Plants of the Palaeozoic Period

Charophyta (Stoneworts).—There is evidence of the presence of plants allied to the stoneworts in the Devonian ; Palaeonitella, from the Aberdeenshire chert, with its whorled branch segments, is very like a small Nitella.

Fungi (Moulds, etc.).—Abundant remains of fungal filaments and resting spores have been found in the Aberdeenshire chert and establish the presence of Phycomcycetous fungi in the Middle Devonian. Some of the filaments have numerous septations which as Kidston and Lang have suggested may indicate the presence of higher fungi (Eumycetes) as well. There are several records of Phycomycetes from the Carboniferous and Permian while F. E. Weiss has demonstrated the presence, in a coal-ball, of roots be longing to some gymnosperm infected with fungal hyphae. The distribution of the fungus in the tissues suggests that here is an example of the symbiotic relationship of fungus to higher plants which is known as mycorrhiza.

Bacteria.—A number of supposed fossil bacteria have been de scribed from Palaeozoic rocks but they are of practically no scientific value. It is however practically certain that bacteria existed because petrified plant remains are found which show clear evidence of decay and no traces of fungi are visible.

Thallophyta of Uncertain Systematic Position. Nematophyton. In the Silurian and Devonian, in several parts of the world, frag ments of a large plant are found the whole bulk of which was built up of loosely packed tubes, the larger running parallel to the axis while finer ones are found forming an inter-lacing system between them. In one specimen in which the outer surface is preserved there is a narrow zone where the tubes turn outwards and meet the surface at right angles. This type of construction is typically algal but the size of the fossils, some of which reach a thickness of three feet, show that they are quite unlike any living alga. In Sporocarpon from the Upper Devonian of America only the tips of the branches of what must have been a thalloid plant have been found. These tins are forked and rows of isolated tetrads of spores are found embedded in the tissue in each branch. The resistant nature of the spores suggests that they may have been cutinised like those of the Pteridophyta. Cutinised spores are also found in the lower Devonian plant Parka, the "puddock spawn" of the Forfarshire quarrymen, which consists of small circular thalli with small rounded masses of spores embedded in them.

Bryophyta.

Mosses and liverworts are found in almost every type of vegetation at the present day and it is surprising that they are so rare in the fossil state. On structural grounds they must be considered a relatively primitive and simple type of plant. Several fossil liverworts are known from the Upper Car boniferous of England. Hepati cites Kidstoni is a very small plant (fig. 2, B) with two paral lel rows of leaves one on either side of a relatively stout axis and two rows of smaller scale-like leaves along the surface of the axis. In H. W illsii (fig. 2, A) and H. Langii the plant is thalloid consisting of a ribbon-shaped body which forked repeatedly. In

H. metzgerioides a strand of con ducting cells is present along the middle of the thalloid body of the plant. All these liverworts appear to be most closely related to the anacrogynous liverworts and compare very closely with some of the living ones in their vegeta tive structure. There is no evi dence of the presence of acrogy nous liverworts. Small shoots have spirally arranged leaves preserved as incrustations. Muscites polytrichaccus, and a very small petrified stem, which has absorp tive hairs with oblique cross walls, M. Bertrandi, have been found in Upper Carboniferous rocks in France, demonstrating fairly con clusively the existence of mosses at that period.

Pteridophyta.

Psilophytales.—Among the several very incom pletely known plants of the Lower Devonian, Psilophyton princeps may be cited as representing one of an important group of plants which have certain primitive attributes. The name was given to some specimens from Canada by Sir J. W. Dawson more than half a century ago. Owing to the discovery of large masses of silica containing petrified plants in beds of Middle Devonian age in Aberdeenshire Kidston and Lang have been able to recon struct, almost in their entirety, some plants which are closely related to Dawson's Psilophyton princeps. The plants are found petrified in their position of growth and five species grouped in three genera have been distinguished which with Psilophyton con stitute the Psilophytales. One of them, Rhynia Gwynne-Vaughani (fig. 3, B) a small plant with cylindrical shoots, about 8 in. in height, is remarkable for having no leaves or roots. The lower parts of the shoots were horizontal and were furnished with small tufts of absorptive hairs on the under surface ; while the presence of stomata on the vertical parts bears witness to their having been aerial and green. Another species, Rhynia major, was a slightly larger plant but both species of Rhynia must have been much alike in appearance. Sporangia are found on the tips of some of the aerial branches containing numerous spores (Pl. I., fig. 2) which were produced in tetrads as in other Pteridophyta. The internal structure of the shoot is simple (Pl. I., fig. I) ; a single strand of xylem (wood) in the centre surrounded by a sheath of phloem (bast) constituted the vascular cylinder or stele. The cortex is wide in relation to the vascular tissue and consists of an inner zone of thin walled cells with large intercellular air spaces and an outer zone, two or three layers deep, of large cells which are elongated at right angles to the surface. The epidermis con sists of a single layer of smaller, slightly thicker-walled cells and had a well developed cuticle. The stomata are infrequent and each consists of two simple guard cells surrounding a pore. Small adventitious branches are found attached to some of the shoots and as there is no vascular connection between the adventitious branch and the shoot it is probable that these branches were easily detached and served to propagate the plant vegetatively.