\CJuip. LU PLANTS OF THE PAST 727 



Pre-Cambrian and Paleozoic floras. The fossil record has also shown 

 that the simplest and least organized plants were the first on the earth. 

 Studies of the Archeozoic and Froterozoic rocks have up to this time 

 yielded no evidence of organisms more complex than the bacteria and 

 blue-green algae. But early in the Cambrian there is evidence that the 

 blue-green, the green, the red, and the brown algae — all lime-depositing 

 species — were present, and it is probable that they had been present 

 also in pre-Cambrian waters. So meager are the facts about the algae 

 of the Cambrian and Ordovician that any description must await the 

 disco verv of better-preserved fossils. 



Silurian rocks contain fossils of vascular land plants which indicate 

 a long ancestry of terrestrial plants. Their dichotomous thalloid vege- 

 tative body bore apical sporangia containing four spores in a tetrad. In 

 plants of today, the formation of spores in tetrads is associated with 

 reduction division and sexual reproduction. These plants have been 

 distinguished as Psilophvtes ( Fig. 361 ) . 



The middle Devonian Psilophytes included plants with upright di- 

 chotomouslv branched stems, having stomates, scale-like appendages, 

 and terminal sporangia. Moreover, the growing stem tips uncoiled like 

 those of fern leaves. Some species had spirally arranged scales, others 

 were naked. The stems consisted of distinct tissues: epidermis, cortex, 

 phloem, and xylem. The spores occurred in tetrads in terminal sporangia. 

 The upright stems developed from rhizomes, and some were several 

 inches in thickness. These are a few of the characteristics of early land 

 plants known from only a few scattered localities in Australia, North 

 America, and Europe. 



Primitive lycopods were also present in the middle Devonian, and 

 the tree species became the dominant plants of Carboniferous time. 

 Lepidodendrons and Sigillarias are the best known of these plants ( Fig. 

 360). They had lance-shaped leaves, sporophylls arranged in terminal 

 cones on branches, and their stems possessed cambiums by which sec- 

 ondary thickening occurred. The leaf scars and their imprints are among 

 the commonest and most beautiful coal shale fossils. 



The ancestral plants of present-day equisetums, the Calamites, were 

 present also. Some of them were trees with jointed stems up to 100 

 feet tall. Apparently they were common in the Devonian and abundant 

 in the Carboniferous. Their branches occurred in whorls. Leaves were 

 simple and the sporophylls were arranged in cones. Some had large 

 underground rhizomes. 



