3. CARBONIFEROUS PLANTS AND THE FORMATION OF COAL The carboniferous time, or the period when the earth was covered with huge forests of strange shrubs and trees, most of which were unlike their modern successors, apparently had a climate so nearly uniform and seasonless that fossil remains of these plants have been found throughout the world. Even in the Arctic the rock strata show the flourishing of forests that must have needed a climate very different from the frigid condition there to-day, and furnishing indisputable evidence of a warm, most probably frostless, climate practically throughout the world. [Illustration: FIG. 108.--HORSETAIL (_Equisetum hyemale_) A modern horsetail or scouring rush, common in the north temperate zone. Ancestors of these formed huge forests at the time that coal was being formed.] The giant club mosses have already been mentioned, with their persistence to the present day in much reduced number, and vastly reduced sizes. No one can picture the grandeur of those ancient forests, peopled with queer animals long since extinct and with dragon flies known to have a wing-spread of two feet or more. But with the club mosses were giant horsetails, which in somewhat changed form have also come down to our times, but in much reduced stature and frequently are familiar enough as weedy plants along railway embankments, and sometimes in more natural environments. Most of our modern representatives of the genus _Equisetum_ (Figure 108), or horsetails, are low herbs, but one South American kind still retains the ancient habit of growing to considerable heights, as specimens up to twenty feet high are known. Related to the ancient treelike horsetails were queer vines with slender twining stems, which, judging from their fossil remains, must have been very common. Both the ancestors of our club mosses and the horsetails must have occupied vast swampy areas, as their stem structure indicates a fondness for water, to which, as we have already seen, their still more ancient ancestors were always confined. Vigorously competing with these plants for occupancy of those great swamps were vast quantities of plants that have been called cycadlike ferns from their likeness to ferns on the one hand and plants like the so-called sago palm on the other. The sago palm, or _Cycas revoluta_, is a modern representative of these ancient forms, and retains the remarkable characteristic of having its male fertilizing cell capable of movement as we know to be the case in nearly all cryptogamous plants. Yet _Cycas_, with its related genera, which are found in nearly all the warmer parts of the earth to-day, are true flowering plants which bear cones. We see, therefore, in these old cycadlike ferns one of the first, almost experimental, evidences of the seed habit, and consequently the breaking away from the spore habit which overwhelmingly characterized the reproductive processes of its ancient associates.