Kidney Bean Seed – the embryo.
It is time to dissect a Kidney Bean seed, one that has been soaked in water. Compare this seed to another that is dry. The soaked Kidney Bean seed are now larger, heavier, and softer from having imbibed (soaked in) water. These wet and soft seed are “mobilizing resources” and preparing to germinate. At the end of the exercise, you will plant another soaked seed in the pots. Because those seed have also been soaked, they will germinate more quickly than dry seed would.
Each seed you examine is covered with a red skin, which is the testa, also called the seed coat. The testa is part of the original ovule and has the same genetic makeup as the mother plant. The new plant (the embryo) is inside.
Gently peel off the testa, and allow the bean seed to open on the surface in front of you. The cream-colored seed is the embryo, a genetically new plant. It probably fell onto the work surface into two parts, two kidney-shaped halves which are its two seed leaves (also called cotyledons).
If you are fortunate, the young root and growing tip remained attached to one of the leaves, and are visible with normal eyesight. It is helpful to have a magnifying glass, a hand lens, or a dissection microscope to study this structure – but not necessary.
This embryo is a plant. It has everything a plant needs: roots, stems, and leaves. But its first two leaves are the huge halves of the bean, nothing like normal leaves. We call them “seed leaves”, or cotyledons. They were produced by the embryo based on provisions from the mother plant, and their destiny is to provide food and nutrients the embryo will need to set up housekeeping (to germinate and establish). You will see a structure that looks like a tiny pair of feathers – the plumule. This is the new stem, and it already has two tiny leaves that will grow into normal green leaves in short order. The small simple tube pointing in the opposite direction from the plumule will grow into the root; we call it the radicle. (A more complete written description of the bean embryo is given at the end of this document.)
Once students see these things, they could wash the bean and eat it! You might want to take caution in this area; there could be people in the group who are allergic to raw beans…
PLANTING ACTIVITY – Germinating a Kidney Bean:
You will need to:
- Review this entire written exercise (with photos) and check the setup so as you know where all of the needed materials are positioned.
- Ensure one or two of the large plastic boxes are labelled for each student group. Each box can accommodate 24 pots, which means most classes will require at least two boxes.
- Organize the students around their work tables
- Help the students work through steps below.
- At the end of the visit, ensure the boxes are filled with pots (even empty pots so the plants stay upright), the lids and class labels are installed, and the boxes are taken back to the classrooms.
Students are asked to set up and plant at least one pot each, using a bean seed. (Extras may prove useful later) They will need to follow these instructions:
- Write the student’s name on a sticky label and affix it toward the top of one of the pot sides.
- Take one wick string (the strings come in groups of four) and fit it through the bottom of your pot. To do this, fold the wick in half, and push the fold insidethe pot through one of the holes in the bottom, so you can pull the folded portion of the string through the bottom. There are chopsticks or skewers available to help push it through. Leave the two ends of the wick loose in the pot, along the inner wall. The loop will hang out the bottom by one to two inches.
- Keeping the wicks along the pot wall, fill the pot loosely, so there is about one inch of free space at the top.
- Place one of the remaining soaked beans on the soil in the pot.
- Scoop more soil on top of the bean, almost filling it to the rim.
- Using the bottom of an empty pot, gently press the soil down so the soil and bean are firmly in place, and the soil level is at least one half inch below the top level of the pot.
- Place your finished pot in the plastic box labeled for your class.
- Water the plants.
- When the exercise is completed, use empty pots to fill out the boxes so that plants will not fall over.
The following photos show stages in this process.
The photo above shows materials we use. Labels and a pen. Pots. Wicks (which are strands we cut from a cotton mop). Soaked beans. Soil mix. Plastic gang box.
Using a small stick, students push the fold of the wick through one of the larger drainage holes in the bottom of the pot.
A pot that has been filled and labelled. (students should write their names on the labels before sticking them on the pots)
A soaked bean is placed on the loose soil. It is then topped with more soil, and (using an empty pot) the soil mix is compressed slightly (not too much!)
The pots are set in a tub that has been labelled for the classroom. Empty spaces are filled with empty pots to keep things from falling over.
The pots are watered gently, just enough to soak thoroughly and for a small volume to remain as the reservoir.
Lids can be placed on the tubs, at least until early germination. Once the plants have germinated, you’ll want to remove the lid for air circulation.
- Place the plant containers in a bright spot and remove the lid by the time seed germinate.
