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The cycads (Fig. 1) constitute remnant species of an ancient class of gymnosperms, the cycadophytes, that evolved from the free-sporing progymnosperms, which also gave rise to the coniferophytes. According to Gifford & Foster (1989), the cycadophytes have included 3 orders of plants, the extinct Cycadeoidales and Pteridospermales (seed ferns), that are known only from the fossil record, and the Cycadales, that includes the cycads. The cycadophytes are supposed to be ancestral to the Gingkoales and possibly, the Gnetales (Crane & Upchurch, 1987; Taylor & Taylor, 1993). The Cycadales are believed to have originated during the Permian era, and to have flourished during the Mesozoic period, probably peaking in distribution and success during the Jurassic period. They have been referred to as “living fossils” (Gilbert, 1984). Because of their great antiquity, the living cycads have been considered to be invaluable for the study of developmental events (Norstog, 1987). Currently, the Cycadales are comprised of only 3 families, the Cycadaceae, the Stangeriaceae (monogeneric) and the Zamiaceae.
citations This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically). | 10 | |
popularity This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network. | Average | |
influence This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically). | Average | |
impulse This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network. | Average |