Introduction to gymnosperms, general characters, life cycle, diversity and origin of gymnosperms.

Abstract

Gymnosperms are a diverse group of seed-bearing plants that are distinct from flowering plants in that they do not have carpels that encapsulate their seeds as flowering plants do. It is believed that all gymnosperms originated from a common ancestor in the Devonian period known as "progymnosperm stock." Gymnosperms were able to exploit upland and drier habitats in the late Paleozoic because of key adaptations that involved the retention of the megagametophyte within a protective coating to form a seed on the parent sporophyte, the dissemination of the microgametophyte in durable pollen, the production of complex root systems, and extensive development of secondary xylem in the stem. These habitats had not been occupied by earlier free-sporing In the late Paleozoic, gymnosperms underwent a period of diversification and were the primary donors of organic matter to some of the greatest coal deposits in the world, which were accumulating at that time. After going through a period of widespread extinctions at the end of the Permian, gymnosperms re-radiated in the Triassic and continued to dominate the global floras until the middle of the Cretaceous.