- Make certain the pots are upright and the wicks are in place.
- Ensure all of the pots were well-watered and the soil is moist. There should be about one half inch of water in the bottom, covering all of the wicks.
- Add water periodically to maintain about one half inch of water at the base of the pots. Do not let the water level rise higher.
Two points that can be discussed with students:
- Soil is the medium that supports the plant, and provides both water and nutrients.
- If soil, sun, water, and air are available, and growing conditions are suitable, the beans will germinate, produce green leaves, and make their own food. They do this by capturing the energy of sunlight, splitting water for electrons, and building sugars out of carbon dioxide from the air. This is called photosynthesis, and it is the source of all the sugars plants use to grow, and all of the food we eat.
In one to two weeks, the beans should begin to germinate, and students can now examine how they grow, most especially what happens underground – in the pots.
When the first seed push up from the soil, it is time to inspect a sample. If you planted extras, students will unapt one to inspect root growth. You will want to have them work on a tray or large paper plate, because they will be un-potting their specimens.
To un-pot, each group could “very gently” turn a pot on its side, tap it softly, and carefully tease away the moist soil. A small paint brush or artist brush will be useful. Treat this like an archaeological dig. You want the students to see everything that is happening underground. If stud ents have cell phones, let them take photos of what they see. Or consider asking them to make a drawing. They might even consider measuring the length of the main roots. Once each group has dissected a specimen, bring students all together to compare their samples. Did they all do about the same thing? Have the roots grown more than the stem and leaves? Of is it the other way around?
If students handle the plants carefully, they can be repotted and will continue growing with no issues.
Each week, for three more rounds, you will want to go through the same exercise, sacrificing (if necessary) another set of plants. This is the reason you need to make certain some evidence was collected on the previous “exhumations” – so students can observe how the germination and “establishment” process occurs.
There are plenty of observations that can be made during this 4-week trial.
Remember, everything you see emerge (the seed leaves, the first normal leaves) is something you already saw in the embryo. It was all there, and now expands and begins to function.
How many days did it take for the two seed halves (the seed leaves) to bend up and open to the sunlight? When did the first leaves begin to expand?
When the leaves turn green, the plant is doing its own photosynthesis. Until that point, all of the food provided was present in the two seed leaves.
When we eat beans, we are eating seed. What does this tell us about when we chose to harvest certain kinds of plants to gain the most nutritional value?
At each stage, it would be fun to soak some Red Kidney beans and ask students to compare the embryo to the expanded and green plant.
Germination: 7 days
A week after students have planted the soaked Kidney Beans, you should notice signs of germination (especially if the pots have been kept in a warm place).
Growth at this point is very quick and this section photo-documents several different seed, all of which are at a very slightly different stage.
The class has choices now. You could sacrifice all of the seed and let each student examine his/ her own sample. Or, you could follow the original directions and exhume only one quarter of the samples; letting students work together in groups to study the growth.
A wonderful add-on would be to have soaked more Kidney Beans the day before, which would allow students to compare the fresh bean to the week-old seedling. Every part of the embryo is still easily compared, providing first- hand knowledge of how an embryo germinates. Remember that both the seed, and now the “bean sprout” are edible.
Students will notice the roots and shoots are going through contortions…. Somehow, the roots know to grow down and the shoots have a system to grow upward. The problem, of course, is that each seed was planted in a different position, so students will see the radicle and plumule had to do some twisting around to get going in the right direction.
Also, students will see the radicle grew into a tap root, but new side roots formed close to the base and quickly became as large and important as the main (tap) root. So the bean does not develop a massive taproot like we see in a carrot or radish.
Beans develop a branched root system early in germination.
Left arrow: Cotyledon (the seed leaf); Right arrow: First normal leaves
The arrows point to the following parts, top to bottom
- 1st normal leaf
- Apical meristem (growing tip)
- Cotyledon (the seed leaf)
Young Plant – Two Weeks
(Soaked 27 Jan, Planted 28 Jan, Examined 10 February 2018)
Two weeks after soaking and planting, our Kidney Bean plants are standing tall. The photo shows one sample, with its withering cotyledons, its first pair of simple, heart-shaped leaves, and its first normal leaf – which is “trifoliate.”
Note that the cotyledons are paired (on opposite sides of the stem), as are the first simple leaves. Both pairs of leaves were visible in the embryo. The cotyledons were the two cream- colored halves that made up most of the seed. The simple green leaves made up the “plumule.”
