Plants of the Palaeozoic Period

PLANTS OF THE PALAEOZOIC PERIOD In this great division of Geological time the oldest rocks which contain evidence of contemporary plant life were laid down. The various groups of plants present in the Palaeozoic will be treated in systematic order while the distribution and succession of the floras will be briefly dealt with at the end of this section. The Glossopteris or Southern type of Carboniferous and Permian flora will be considered in the section on the Mesozoic Period.

Thallophyta.

Algae (Seaweeds, etc.).—As a class the Algae are difficult subjects to investigate in the form of fossils. Except in the case of a few of the groups of this large division of the plant kingdom the plant body is soft, consisting of a very large proportion of water, and therefore offering a very small amount of solid matter for the making of a fossil. In external form so many Algae are alike ; they are often undifferentiated into stem and leaves, i.e., thalloid, and unless there is information available about their method of reproduction external form alone is of little value in determining their systematic position. A great many supposed fossil-algae have been described and named but it can often only be said about them that the outline of the mark they make on the rock suggests the outline of the frond of a sea weed. If none of the carbon aceous constituent of the plant is left and no cellular structure pre served such a fossil is of no scien tific value. Some purely physical causes such as the flow of a small trickle of water over a mud sur face can produce shapes like branching fronds and it has been shown fairly conclusively that certain marks which at one time were supposed to represent Algae in some early rocks are no more or less than the tracks of worms or some other animals in the mud which was finally consolidated to form the rock. Where organic residue is present and there is evidence that the structure is really of vegetable origin fur ther information cannot be got without cellular structure as a guide as there are other plants beside algae which have a thalloid form, e.g., some Liverworts.

The Schizophyceae were present in all probability as early as the Cambrian. Marpolia spissa, one of several thalloid and fila mentous types found in Middle Cambrian rocks in British Colum bia shows structure which indicates that what appear to be branch ing filaments are really compound, consisting of several cellular filaments united in a sheath. The cells of the filaments are ex ceedingly small. These features are also exhibited by the living

genus Schizothrix of the Schizophyceae. Archaeothrix oscillatori.

formis discovered by Kidston and Lang in the Middle Devonian chert of Aberdeenshire is an Alga consisting of cellular filaments which are so like those of some living Schizophyceae that there is no doubt of its systematic position in that group. Pachytheca, a small spherical algal colony built up of filaments of a similar type is a characteristic fossil of the Silurian and Devonian and appears to be related to the Schizophyceae.

The Clzloropliyceae (Green Algae) are not certainly known as fossils but it seems probable that they may have existed in the Palaeozoic. A considerable deposit of a combustible material is known in Silurian rocks to the east of the Baltic. This deposit (Kuckersite) consists of a f ossil-alga Gloeocapsomorpha (fig. C). The affinities of this plant are doubtful but it may well be a green Alga. It has been shown that many bituminous shales and cannel coals are mainly formed of a deposit of small algal colonies. Pile and Reinschia (fig. 1, A and B) form the main algal constit uent of many shales and coals of Carboniferous and Permian age. The building up of combustible deposits of a similar nature is taking place at the present day : a green alga, Botryococcus coorongiana, almost identical in the form of its cell-colonies with Reinschia, forms masses of tough bituminous matter on the shores of certain South Australian lagoons. While alive these algae store up oil in their cells and it is this oil which on the death of the organism permeates the whole mass and gives the bituminous properties to this "coorongite" and presumably also to the fossil deposits built up of algae with a similar cellular structure such as Reinschia. The existing families of the Codiaceae and Dasy cladaceae are represented as far back as the Ordovician and pro vide us with the longest and perhaps most complete historical record of any living group of plants. In several of the genera of both families a thick calcareous deposit accumulates on the out side of the cell-wall and it is in virtue of this deposit that they have been preserved and that some are important constitutents of limestones formed during various geological periods. Dimor phosiphon, Paiaeoporella and Cyclocrinus are some of the Ordovician genera. Dimorphosiphon is remarkably like the living genus Codium but differs in having a secretion of lime on its surface.