Biology-Mr.Willets
Lab-Stems
The stem is the part of the plant that connects the roots to the leaves. The leaves are up high to get more light to make food. The roots are in the ground to anchor the plant and to get water and minerals. Water must move up the stem to the leaves, food must move down the stem to feed the rest of the plant. The stem must contain vascular tissue (to move water up and food down)and some amount of support tissue (to give strength to the stem). Exactly how these two tissues and some other tissues are arranged in the stem varies. Plants that live and grow year after year have a different arrangement compared to those that live for only one growing season. Also, monocots and dicots differ in how the vascular tissue is arranged in the stem.
You will look at two slides to compare these arrangements. One slide shows the stem of a woody dicot. This is a dicot that grows bigger each year- trees, bushes etc. The other slide shows the stems of two
non-woody (herbaceous) plants: one a dicot, the other a monocot. Herbaceous plants grow for one season and then die or become dormant during the winter and grow anew in the spring.
Part I- Woody Dicot Stem - Tilia
This slide shows a two year old stem. Examine it using the stereo scopes (20X). You should then switch to using the compound scopes (scanning power) to see more clearly.
A) The first thing to notice is that everything is in a circular pattern. This is characteristic of a dicot stem. In the very center are a group of cells called pith. These are used for storage.
B) Around the pith are two rings of xylem, the vascular tissue that transports water up the plant. This is the wood of the plant and also serves as support tissue. Look closely at these rings. The inner part of each ring has larger cells than the outer part. These larger cells were made during the spring and that part of the ring is called spring wood. The smaller cells of the ring were made during the summer. Each ring has spring wood and summer wood. Each year, this plant makes a new ring of xylem. The age of many woody plants can be determined by counting the rings in its stem.
C) Around the xylem (wood) rings are several different types of cells. Most of the red stained cells are phloem - the vascular tissue that transports food down the plant. There is only one layer of phloem. It dies each year and is replaced in the spring.
D) In between the phloem and the xylem is a very thin layer of cells that you may not see clearly. This tissue is called vascular cambium. Cambium is a growth tissue and these cells are the ones that divide to produce the new phloem each year and the new ring of xylem each year.
E) Mixed in with the phloem are some cortex cells that are used for storage. These will probably not have been stained red.
F) The very outer layer of cells is cork. You would call this the bark but officially it is called the outer bark since the full bark includes the phloem also. Just inside the cork is another thin layer of cells called the cork cambium. These cells divide to produce more cork each year.
G) On the front of the question sheet, make a simple diagram showing the tissues underlined and in bold face print above. Label each tissue. Label the diagram "woody dicot stem"
Part II. Non-Woody (herbaceous) Stems
Get a slide labelled "monocot and dicot stems" On the slide are three cross sections of stems - two small ones and one larger one. The two smaller ones are from the same plant.
These are stems from herbaceous plants which means that they only grow for one season and then die. There is no build up of xylem (wood) year to year. Hence the name "non-woody".
Herbaceous stems do not have rings of xylem and phloem but rather have "bundles" of cells. These bundles contain some xylem cells, some phloem cells and some support tissue. They are called vascular bundles.
A) The larger cross section is of a monocot stem. Look at it using the stereo scope. You will see the bundles are scattered throughout the stem in a random pattern. That random distribution of bundles is a characteristic of monocots.
B) Now look at the smaller cross section. This is from a dicot. You will see that the bundles are arranged in a circle around the edge. This circular pattern is a characteristic of dicots. The red cells in the bundle are phloem, the xylem cells in the bundle are larger than the phloem.
C) In both stems, there is an outer layer of cells called the epidermis. Since the stem only lasts one season, this outer protective layer is not as thick as bark would be.
D) In both stems, all the other cells surrounding the bundles are pith cells and are used for storage.
E) On the data sheet, make a diagram of a dicot stem showing several vascular bundles, pith cells and epidermis. Label those 3 structures. Label the diagram "herbaceous dicot stem" Make another diagram showing and labelling the same things in a monocot stem. Label this "herbaceous monocot stem".
F) Use the compound scope (low power) to get a good look at one bundle from the monocot stem (the larger one). In it you will see three large cells. These are the xylem cells. There will also be a group of one or two dozen smaller cells at the other side of the bundle. These are phloem cells. The rest of the bundle is support tissue.
G) On the data sheet, make a diagram of one vascular bundle. Label xylem, phloem and support cells. Label this diagram "monocot vascular bundle"
Biology- Mr. Willets
Lab-Stems
USE A PENCIL FOR ALL DIAGRAMS
1) In dicot stems, the vascular tissue is arranged in what pattern? _______________________
In monocot stems, the vascular tissue is arranged in a _____________________ pattern
2) Why does spring wood have larger cells than summer wood? (think about the weather conditions) _____________________________________________________________________________
3) A tree would die if a ring of bark were removed because the bark includes ________________. If this were removed, the cells below would not be able to get any _____________
4) What is the function of phloem? __________________________________
5) What is the function of xylem? _______________________________________
6) The wood of a tree is rings of ________________
7) Xylem and phloem are both ______________________ tissue
8) The tissue that makes new xylem and phloem is called __________________________________
9) The tissue that makes new cork is called ___________________________________________
10) Look at your answers to #8 and #9. What is a general name for growth tissue in plants? ______________________________________
11) Another word meaning non-woody is _________________________
12) In non-woody plants, the vascular tissue is found in _____________________bundles. What three types of cells are in these bundles? ___________________________________________