 species, development stops at the leaf primordium stage, and in 52 species, mature leaves are up to 500 μm tall (distinctly larger than primordia) but consist of just ground tissue and ordinary epidermis cells. Mature leaves in 71 species are elongate (500–2500 μm) cones of epidermis and ground tissue, with xylem near the adaxial side (phloem was detected in only four species); most conical leaves have abaxial stomata. Chlorenchyma is universally present in mesophyll; mucilage and druses occur in some species. Leaf reduction may have reduced auxin production: long, complex leaves occur on columnar plants; short, simple leaves are on globose plants. Expression of genes such as KNOX, PHANTASTICA, and YODA were probably modified during leaf evolution in cacti.
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