The three main organs in a plant are roots , stems, and leaves. Roots absorb water and dissolved nutrients from soil. They anchor plants to the ground. Roots are able to do all of these jobs because they grow , they develop complex branching networks that penetrate the soil and grow between soil particles. Stems hold a plants leaves up to the sun. Leaves are the organ in which plants capture the suns energy. These tissues must be protected against water loss to dry air. That’s why most plants are covered in waxy coating called cuticle.
Plants have a well developed two way plumbing system consisting of xylem and phloem Xylem is primary responsible for carrying water and dissolved nutrients from the roots to stems and leaves. Phloem carries product of photosynthesis and certain other substances from one part of the plant to another. Seed plants do have alternation of generation. However life cycles of seed plants are well adapted to the rigors of life on land. Flowers and cones are specialized reproductive structures of seed plants.
Because they developed within the sporophyte plant, neither the gametophyte nor the gametes need standing water to function The entire gametophye of seed plants is contained in a tiny structure called a pollen grain. The entire pollen grain is carried to the female gametophyte by wind, insects, birds, small animals, and sometimes even bats. The carrying of pollen to the female gamtophyte is called pollination. Seeds are structures that protect the zygote of seed plants. After fertilization the zygote grows into a tiny plant called an embryo. A seed coat surround the embryo and protects it and the food supply from drying out.
The first seed bearing plants appeared during the Devonian period. The most ancient surviving seed plants belong to the three classes: the Ginkgoae, the Coniferae, and the cicadae. In these plants leaves evolved into specialized male and female gametophyte called scales. Scales then are grouped into larger structures called male and female cones. Male cones produce pollen and female produce eggs. Each seed is protected by a seed coat bu the seed is not covered by the cones. Because their seeds sit naked on the scales cycads, ginkoes and conifers are called naked seed plants or gymnosperm.
Cycads are palmlike plants that first appeared in the fossil record during the Triassic period, 225 million years ago. They can be found growing in tropical and subtropical places. The living ginkoe species looks almost exactly like its fossil ancestor. It might be the oldest plant seed alive today. Conifers are commonly called evergreens are the most abundant gymnosperm today. Pines, spruce, fir, cedars , sequoias, redwoods, and yews are all conifers. Some conifers live more than 4000 years and can grow more than 100 meters tall. The leaves of conifers are long and thin and are often called needles.
Both male and female gametophyte are very small. Males cones, called pollen cones, produce male gametophyte in the form of pollen grains. Female cones called seed cones. Some species of conifers produces male and female cones on the same plant. Angiosperm are all the flowering plants. All angiosperm reproduce sexually through their flowers in a process that involves pollination. Angiosperm seeds are contained within a protective wall that develops into a structure called a fruit. Angiosperm can be separated into two sublclasses the monocot and dicots. The monocots include corn, wheat, lilies, dafflodils, orchids, and palms.
The dicots include plants such as roses, clover, tomatoes, oaks, and daisies. The leaves of the embryo are called cotyledon. In some species of cotyledon are filled with food for the first leaves to carry on photosynthesis for the germinating plant. In monocots stems, xylem, and phloem tissues are gathered into vascular bundles that are scattered throughtout the stem. In dicots they are arranged in a ring near the outside of the stem. The process by which two organisms evolve structures and behaviors in response to changes in each other over time is called co-evolution.
Flowers pollinate by wind. Wind pollinated plants usually have a small plain simple flowers with little or no fragrance. Most angiosperm are not pollinated by the wind. They are pollinated by insects, birds or mammals . In return the flowers provide pollinators with food. Vector pollination or pollination by the actions of animals is a very efficient way of getting the male gametophyte to the female gamete. It is more efficient than wind pollination because it does not waste enormous amount of pollen. The process of distributing seeds away from parent plants is called seed dispersal.