Course 6.2Lesson 1 of 3
The plant body has three tissue systems. The epidermal system is the outer skin, with the cuticle, stomata, root hairs and trichomes. The ground system fills the body, as cortex, pith, medullary rays and mesophyll. The vascular system carries water and food, in open or closed, conjoint or radial, endarch or exarch bundles.
Try the code and the simulations beside the lesson.
In the last course you met the tissues one by one. A real stem or leaf does not use them one at a time. It groups them into three tissue systems, and each system runs through the whole body, from the root to the leaf.
Think of a house. The epidermal system is the outer wall with its doors and windows. The ground tissue is everything inside the walls: the rooms, the floors and the stores. The vascular tissue is the pipes that carry water and food through the house.
This system forms the outermost covering of the whole plant. It has the epidermis, the stomata and the epidermal hairs.
The epidermis. It is usually a single layer of closely packed parenchyma cells. Each cell has a little cytoplasm and a large vacuole. The epidermis of the aerial parts is covered with a waxy layer, the cuticle, made of cutin. The cuticle cuts down the loss of water. It is absent in roots.
The stomata. The epidermis of a leaf has small pores called stomata. They let gases move in and out, and they let water vapour escape in transpiration. Each stoma is guarded by two special cells, the guard cells.
Some epidermal cells near a stoma become different in shape and size. They are the subsidiary cells. The pore, the guard cells and the subsidiary cells together make the stomatal apparatus.
Root hairs. A root hair is a long, thin outgrowth of one epidermal cell of the root. It is unicellular. Root hairs take up water and minerals from the soil.
Trichomes. These are hairs on the epidermis of the stem. They are usually multicellular. They may be branched or unbranched, soft or stiff, and some are secretory. Trichomes help to prevent the loss of water by transpiration.
The ground tissue system is everything except the epidermis and the vascular bundles. It is made of simple tissues: parenchyma, collenchyma and sclerenchyma.
In a young stem and root it forms these parts.
The vascular system is made of xylem and phloem. Xylem carries water and minerals. Phloem carries food. The two are put together in vascular bundles. A bundle can be described in three ways.
1. Open or closed. In a dicot stem, a strip of cambium lies between the xylem and the phloem. Such a bundle is open. The cambium can make new xylem and phloem, so the stem can grow thicker. In a monocot stem, there is no cambium. The bundle is closed, and the stem does not grow thicker in this way.
2. Conjoint or radial. If the xylem and the phloem lie together on the same radius, the bundle is conjoint. This is the rule in stems and leaves. In roots, the xylem and the phloem form separate patches on different radii, one after the other. This is the radial arrangement.
3. The place of the phloem. In most conjoint bundles, the phloem lies only on the outer side of the xylem. Such a bundle is collateral. In a few plants, such as the pumpkin, phloem lies on both sides of the xylem. This is a bicollateral bundle.
The side of the first xylem. The first vessels to form are narrow. They are the protoxylem. The wider vessels that form later are the metaxylem.
You should find that stomata on each square millimetre give stomata on a patch of square millimetres. They carry guard cells. A single leaf can have a very large number of pores.
The program sorts tissues into their three tissue systems.
You should see cambium in the vascular system with xylem and phloem. A trichome is an outgrowth of the epidermis, so it appears under the epidermal system.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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