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Evolution of Micro-organisms and Plants


Permian Period, 298.9 - 252.2 MYA

During the Permian period, great changes occurred in the earth's climate and appearance. The warm zone spread in the northern hemisphere, where extensive dry deserts appeared. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover (see Figure 18). The old types of plants and animals died out. Ferns were no longer the dominant plant species; they were succeeded by gymnosperms, including conifers such as Walchia. These were very widespread and, unlike ferns, were able to grow in dry areas. The largest group of gymnosperms is the cone-bearing conifers, which have needlelike leaves that are well adapted to not only not summers but also cold winters and high winds. Most gymnosperms are evergreen trees.
Permian Conifer The reproductive pattern of conifers has several important innovations for adaption to the dry environment. First, the gametophyte (haploid state) is not dependent on external moisture. The non-motile sperm are enclosed within the pollen grain and are delivered to the egg with the growth of the pollen tube. Therefore, fertilization does not require external water for swimming sperm. Second, the female gametophyte is protected from drying out by remaining within the ovule, which still is attached to the cone. Third, the immature sporophyte (diploid state) is dispersed by the

Figure 18 Permian Plants [view large image]

Figure 19 Conifer Life-cycle
[view large image]

seed, within which it is enclosed. The seed need not germinate until conditions are favorable to support growth (see Figure 19).

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