Division: Pteridophyta (Filicophyta)
Class: Pteridopsida (Filicopsida)
The order Cyatheales includes most of the tree ferns. The few tree ferns not in the Cyatheales generally have trunks no higher than two meters tall, while some tree ferns of the Cyatheales have been known to have trunks fifteen meters tall.
The cyatheoid ferns appear to be a unique group of fairly old derivation for the leptosporangiate ferns. It has frequently been suggested that the Dennstaedtioid ferns are allied with the Dicksonioid portion of the Cyatheales (that is, Dennstaedtioid sensu strictu, maybe or maybe not including the Hypolepoid ferns). This is possible, given that there is reason to propose an ancestral count of n=12 for the Dennstaedtoids (but generally not the Hypolepoids). The split, however, would not have been recent. The cyatheoids have several significant primitive characteristics not shared by the dennstaedtioids, which in turn share advanced characteristics with other more advanced ferns, characteristics not seen in cyatheoids. There is a significant possibility of convergent evolution, however.
The chromosome count origins of the Cyatheales are not clear. Almost all members of the family Cyatheaceae have a count of n=69, with just a few of n=70; the family Dicksoniaceae has counts of n=56, 58, 64, 65, 66, 68; Lophosoria quadripinnata, the sole species of the Lophosoriaceae, has a count of n=65; the Loxomaceae has counts of n=46 and 50 and an uncertified count of n=47; Metaxya rostrata, the sole species of the Metaxyaceae, has a count of n=96, and the Plagiogyriaceae has a count of n=66 and approximate counts of n=75, 100, 125 and 132. I would propose that the ancestral count was n=12, increased from the n=11 of the ancestral stem line. The count of the Cyatheaceae could be explained by tetraploidy to n=24, reduction to n=23, then hexaploidy to n=69. The counts of the Dicksoniaceae and Lphosoria could then be reductions from this count. Metaxya could be derived from n=12 by hexadecaploidy (that is n=12->n=24->n=48=>n=96). The Loxomaceae is less clear.
Families of Cyatheales:
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This page was last revised on 11-10-1997.