Plants of the Palaeozoic Period

Gleicheniaceae. The evidence for the presence of Gleicheniaceae Chansi district in China also of Upper Carboniferous age has as many as 20 sporangia in each group or sorus. It is not until the early part of the Mesozoic that undoubted Gleicheniaceae are encountered.

Pteridosperms.

A considerable number of fossils are known which have in their vegetative structure such similarity to the Ferns and indeed may well have evolved from an ancestral group of Ferns by developing in the course of their history an advanced type of heterospory in which seeds (i.e., integumented megaspo rangia) were produced and pollination or the provision of pollen grains (microspores) to the seed took place before the seed was liberated from the parent plant. No Pteridosperms have survived to the present day, they were apparently extinct by the middle of the Jurassic period. Eospermatopteris, a plant rather like a tree-fern in habit with large fronds consisting of finely divided rachises and practically no lamina, has been found in the Upper Devonian rocks of New York with seed-like bodies attached to its fronds. Hostimella racemosa a Middle Devonian fossil from Scotland consisting of branching linear stalks bore, on short lateral branches, oval-bodies which though certainly sporangia might con ceivably be seeds while fronds called Aneurophyton (fig. 14) have been found in the Middle Devonian of Germany closely resem bling the fronds of Eospermatopteris: so that there is a certain amount of circumstantial evidence for the presence of Pterido sperms as early as the Middle Devonian. In the Lower Carbon iferous Pteridosperms form an important constituent of the vege tation and maintain that position right through the Carboniferous, Permian and Triassic ; in the Jurassic they disappear. The Pteridosperms form a large and varied group but agree in having fern-like fronds and secondary woody thickening as a normal feature in their vascular construction. Lyginopteris Oldhamia the most completely known Pteridosperm is of Upper Carbon iferous age and is of common occurrence in the coal-measures round Oldham and other parts of Lancashire. It is found petrified in the coal-balls and also as impressions in the shales forming the roofs of the seams. The frond (Plate I., fig. 3) by an equal fork ing of the leaf-stalk is divided into two equal divisions. The stem (fig. 15) varies from 2 mm. to 4 cm. in diameter. There is a pith forming the core of the stem surrounded by a ring of scattered vascular strands which are connected with the leaf supply system. In older stems the ring of strands is sur rounded by a considerable quantity of secondary wood and bast. The outer cortex contained long anastomosing bands of fibres which formed a reticulum just below the epidermis. In Heter angium, a closely allied genus the centre of the stem is occupied by a solid core of primary xylem and the strands of the leaf-trace system were incorporated in the outer part of the primary xylem. Oliver and Scott in 1903 were able to identify a petrified seed found in a coal-ball, as the seed of Lyginopteris by the presence on its husk of glandular spines exactly like those on the frond and stems of Lyginopteris. The seed, Lagenostoma Lomaxi (fig. 16 A and B), is enclosed in a lobed husk or cupule ; it is barrel shaped and its integument is furnished with a vascular system.

One part of the nucellus (sporangium wall) is modified to form a complicated pollen chamber or cavity in which the pollen be came lodged. In structure the seed is very

similar to those of the living Cycads or Ginkgo but the pollen-chamber is rather more complicated than any of these living plants. Kidston described some tassels of microsporangia (fig. 17) attached to small pieces of fronds which bore leaflets of the same shape as those of Lyginopteris and there is no doubt that they represent the pollen producing organs of this plant.

In most seed-bearing plants the micro sporangia are borne on highly modified structures, stamens, which are quite unlike leaves in appearance; but in many Pteridosperms the fronds which bear the microspo rangia are very like the sterile foliage fronds and in this respect the Pteridosperms are primitive.

The Medulloseae, another important group of Pteridosperms, of Carboniferous and Permian age had fern-like foliage belonging to the form-genera Neuropteris and Alethopteris. The stems, Medullosa, which are known to belong to the group are peculiarly complicated in structure ; in place of the usual single vascular column they had several. Each vascular column was in structure like that of Heterangium and the stem could be briefly described as a polystelic Heterangium. The spirally arranged leaf stalks are of large size and contain a large number of small collateral vascular strands. The appearance of these petioles in section has given rise to the erroneous report that Monocotyledonous Angio sperms were present in the Carboniferous ; they are however very like the petioles of Stangeria, one of the living Cycads. Another genus Sutcliffia had a large central stele with other smaller ones forming a system of meshes round it. Seeds have been found attached to fronds of Neuropteris (fig. 18, B) and Alethopteris. Trigonocarpon is a genus of seeds which were borne on some species of Alethopteris; it resembles the seeds of living Cycads in having a testa consisting of both soft and stony layers. The micropyle was of very considerable length. The pollen-producing organs of these plants (fig. 18, A and C) are still imperfectly known. Potoniea (fig. 18, C) very probably represents the pollen bearing flower of a Neuropteris. The structures with toothed margins were originally cup-shaped with microsporangia placed on the inner surface of the cup. Linopteris an allied frond-genus had similar microsporangiate fructifications. Another fructifica tion Whittleseya had large cups more than an inch deep and large pollen grains have been isolated from them. In Telangium the microsporangiate fructification consists of synangia of from 6 to 25 fusiform sporangia attached to small terminal expansion of the divisions of the fertile frond. In Telangium bifidum and Telan gium teilianum, both from the Lower Carboniferous, the fertile part of the frond branched repeatedly at wide angles. In T. teilia num the fertile part was at the extremity of the frond (fig. 19) and two large pinnae branching off from the rachis on either side acted as the foliage part of the frond. Several specimens have been found in which there is a small abortive branch in place of the fructification and the frond was entirely vegetative in function. In T. teilianum the pollen producing synangia are known to have been carried on the frond in this manner while in T. bifidum, a closely related species small seed-husks have been found similarly placed. The nature of the stems of Telangium is unknown.