advanced Cycadean type, the microsporangiate organs were more
like those of a Fern, the reproductive organs thus showing the
same combination of characters which appears in the vegetative
structure. The family Calamopityeae, allied anatomically to Lyginodendreae,
is of Devonian and Lower Carboniferous age.
 |
| ||||||||||||||||||||
| (After Oliver. Scott, Studies.) | |||||||||||||||||||||
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Fig. 26.—Lagenostoma Lomaxii. Diagram of seed in median longitudinal section. | |||||||||||||||||||||
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| (After Arber. Scott, Studies.) |
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Fig. 27.— Lagenostoma Sinclairi. Two seeds, enclosed in lobed cupules and borne on branches of the rachis. (× 5.) |
Cycadoxyleae.—A few Coal Measure and Permian stems (Cycadoxylon and Ptychoxylon) resemble Lyginodendron in the general character of their tissues, but show a marked reduction of the primary wood, together with an extensive development of anomalous wood and bast around the pith, a peculiarity which appears as an individual variation in some specimens of Lyginodendron oldhamium. It is probable that these stems belonged to plants with the fructification and foliage of Cycads, taking that group in the widest sense. It is only quite at the close of the Palaeozoic period that Cycads begin to appear. The Lyginodendreae type of structure, however, appears to have formed the transition not only to the Cycadales, but also to the extinct family Cordaiteae, the characteristic Palaeozoic Gymnosperms (see p. 107).
|
| (From a sketch after Kidston. Scott, Studies.) |
|
Fig. 28.—Crossotheca Höninghausi, the male fructification of Lyginodendron. Fertile leaflets, bearing sporangia, and sterile leaflets on the rachis of the same leaf. (× 2.) |
Medulloseae.—In some respects the most remarkable family of the Cycad-fern alliance is that of the Medulloseae, seed-bearing plants often of great size, with a fern-like foliage, and a singularly complex anatomical structure without parallel among recent plants. Some of the Medulloseae must have had a habit not unlike that of tree-ferns, with compound leaves of enormous dimensions, belonging to various frond-genera—especially, as has now been proved, to Alethopteris and Neuropteris; these are among the most abundant of the Carboniferous fronds commonly attributed to Ferns, and extend back to the Devonian. In habit some species of Alethopteris resembled the recent Angiopteris, while the Neuropteris foliage may be compared with that of an Osmunda. The Medullosa stems have been found chiefly in the Permo-Carboniferous of France and Germany, but a Coal Measures species (M. anglica) has been discovered in Lancashire. The great anatomical characteristic of the stem of the Medulloseae is its polystelic structure with secondary development of wood and bast around each stele. In M. anglica, the simplest species known, the steles are uniform, and usually only three in number; the structure of the stem is essentially that of a polystelic Heterangium. In the Permo-Carboniferous species, such as M. stellata and M. Leuckarti, the arrangement is more complicated, the steles showing a differentiation into a central and a peripheral system; the secondary growth was extensive and unequal, usually attaining its maximum on the outer side of the peripheral steles. In certain cases the structure was further complicated by the appearance of extrafascicular zones exterior to the whole stelar system. The spirally arranged petioles (Myeloxylon) were of great size, and their decurrent bases clothed the surface of the stem; their structure is closely similar to that of recent Cycadean petioles; in fact, the leaves generally, like those of Stangeria at the present day, while fern-like in habit, were Cycadean in structure. In the case of Medullosa anglica we have an almost complete knowledge of the vegetative organs—stem, leaf and root; Cycadean characters no doubt predominate, but the primary organization of the stem was that of a polystelic Fern. In the new genus Sutcliffia, also from the Coal Measures of Lancashire, the stem had a single, large central stele, from which smaller strands were given off, forming a kind of network, which gave rise to the numerous concentric leaf-traces which entered the petioles. This plant may be regarded as anatomically the most primitive of the Medulloseae.
|
| (After Kidston. Scott, Studies.) |
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Fig. 29.—Neuropteris heterophylla. Seed, attached to a branch of the rachis bearing two vegetative leaflets. (× 2.) |
In one member of the Medulloseae, there is direct evidence of reproduction by seeds, for in Neuropteris heterophylla Kidston has demonstrated that large seeds, of the size of a hazel-nut, were borne on the frond (fig. 29). In this case the internal structure is not known, but another seed, Trigonocarpus Parkinsoni, associated with, and probably belonging to, the Alethopterid species, Medullosa anglica, occurs in the petrified condition and has been fully investigated. This is a large seed, with a very long micropyle; it has a beaked pollen-chamber, and a complex integument made up of hard and fleshy layers, closely resembling the seed of a modern Cycad; the nucellus, however, was free from the integument, each
