Cycadopsida


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Cycadopsida

[sī·kə′däp·sə·də]
(botany)
A class of gymnosperms in the plant subdivision Cycadicae.

Cycadopsida

 

a class (also an order and family) of gymno-spermous trees usually having tuberous or radish-shaped trunks that are hidden in the soil. A few species have above-ground columnar trunks reaching a height of 18–20 m. The trunks are topped by a bundle of rather large (up to 3 m long), fernlike, pinnate leaves, which are stiff and leathery. The blades die after one or two years, while the bases of the thickest petioles, together with other thick, heavily tomentose, and squamiform leaves, form a sort of carapace on the trunk.

Cycadopsida are dioecious wind-pollinated plants. The microsporophylls have numerous sori on the underside and are closely arranged on the microstrobiles (male cones), which reach 80 cm in length and 20 cm in diameter. Each sorus contains two or three (sometimes five) microsporangia, in which grow numerous microspores. The microspores disseminate after forming three cells. The megasporophylls sometimes resemble small leaves (Cycas) but lack chlorophyll and are densely pubescent; most often they are gathered into megastrobiles (female cones), which measure up to 1 m long and weigh up to 40 kg. The ovules, of which there are usually two, are generally large, reaching a length of 5 cm. With the development of the megaspore, a group of reduced archegonia is formed. The male gametophyte develops in the pollen chamber, which is filled with a sugary fluid and which is also the formation site of two or more large spermatozoids. After fertilization, an embryo with two cotyledons is formed.

Cycadopsida include nine or ten genera, which embrace about 100 species. The plants are distributed in the tropics and subtropics of both hemispheres. The uniqueness of their structure, process of fertilization, and geographic distribution (most Cycadopsida have a discontinuous area of distribution), as well as their fossil remains, are evidence of the extreme antiquity of Cycadopsida.

REFERENCES

Takhtadzhian, A. L. Vysshie rasteniia, vol. 1. Moscow-Leningrad, 1956.
Schuster, J. Cycadaceae. Leipzig, 1932.
Arnold, C. A. “Origin and Relationships of the Cycads.” Phytomorphology, 1953, vol. 3, nos. 1–2.

M. E. KIRPICHNIKOV