The stem arising from the simple leaves appears somewhat different from the lower stem. This stem will twine, becoming a vine.
And the growing point of
the stem will continue to produce leaves, but those new leaves will appear one at a time (not in pairs.) And those new leaves will be “compound” – they will each have three leaflets.
Hold the potted plant over a shallow tray (to catch the soil). Then remove the plant gently from its pot by tapping the side and slipping the plant and rootball from the pot. Carefully, allow the loose soil to fall from the roots, protecting them from damage. Then slip the plant into a pail or bowl of water and wash the soil from the roots. The goal is to clean away the soil while keeping the roots intact for examination. A small artists brush could be helpful.
We see that the main branches continue to develop along with the “taproot” – such that smaller roots completely explored the available soil.
If the bean had been planted in a garden, where the roots are not confined in a pot, the side roots would have grown out more horizontally, reaching as far from the plant base as possible to support the plant, and contact needed water and nutrients.
So what can we do with young plants like this? What kinds of trials and tests could we conduct that would answer questions about how a bean grows?
One easy trick to pull is to lay a plant on its side. Below you will see what happened when we left the plant stripped of soil lying in a pan of water for the day. Within just a few hours, the stem tip (which is growing rapidly) had perceived the new posture and began growing upward. Students will see that the older stem does not bend, because it is no longer growing rapidly. But the portion that is still elongating now orients so it is growing vertically. That raises some questions!
Is this a response to light? Or to gravity? What might have happened had the plant been set in the dark (or covered by a box) for those few hours?
How can we learn if light affects how a plant grows? Now that you have seedlings that are growing so quickly, what kinds of experiments could students design to test how plants determine the direction and speed of their growth?
Is there a simple test students could devise to determine which part of the stem is growing, and how fast it is growing? Let the plant grow longer, turning the stem more than once…..
Flowering – Six Weeks
Photos below document the bean plants six weeks after planting. The plants have grown to a mature form, vining, flowering, and even fruiting.
The white flowers have the same shape as those of every other pea or bean. A large petal (the “standard”, or “banner”) stands upright, while two pairs of petals project below. One pair (the “wings”) loosely wrap around other flower parts, while another pair (the “keels”) create a boat-shaped envelope which surrounds the stamens and pistil. You can look up “pea flower” on the web and find many illustrations of the floral structure.
The pistil, once pollinated and fertilized, quickly grows into the bean, a seed- containing fruit called a legume.
Other Ideas …
Other studies with Kidney Beans
Bean and pea have two meanings. The entire fruit, the pod (the legume) is called a bean (like a green bean) or a pea (like a Sugar Snap pea). But each individual seed inside the pod is also called a bean or a pea. People will even use bean or pea to refer to the whole plant. Unfortunate but true. It takes a bit of context to keep things straight. For the moment, I want to talk about a seed, one called a Kidney Bean. These are available at practically any grocer, and are quintessentially useful.
Start with some dry Kidney Beans. Take a sample and soak those seed in water overnight (or in hot water for a few hours). On the following day there are many lessons to be learned from those beans. First, line up some dry beans and a similar rank of beans that were soaked in water. What a difference a day makes. As seen below, if you have a set of good scales available, you might actually weigh the beans dry, and then weigh them again after being soaked. This way you can quantify how much water has been absorbed.
But you may have explored this Reader sufficiently to know that botanists do not simply say that seed take up water. And it is not at all useful to say any seed, or any plant, drinks water. Seed “imbibe” and we call the absorption of water imbibition. I know that we sometimes refer to people drinking alcohol as “imbibing” as a gentle method of indicating they have been drinking – so it is a bit fussy to avoid use of the word “drinking”. But botanists can abide seed imbibing while not accepting the idea of plants drinking. Regardless, imbibition happens.
And the results are dramatic.
When you plant a bean, imbibition must happen first. A seed will not germinate until the tissue has absorbed water. The cells must be rehydrated for biological reactions to gear up, and the tissue needs to soften for cells to elongate and growth to resume. Some gardeners have taken to soaking pea seed overnight before planting, just to give them a head start.
In the following photographs, I can illustrate the trial mentioned above. Using a fine marker, I numbered ten Red Kidney Beans. (It is easy to do this, but do not touch the numbers with your hands – they will rub right off.) I measured their mass and then made two simple photos to document their size – one on inch-square graph paper and the other alongside a rule. I then soaked the seed in tap water for two days and repeated the measurements.
