Lower Vascular Plants
Lower vascular plants include divisions Lycophyta, Sphenophyta, and Pterophyta. Division Lycophyta includes club mosses and ground pines. Sporophytes of these have stems covered with small, scaly leaves. Sporangia appear on the upper surface of the leaves and are structures that produce spores. Lycopods have small leaves that each contains one vein that brings water to the leaf from the roots and carries away extra food. Division Sphenophyta is made up of horsetails or scouring rushes. Similar to the lycopods, horsetails have an underground rhizome that produces tiny roots and stems. A small ring of leaves grows around the stem at each joint.
Another similarity to Lycopods is their environment, carboniferous forests. Division Pterophyta is ferns. Ferns have large, multiple-veined leaves. Leaves of ferns are called fronds and generally arise from a rhizome. Ferns can range many heights depending on their environment. Tropical ferns can grow very high while ferns in temperate areas can be rather small. Sporangium is generally found on the underside of fern leaves. Cells in the sporangia undergo meiosis and form haploid spores that are released into the air.
Gymnosperm, meaning naked seed includes some of the tallest and oldest plants in the world. It differs from lycopods, horsetails and ferns because they’re fully adapted to life on land and live in dry places. An evolutionary achievement is their strong, woody tissue, made up of xylem. Another evolutionary advance is that gymnosperm reproduction doesn’t require liquid water; instead they have wind-borne pollen and seeds. A good example of this is the reproduction of a pine tree. Pine trees form two different kinds of spores; microspores and megaspores. A microspore is the small pollen cones and the megaspores are the larger seed cones. The megaspores have the female gametophytes and the microspores have the male gametophytes. Pollen grains are the immature male gametophytes.
The pollen grain completes its development when the wind carries it to a female seed cone. It grows a thin pollen tube towards the female gametophyte. The haploid nuclei divide and form two sperm nuclei. When the sperm nucleus leaves the pollen tube and fuses with the egg nucleus fertilization occurs. Now the zygote and surrounding structures develop into a seed. This seed has three main parts: embryo, food supply and seed coat. The embryo develops through mitosis from the zygote. The food supply surrounds the embryo and will provide energy for the embryo to grow. The seed coat develops from the parent sporophyte and is around the outside.
Division Anthophyta: Angiosperms or Flowering Plants
Flowering plants are very diverse; they can range from many sizes and live in many different environments. Flowering plants are very useful and are essential to existence of humans. Almost all the food that comes from plants is from flowering plants. Flowering plants provide us with many things as well as food, such as housing, clothing, dyes, medicines, and spices. The flower is the most distinct feature of angiosperms. The flower includes male and female reproductive structures. The flower is an example of a modified leaf. The reproduction of flowering plants relies on the wind or animals to transport pollen grains. Seeds develop inside the ovaries which grow from female flower parts. Flowering plants use double fertilization which is when the one of the sperm from the pollen tube fertilizes the egg and the other fuses with two nuclei near the egg. The new nucleus develops into endosperm tissue (seed’s food supply.)