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How do plants grow?--by both cell division and cell expansion. But cells do not just get bigger they also change in shape to perform particular functions in the plant--they undergo differentiation. Cell division is concentrated in regions known as meristems where there are undifferentiated cells. At the shoot and root tips are the apical meristems. From these are derived the primary meristems:
The protoderm produces cells for the epidermis,the outermost cells of shoot and root
The procambium produces cells for the vascular or conducting tissues (xylem and phloem)
The ground meristem produces cells for the cortex, between the vascular tissues and epidermis, and the pith, if present.
The apical meristems are already visible in the embryo of the mature seed and they persist as long as a root or shoot continues growing.
Wheat (Triticum) grain, L.S.
As in other members of the grass family (Poaceae) the grain of wheat is a fruit rather than a seed. It is a single-seeded fruit, similar to an achene which is sometimes given the special name "caryopsis". Thus the outer coat is really pericarp (equivalent to the fleshy part of a peach), rather than a seed coat since it is derived from the ovary wall. The seed coat fuses with the inner layer of pericarp and is not visible as a separate region.
Make an outline diagram of the embryo end of the section and look at a germinating wheat seedling. Try to label the diagram with the words in bold type and with the aid of the following prompts and questions:
- The root apical meristem is surrounded by a sheath,
the coleorhiza. (Look to see which end the root comes
out of the seed.)
The shoot apical meristem already shows several leaf primordia and is surrounded by a sheath, the coleoptile.
What is the first part of the shoot to emerge fromt he germinating seed?
Most of the seed is used for storage of starch in the dead cells of the endosperm. Outside this is a thin layer, the aleurone that secretes enzymes into the endosperm on germination. The seed is enclosed in the ovary wall or pericarp. The one cotyledon in this seed takes the form of a scutellum. Sugars are transferred from the endosperm to the embryo through the scutellum.
Chromosome squash
- Collect two onion root tips and trim so that you have about 2 mm of the very tip.
- Soak the tips in a few drops of 1 M HCl in a watch glass for about 10 min.
- Transfer the tips to a few drops of acetocarmine stain for about 15 min.
- Transfer one tip to a few drops of water on a slide.
- Lower a cover slip onto the tip and press it down gently so that the tip squashes out to about 7 mm (1/3 inch) around.
- Look for mitoses under high power (X40). Let me know if you
strike it lucky!
(If this fails for any reason try your other root tip)
WARNING: Acetocarmine is corrosive and stains skin and clothes. It does not come out. Take care in handling it.
Coleus stem apex (prepared slide)
Stems are more complicated than roots and it is no surprise than the stem apical meristem (SAM) is more complicated than the RAM). There are two distinct areas of cell division at the apex:
- the tunica which gives rise to the protoderm and some of the underlying ground meristem (on the outside, think of tunics!)
- and the corpus that give rise to the procambium and the rest of the ground meristem (corpus is latin for body, think of corspe!)
Choose a slide which is either marked "EARLY" or unmarked. You can look at the "LATE" slides later on in order to see further stages of leaf development. Use the X4 objective to get a general view of the section. Make an outline drawing and without looking at the text try to mark the following:
- apical meristem
- bud primordium
- leaf primordium
- protoderm
- ground meristem (2 areas)
- procambium
On a higher power try to make out the tunica and corpus in the apical meristem.
How many cell layers are there in the tunica?_________________________
What is different about the arrangement of cell walls in this region from that in the corpus? What does this suggest about the pattern of cell divisions in the two regions (This makes sense if you think what the plant surface would look like if cell divisions in the tunica were the same as in the corpus)?
______________________________________________________________________________________________
There are various other bits and pieces on these sections. What do you think the large pieces of (apparently detached) tissue in the middle of the sections are?
________________________________________________________________________________________________
This plant has two kinds of trichomes. What do they look like? ________________________________________________________________________________________________
Raphanus (radish) stem apex
In order to get a view of a fresh stem apical meristem: Remove the cotyledons from a radish seedling. Use mounted needles under the dissecting scope to remove developing leves, until you come to the apical meristem. You can also try this with Coleus but it is much more difficult.
Brassica cultures
Remove the lids from your cultures for a final examination of the growth. Use the dissecting scope to examine strucures more closely.
The roots are probably covered in a "fuss". What is this? _____________________________________
On some of your plates you may be able to see dome like structures with little bumps. If so where can you see them? _________________________________________________________________________________________________
What are the domes? ______________________________________________________________
And what are the bumps?____________________________________________________________
Brassica rapa stem cell types
Over the next few weeks you will be looking at transverse sections of plant parts and trying to figure out their organiztion and function. It should help to have other views of cells that you will encounter in sections. The simple technique of tissue squashes can show most of the types of differentiated cell.
Collect a piece of RCB stem which has been soaked in sodium carbonate solution for a few days. This softens up the cell walls so that tissues break wasily and you can see cell types. Cut the stem lengthwise and use needles on the slide to tease apart the tissue in a few drops of water. Cover with a cover slip and examine under low power. Use high power to get a better view of promising areas
What kind of cells can you make out? ____________________________ ___________________________
____________________________ ___________________________
____________________________ __________________________
What kinds of thickening can you see in the xylem vessels?
____________________________ ___________________________
Raphanus (radish) hand sections
If we had unlimited time and materials you could explore plant structure from scratch, making your own sections. Before we look at what the professionals can do you might like to try your hand at cutting sections.
Use a new razor to try to cut a thin section of a young radish stem. Keep the tissue and blade moist as you do this and float the sections onto water in a watch glass as you cut them. Select what seems to be the thinnest and make a wet mount of it. You do not have to get a complete section like the prepared slides and oblique sections cqn be successfully viewed at their thin edges. See if you can make out any tissue types. Make an outline drawing of your section.