Their mass more than doubled; read the scales. And the length of the seed increased by just over 30%, from 6.5” for the row to 8.5”.
Check it out:
Mass: Dry vs. Imbibed
Size: Dry vs. Imbibed
Dry, strung out along a ruler – totalling 6.5 inches:
Imbibed, lain out against the same ruler the following day, totaling 8.5 inches length:
Now that you have a batch of hydrated bean seed, there is a lot more to be discovered. Working over a table, gently peel off the red seed coat, letting the two halves of the embryo rest on the table surface. The red part you peeled off is genetically different from the embryo on the table. That seed coat (also called the testa) has the same genetics as the bean pod – which means it has the same genetics as the flower and the parent plant. The creamy white embryo is a new generation. The embryo has its own genetics because it grew from an egg cell in the ovule that was fertilized by a sperm nucleus from a pollen grain. Importantly, each bean or pea in a pod is genetically different because each started with a different egg and was fertilized by a different pollen grain – laying waste to the phrase “like two peas in a pod.” (In humans, we would call these “fraternal” twins, or triplets, etc.)
The most important and useful discovery is visible at the core. The two “halves” of the embryo that came out of the seed coat were of course attached before you disrupted the arrangement. They are the two seed leaves (the cotyledons), and almost every time I open a kidney bean, one cotyledon falls free entirely, leaving the fancy parts of the embryo attached to the other seed leaf. Even with the naked eye, the embryo is visible, but it is a thing of glory under a nice dissection microscope at about 6-10× power.
Everything is there – shoot and root. This is an entire plant, and even lacking one of its cotyledons, this seed remains viable. You are looking at all a bean is, and everything it can do. Plants only make roots, stems, and leaves – the three parts you can see in this embryo. The radicle, which is obvious in this bean seed, is the beginner root, and it only knows how to make roots. Its growing tip is right there to see – in that tip is a root apical meristem that will generate the entire mature root system. Charles Darwin and his son Francis reported experiments that indicated you can halt the response a root has to gravity by cutting of the very tip. Roots will stop growing downward until a new tip has been generated.
The more decorative component, called the plumule because it resembles a small feather, is the beginning shoot for the new plant. You can see the stem (particularly that curious piece of stem, the hypocotyl, between the cotyledon and the root) and
the newer embryonic leaves. We cannot say that the plumule has the first leaves, because the cotyledons are the first two leaves the embryo produced. But the tiny leaves visible in the plumule will expand to become the first regular-looking, heart- shaped green leaves the growing bean produces.
Tucked hidden inside the plumule is the stem growing tip, which includes the shoot apical meristem. That growing tip produces all future stem, as well as its sidekicks, the leaves. The tip is responsible for branching and plant structure (architecture), as well as short stems that produce specialized leaves that are the flower parts – sepals, petals, stamens, and pistil.
You do not have to stick with the Kidney Bean. With a quick trip to any organic grocer, you’ll likely find a host of different pulses (beans, peas, lima beans) available in open stock. I have sometimes purchased small samples of many of the seed available (especially huge lima beans) and soaked some of each. Students would have a great exercise peeling and comparing the embryonic structure of many different pulses; they are beans too (check on students and concerns about Peanut allergies before going down this path).
Inside the Seed:
Ask students to identify the following parts of the Pinto Bean Embryo, based on arrows in the image above (the list below is in clockwise order based on the image):
Break…. Point at which the left seed leaf was originally attached to the embryo
Seed Leaf (Cotyledon)
Energy Storage: Bean and Pea cotyledons are elephantine compared to the rest of the embryo, because they are the storage site for starches, oils, and proteins that will be mobilized to support seedling establishment. We take advantage of this storage by using seed as major sources of food for ourselves and our livestock.
Importantly, beans and peas are particularly important to us because the entire bean family (we also use the term pulses for these edible legumes) has another significant trick that causes the seed to be higher in proteins than the seed of other kinds of plants.
Proteins are made of amino acids, which are nitrogen-based. Humans obtain nitrogen as part of proteins we eat, but plants make their proteins from scratch, starting with simple compounds to make all of their own amino acids. Some kinds of beans are so packed with proteins that we can extract useful products, such as when we soak soybeans to make tofu.
