Spore cones of horsetail trees bear sterile leaflets (bracts)
in whorls.
The sporangia sit between these whorls. Two types are distinguished based
on the way in which these sporangia are attached to the stem:
Palaeostachya , which has the sporangia
in the axils of the bracts
Calamostachys, which has the sporangia
at right angles between the whorls
Palaeostachya
Calamostachys
(drawings after Stewart & Rothwell 1993)
It is often difficult or impossible to determine the type of a cone.
Still attached spore cones of a horsetail
tree.
Sobernheim (Germ). Lower Permian. Length of the long cone 5
cm.
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