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Evolution of Micro-organisms and Plants

Tertiary Period, 66.0 - 2.588 MYA

Tertiary Grass Vast swampy forests were a marked feature of the Tertiary period, appearing in many of the low-lying land areas that emerged as a result of extensive mountain-folding. To give themselves additional support, conifers grew long prop roots, while the lower trunks of deciduous trees became cone-shaped (see Figure 26). As the original carbon-containing material of the forests became buried, it was subjected to enormous pressure. With the Tertiary swamp deposit, the carbonization process only went to the stage of lignite, or brown coal. This contains much

Fig. 26 Tertiary Plants
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Figure 27 Grassland
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less carbon than the earlier Carboniferous deposits, which themselves vary in grade from bituminous (soft) coal to anthracite (hard coal). The appearance of grasses, which belongs to the angiosperm class of monocot, changed the Tertiary landscape as well as the diversity of animals.
The earliest grasses appeared in the early Tertiary but didn't really become a significant and distinctive presence on the land until much later. They probably spread across the plains in response to drier and cooler temperatures. The success of grass on the world's plains and savannas owes much to its structure, growth habits and biology. Unlike most plants, grasses do not grow from the tips of their shoots; instead they grow from the base. So, even when grass leaves are damaged, the plant continues to grow. The root system of grasses also ensures their durability: up to ninety percent of the weight of a typical grass plant lies underground, forming a densely tangled root system. Such an underground network helps grasses survive cold and dry spells, raging fires and herds of thundering grass-eaters.

Grasses, which cover up to one-third of the earth's land surface, have not only nurtured great herds of mammals but entire human civilizations. Characterized by narrow leaves with parallel veins and small inconspicuous flowers, grasses come in about nine thousand different species. Grasses such as rice, wheat, maize, oats, barley, millet and sorghum remain important staples of the human diet. Not only is grass widespread, but it grows in virtually every environment including the coldest and driest of regions. Grasslands are also referred to as savannas, parklands, or prairies (see Figure 27).

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