Stems. But in nearly all perennial Dicotyledons, in all dicotyledonous and gymnospermous trees and shrubs and in fossil Pteridophytes belonging to all the great groups, certain layers of cells remain meristematic among the permanent tissues, or after passing through a resting stage reacquire menstematic properties, and give rise to secondary tissues.
A thorough examination of cycadean seeds has recently been made by Miss Stopes, more particularly with a view to a comparison of their vascular supply with that in Palaeozoic gymnospermous seeds (Flora, 1904).
It is of interest to note that the leaves of Gnetum, while typically Dicotyledonous in appearance, possess a Gymnospermous character in the continuous and plate-like medullary rays of their vascular bundles.
The seed-like body was detached as a whole from the cone, and in this condition was known for many years under the name of Cardiocarpon anomalum, having been wrongly identified with a true Gymnospermous seed so named a seed are obvious; the which is not tubular, but forms a long crevice, running in a direction radial to the strobilus.
Gymnospermous remains are common in Palaeozoic strata from the Devonian onwards.
The modern Gymnospermous orders have but few authentic representatives in Palaeozoic rocks.
Among Pteridosperms it is the family Medulloseae which is especially characteristic. Cordaiteae still increase, and Gymnospermous seeds become extraordinarily abundant.
The Upper Carboniferous and Permian plants may be grouped together as constituting a Permo-Carboniferous flora characterized by an abundance of arborescent Vascular Cryptogams and of an extinct class of plants to which the name Pteridosperms has recently been assigned - plants exhibiting a combination of Cycadean and filicinean characters and distinguished by the production of true gymnospermous seeds of a complex type.
In Bennettites the ovules are left exposed at the apex, but they are by no means so distinctly gymnospermous as.