466 BOTANY PART n 



preservation, among the best known ; of these, forms are known from the Tertiary 

 strata back to the Silurian, while the Corallineae, which are calcareous Red Sea- 

 weeds, appear from the Upper Jurassic onwards. Among unicellular Algae the 

 Diatomaceae, which have a silicified cell wall, are well preserved. The fossil forms 

 frequently belong to existing genera, and occur from the Jurassic onwards. 

 especially in the Cretaceous and Tertiary strata, often forming thick layers of 

 Kieselguhr (diatomaceous earth). From Tertiary times onwards Characeae are 

 abundant, and occasional remains are found as far back as the Muschelkalk. Most 

 of the existing groups of Algae are only to be distinguished with certainty from 

 Tertiary strata onwards. 



Bacteria must have played their part in causing the decomposition of organic 

 substances from very early times ; they can be distinguished in vegetable remains 

 from Carboniferous rocks. The Eumycetes and probably the Myxomycetes also, were 

 present in Carboniferous times ; Ascomycetes living on leaves and stems are found 

 from the Carboniferous onwards in all strata. Remains of existing genera of 

 Lichens appear in the Tertiary rocks. 



II. Bryophyta. Most of the examples of this group, which is rarely met with 

 in the fossil condition, come from the Tertiary strata and resemble existing genera 

 closely. Only isolated examples of Liverworts and Mosses occur in the older rocks 

 (Jurassic, Upper Triassic). 



III. The Pteridophyta extend back to Silurian times, but were most highly 

 developed in the Carboniferous period, when they formed the main mass of the 

 land vegetation. With the advent of Gyninosperms, and later of Angiosperms, 

 their relative importance in this respect diminished. 



1. The class of Filicinae was represented by forms belonging to both sub- 

 classes at the end of the Silurian period, and large numbers of species occur in 

 the Carboniferous strata. The organisation of these ancient types is essentially 

 similar to that of the existing forms. Most of the recent families were represented, 

 some (e.g. Marattiaceae) more fully than at the present time. The Cycadofilices, 

 fern-like plants with secondary thickening, which already bore seeds, appear to 

 have given rise to the Phanerogams, or at least to the Cycadaceae. The other 

 classes of Pteridophyta do not appear to have given origin to higher forms. 



The Hydropterideae are known with certainty from the Tertiary rocks, but 

 Salvinia and Marsilia can be traced back to the Chalk. 



2. The class of the Equisetinae represented at present by the single genus 

 Equisetum, which can be traced back to the Triassic period, was much better 

 developed in Palaeozoic times. The large order Calamarieae consisted of plants 

 resembling the Horse-tails in general habit, but in some cases attaining the size 

 of trees 30 metres high ; the hollow stem bore whorls of branches at the nodes, 

 was covered with a periderm, and underwent secondary thickening. The leaves 

 (Annularia] stood in alternating whorls : their form was narrowly lanceolate and 

 at their bases they united into a sheath. In the most ancient type, Archaco- 

 calamites, they were dichotomously divided. The cones or flowers (Calamost<iclii/s> 

 had in some the same structure as those of Equisetum ; in most cases they w ere 

 more complicated, whorls of scale-leaves alternating with the sporophylls. At least 

 some of the Calamarieae were heterosporous. 



3. The Lycopodinae were also abundantly represented in Palaeozoic times, 

 especially by the two great extinct orders, the Sigillarieae and the Lepidodendreae. 

 The Sigillarieae, found from the Culm onwards, are most numerous in the Car- 

 boniferous period, and persist with one species in the Bunter Sandstone. They 

 were stately trees, with but little branched, pillar-like stems, which grew in thick- 



