.SECT. I CRYPTOGAMS 471 



frequently helong 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 Upper Cretaceous. 

 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 Phycomycetes, Eumycetes, and probably the 

 Myxoniycetes 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 

 back to the Carboniferous. 



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

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

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

 their relative importance in this respect diminislied. 



1. The class of the Equisetlnae represented at present by the single genus 

 JSquisetum, 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, btit 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 

 {Annular ia) stood in alternating whorls ; their form was narrowly lanceolate and 

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

 calamites, they were dichotomously divided. The cones or flowers {Calamostachys) 

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

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

 some of the Calamarieae were heterosporous. 



2. 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- 

 ness. They had long narrow leaves, which Avhen they fell off" left longitudinal 

 rows of hexagonal leaf-scars on the surface of the stem. Long-stalked, cone-like 

 flowers were borne on the stem ; only one kind of spore was contained in the 

 s})orangia which were borne singly on the sporophylls. 



The Lepidodendreae extend from the Lower Devonian to the Rothliegende, but 

 are also best developed in the Carboniferous period. They were tree-like plants 

 attaining a height of some 30 metres with dichotomously branched stems which 

 grew in thickness. The leaves, which attained a length of 15 cm., were spirally 

 arranged and seated on rhombic leaf-cushions. The cone-like flowers [Lepidostrobus) 

 were borne on the ends of branches or sprang from the stem itself; each sporophyll 

 bore a single sporangium, which contained either macrospores or microspores. 



The leaves of Sifjillaria and Lepidodendreae are characterised by the possession 

 of a ligule springing from the upp^r portion of the leaf-base. 



Smaller Lycopodiaceae, the predecessors of the existing species of Lycopodimn, 



