Up to now we've been looking at the exterior of plants. It is time to delve more deeply inside.
What do you see when you look closely at a leaf?
Take this oleander leaf, for example. You can see that is is green, but there are also lighter yellow or white areas. The light line down the middle is called the midrib. It is the main vein of the leaf for moving water and nutrients in and out. The midrib often also adds stiffness or structure to the leaf.
Perpendicular to the midrib are numerous smaller veins. In the oleander leaf they are lined up parallel to each other. The smallest veins form a netlike pattern.
The leaves of certain plants will have some features not found in all plants. For example, the yellow bumps on the leaves above are specialized glands called nectaries. In this case they occur outside of flowers, so they are also called "extrafloral nectaries."
Other plants may have numerous hairs or trichomes.
(Public domain image from Wikimedia)
Leaves are made up of different types of cells. On the right of the illustration is a cluster of xylem (carry water) and phloem (carry food) cells labeled as the vascular bundle. Those are the elements of the veins.
The two types of cells labeled "mesophyll," the palisade and spongy, are the cells that contain the chloroplasts and are responsible for the majority of photosynthesis that occurs in the plant. We will discuss photosynthesis extensively in a future post.
At the bottom is found an opening called a stoma (plural stomata), which is where gases go in and out of the leaf. Because the openings also allow some water to be lost, under certain conditions the cells around the stomata, the guard cells, swell up and close the opening.
(public domain image of a generalized plant cell from Wikimedia)
If we could delve yet deeper, we could see the make up of individual plant cells. Plant cells have some features in common with those of other organisms, such as the presence of a nucleus, endoplasmic reticulum, and mitochondria. Other features are only found in plants, such as the cell wall made up of cellulose.
Activity Suggestion 1. Look closely into plants
- Fresh plant material (non-allergenic and non-toxic)
- Dissecting microscope or hand lens
- Age-appropriate cutting implements, dissecting pins
Examine the plant material under a microscope or hand lens. Dissect stems and leaves into sections to see structures more clearly. Draw and label what you see.
Activity Suggestion 2. Make a model plant cell.
Making 2D and 3D models of plant cells out of a variety of materials has become a classic science activity for middle school/high school students. There are numerous examples online. Start at this Model Plant Cell Pinterest page for ideas, as well as the diagram above and the interactive website.
Activity 3: Water use by plants
We all know we must water our houseplants, lawns, and gardens, but what are plants doing with the water? Primarily, the plants have chemicals in their leaves that can use sunlight, water, and carbon dioxide to make food in the form of sugars. This is called photosynthesis. Plants also use water to move nutrients, to add support to their structures, and to keep cool. All that water moves from the roots through the stems to the leaves via the xylem.
- Clear plastic bag big enough to hold a few leaves
- Twist-tie or chenille
- Tree or shrub (with leaves close enough to the ground to put a bag over)
On a warm sunny day, slip a clear plastic bag over some leaves on the end of a branch of a tree. Tie the bag tightly to the branch with the twist-tie or chenille, trapping the leaves inside and preventing air from escaping. Visit the tree in fifteen minutes and then again in a half hour. What is happening inside the bag?
You should see the bag start to fill with condensing water. Why?
Trees release a lot of water on a hot day through a process known as transpiration. Much of the water comes out of the stomata. You are capturing the water that is being released. which condenses when it reaches the plastic. Some figures suggest that more water enters the air from plants transpiring than from evaporation from the surface of the ocean.
Transpiration cools the plant on a hot day and is also involved in helping the plant move water to the chloroplasts.
How to make this activity an experiment:
- compare the rate of transpiration at different times of the day or at different temperatures (by measuring the amount of water produced in a given time)
- compare transpiration rates between different trees and shrubs
Enchanted Learning has a collection of leaf anatomy activities, including a diagram of a cross-section of a leaf to label.