Seed+Plants

 Seed plants are divided into gymnosperms and angiosperms. Gymnosperms produce seeds on the surface of cones while angiosperms produce seeds within flowers. ** Reproduction **  Seed plants do not require water for the fertilization of gametophytes, so they can grow in a variety of climates. They use flowers and cones, pollination, and seeds to reproduce.  Gymnosperms use cones and angiosperms use flowers to produce and store seeds.  Pollination occurs when a male gametophyte enclosed in a pollen grain is carried a female gametophyte by wind or animals.  Fertilization transforms the zygote in a seed into an embryo. When the embryo is still small, it stops growing and stays dormant until conditions are favorable. It them begins to grow using the energy stored in the endosperm.  A seed includes a plant embryo, an endosperm food supply, and a seed coat which protects the embryo and keeps it from drying out. Some seeds have specialized structures which aid in their dispersal such as sticky coating or fleshy tissues which are eaten by animals. Evolution  The ancestors of seed plants formed many adaptations so that they could live in environments too dry for moss and ferns. The most important adaptation was that of seeds.  During the Coniferous and Devonian periods between 300 and 400 million years ago land environments were much wetter than they are today. This allowed mosses and ferns to spread farther inland than they previously had been. However, when the land became dry millions of years later, most of the moss and fern species died and were replaced by seed plants.  Fossil records reveal that seed plants originated 306 million years ago and spread to all the major landmasses. Gymnosperms  Gymnosperm means “naked seed”. They include conifers, cycads, ginkgoes, and gnetophytes and are the oldest surviving seed plants.  Conifers are the most common gymnosperms. The phylum Coniferophyta contains over 500 species including pines, spruces, firs, redwoods, and yews. Conifers grow in a variety of habitats and are well adapted to dry climates. It is thought that when they originated 250 million years ago they evolved thin needle-like leaves which reduce water loss by decreasing surface area. The leaves also developed waxy coatings and stomata on their undersides to avoid water loss. Most conifers are evergreens which only lose their needles as they grow old.  Cycads are palm-like plants which evolved during the Triassic period 225 million years ago. Today the phylum Cycadophyta only includes nine genera which grow in tropical regions around the world.  Ginkgoes were common during the Mesozoic period. Only one species of the phylum Ginkgophyta remains: //Ginko beloba.// It is one of the oldest seed plants alive and is common in cities in the United States.  Gnetophytes are an odd group of gymnosperms. The phylum Gnetophyta includes 70 species which have reproductive scales organized into cones. Examples include //Welwitschia,// an African plant with two leathery leaves that crow continuously, and the genus //Ephedra// which is known as Mormon tea in the United States.
 * Seed Plants **