Biology -Mr. Willets
Stem Info
The stem is the part of the plant that basically connects the roots to the leaves. The leaves must be above ground to get light, the roots must be below ground to anchor the plant and absorb water and minerals.
The stem must contain special cells that can transport water up to the leaves and other cells to transport food down from the leaves to the rest of the plant.
Cells whose function is the transport of material are called vascular tissue. The two types of vascular tissue are xylem, which transports water up and phloem, which transports food down.
Xylem and phloem along with other cells are arranged in specific patterns within the stem. These patterns differ depending on whether the plant lives and grows year after year (a woody plant- like trees and bushes) or if the plant grows for one season and then dies or becomes dormant and starts all over in the spring. These are called herbaceous (or non-woody) plants.
************
In a woody plant, everything is arranged in rings. Each year there is a new ring of xylem formed. Xylem cells make up most of what we call wood. Each ring has large xylem cells called spring wood and smaller ones called summer wood. You can easily tell by the size difference where the summer wood from one year ends and the spring wood from the next year begins. This is how you count the rings to tell the age of a tree.
Outside the rings of xylem is located one ring of phloem. The phloem is officially attached to the bark and if you remove a piece of bark from a tree, the inner part of the piece is the phloem.
In between the xylem rings and the phloem is a layer of cells called vascular cambium. These cells divide to form new xylem and phloem cells as needed.
The very outside of a woody stem is what you call the bark. Officially it is called cork and it is produced by a layer of cells called cork cambium. The term bark includes the cork, the cork cambium and the phloem.
*************
Herbaceous plants have a very different arrangement of cells in the stem. The xylem and phloem are found in groups called vascular bundles. Each bundle has a few large xylem cells and several phloem cells.
The bundle also has some cells that help support the stem. These bundles are surrounded by pith cells that are mostly used for storage. The stringy things you run into when you eat celery are the vascular bundles that run up the length of the stem (the part of the celery you eat is the stem)
In herbaceous plants called dicots, the bundles are arranged in a circular pattern. In monocots, the bundles are in a random pattern.
Since herbaceous plants do not live more than a few months, the outer protective layer of cells is much thinner than found in woody plants. It is called epidermis.
*************
All stems grow in length only from the tips. This type of growth tissue is called apical or tip meristem.
************
Some plants have modified stems to perform special functions. A white potato is part of an underground stem that is used for storage. It is called a tuber. Strawberry plants produce stems that grow sideways along the ground to produce new plants. These are called runners. The cactus has a fleshy stem which stores water and also does the photosynthesis for the plant. Many plants have thorns that grow from their stems to protect the plant from a variety of dangers.
Biology-Mr. Willets
Stem Questions
1) The basic function of stems is to connect ___________________ to the ____________________
2) The leaves need be where there is _____________ in order to make ______________ and the roots have to be in the _______________ to absorb ___________________________________ and to _______________ the plant.
3) What must be transported up the stem? _____________________ What must be transported down the stem? _______________________
4) What cells transport water?__________________ What cells transport food? ________________
Together these two types of cells are known as _____________________ tissue.
5) A woody plant grows ___________ after ____________. Most woody plants are called ___________
6) Another word for non-woody is ______________________ What happens to these plants each year?
________________________________________________________________
7) In a woody plant, all the cells are arranged in ________________
8) The wood of a plant is made of ________________ cells. There is a new ring of ________________ each year.
9) The part of a xylem ring with large cells is called ____________________ wood. The part of the ring with smaller cells is called ___________________ wood.
10) The larger xylem cells were produced in the ________________ when there was more ___________ and therefore the cells had to be larger. (think about this one)
11) You cannot count the rings of a tree from the tropical rain forest. Why not? (think about it)
____________________________________________________________________________
11) Outside the rings of xylem is a single ring of ______________________.
12) In between the xylem and the phloem is a ring of cells called _____________________________
These cells divide to form new _______________ and _______________ cells.
13) The outside of a woody plant (tree) is commonly called the ____________ but is really called ____________. New cork cells are made by a ring of cells called _____________________________
14) There are two layers of cells whose main job is growth- the names of both contains the same word-_________________________. This word describes any growth tissue within the stem.
15) Herbaceous plants have their xylem and phloem in groups called vascular ____________________
16) Each bundle contains a few large _______________ cells and several smaller ______________ cells.
17) What type of plants have their bundles arranged in a circular pattern? _________________
18) Herbaceous monocot have their cells arranged in a ____________________ pattern
19) Herbaceous plant stems have a thin outer protective layer called ______________________
20) You nail a sign on a tree about 5 feet off the ground and come back 10 years later. The tree is much taller but the sign is still 5 feet off the ground. Why? ___________________________________________
____________________________________________________________________________
21) Give some examples of specially modified stems. _______________________________________
____________________________________________________________________________