People who have taken gardening classes or ecology will have learned the secret to this protein-abundant lifestyle. Legumes have the capacity to form nodules along their roots, nodules inside which nitrogen-fixing bacteria can flourish. They make this happen through chemical magic – producing molecules that sequester oxygen, which preserves an oxygen-free environment inside the nodules that is needed for the bacteria. The benefit of having a native source of usable nitrogen is enormous.
Though the atmosphere is 78% nitrogen, that nitrogen is gaseous N2 and simply inert to both plants and animals. Plants take in nitrogen that is “fixed” – bonded to other elements in the form of nitrate (NO3) or ammonium (NH4). Nitrogen is a crucially important plant nutrient, but most plants have a tough time getting enough. But legumes are set. In fact, farmers rely on legumes such as alfalfa and clover as cover crops – plowed in they provide green manure that enriches the soil with nitrogen. Coffee farmers in tropical regions rely on Fabaceous trees (mainly Inga) to grow shade coffee, thus artificial fertilizers are not required. You can dig the roots of clover or other legumes and readily observe these nitrogen-fixing nodules.
What to do once the examinations are over. If you had a lot of soybeans, you could go to many sources and discover how to make your own tofu. But you simply could take the soaked beans and cook them to make a bean dish. This provides the opportunity for yet another bit of experimentation. Most people will know that beans take quite a while to cook to tenderness. While living in Colombia several decades ago, I quickly learned that everyone cooked dry beans in a pressure cooker, which of course raises the temperature and makes quicker work of the cooking process.
But there is another way to tenderize beans – by making the cooking water more alkaline. Many starchy products benefit or suffer from the trick of adding baking soda to the cooking water (in the case of corn, cooks use lye water). The problem is that this can make the beans too mushy too fast. So I am not recommending baking soda as a standard addition, but you can prove this to yourself by boiling two small batches of beans – one batch in plain water and the other in alkaline water – to appreciate the difference.
Pea (Legume) Flowers, Peas and Beans
There are a lot of sweet things students can do with pea flowers. And there are a lot of pea flowers available for exploration. You can let the Kidney Bean plants grow until they flower, or you can look around. Sweet Pea, Wisteria, and Clover make similar flowers, and there are much larger examples available, like Crotalaria.
So what is a “pea flower”? Well it is the same thing as a “bean” flower – the kind of flower you find in the peas and beans and their nearest thousands of relatives in the Bean Family (the Fabaceae). When you look straight at a pea flower, you see it has a single plane of symmetry. A line drawn from top to bottom shows each half to be a mirror image of the other. We call this bilateral symmetry (or you can say the flower is zygomorphic – because zygo suggests the shapes are “paired”.) Compare looking at a pea flower to a rose, or a lily. In rose and lily the symmetry is radial – like that of a star pattern. You can draw many lines of symmetry through the flowers that have this form (which we call actinomorphic, where actino refers to ray, or radial, or radiating.
In the pea flower, one petal opens upward, and is usually the showiest – this is called the banner. Two petals join together and project forward (from below the banner) as a keel (shaped like the keel of a boat.) Those keel petals enclose the stamens and single pistil. Alongside, or below the keel are two lateral petals that take on many configurations. They are called wings because that is one common appearance they create for pea flowers.
If some of the monster-sized Crotalaria flowers are available, you can make silly puppet hummingbirds out of them by removing the large banner (the upper petal). That doesn’t tell you much about botany, but it is a lot of fun if you need to distract a bunch of kids.
But the most useful aspect of pea flowers is their remarkable structure, and the way they help us explain how a flower becomes a fruit. Grow some garden peas or beans, or find pea flowers and purchase a few snap peas at the grocers. With a handlens, or using a dissection microscope, investigate what is happening inside the keel. You will find a small, single, green pistil surrounded by a pale sheath that ends in a bunch of attached stamens. You will also find a single, free stamen – always. It is consistently like that. Simply amazing.
The pistil is a perfect small version of the mature fruit (the pea pod, or bean – which is called a legume). If you happen to have a growing plant, it is very useful to follow down the stem, from the newest flowers to earlier flowers in which the petals have shriveled or shed, and the pistil is expanding. Lacking that, compare the pistil in the flower to the mature pea pod from the grocers. They are different stages of the same floral organ. I know it does not seem like a big deal, but somehow there are a lot of people who simply do not draw the absolute relationship between a flower and a fruit. If you can do this in a pea, then you know beans too.