It is no secret I love houseplants. I do. Always have. In my younger years,
I used to have quite the collection, but I made a terrible mistake starting out:
I thought all plants liked exactly the same things. Lots of water. Lots of light. And me,
serenading them with my violin music. Besides the violin, which – I can’t
comment on that– I can tell you that treating all the plants with lots of light and lots of
water was a mistake. Maybe some of them liked it. But a lot of them did not. Why?
You know how there are many different types of animals----well there are many different
types of plants as well. They don’t all require the exact same amount of light
and the same amount of water. There is so much diversity in plants – there are over 300,000
estimated species of plants – and there are some fascinating adaptations that have helped them
live in a variety of environments. This video is going to briefly touch on types of plants,
their structure, and some fascinating adaptations.
First, we need to outline two general plant categories. Vascular and nonvascular.
Recall that in the human body, your vascular system includes arteries. Well plants don’t
have arteries…or blood… for that matter. When we’re talking about a vascular system in plants,
we’re talking about two major types of tubes---or vessels--- called the xylem and phloem. The xylem
carries water. Xylem is found throughout a vascular plant---water is absorbed from the
roots of vascular plants and carried upwards. Roots help anchor plants and also to absorb the
water found in the soil that they are in. The xylem continues up from the roots through the
stem to the leaves. Structure-wise, the stem has a function of providing support to the leaves.
Now the other vascular tissue: the phloem. So recall that glucose - a sugar- is typically
produced in the leaves during photosynthesis. All the plant’s cells will need sugar.
Sugaris the plant’s food source. The phloem carries sugar from the leaves where it’s generally made
throughout the plant. By the way, the word phloem might start with a p but it does have
the same “f” sound that food has so it helps me remember that it carries the plant’s food.
If a plant is nonvascular, it does not have vessels like the xylem and phloem. However,
it still needs water and it still produces sugar. It can’t carry water upwards in the xylem because
it doesn’t have one. That means, nonvascular plants typically are limited in size. A giant
tree needs a xylem for water transport; the water is being carried against gravity. Nonvascular
plants instead get their water by osmosis. Kind of like soaking up water like a sponge.
Nonvascular plants are often informally called bryophytes. I say informally because it’s kind
of like when you use the informal word “protists” – the term “bryophytes” is
often a collective informal term that includes three different phyla: a phylum that contains
liverworts, a phylum that contains hornworts, and a phylum that contains mosses. And actually the
phylum that includes mosses is a phylum called Bryophyta so again just be aware of the informal
use of the word bryophytes. As they’re nonvascular - bryophytes are generally small and are more
frequently - but not always- found in areas where there is a lot of moisture. They do not have true
stems nor true leaves nor true roots, although they can have structures that resemble them.
Now, let’s briefly discuss vascular plants – again, vascular plants have xylem and phloem.
These plants also have true stems, leaves, and roots. We can arrange vascular plants into three
general categories. 1. Seedless vascular plants –as their name suggests – they have no seeds.
This group includes a phylum that contains lycophytes- like club mosses – which are
different than the moss in the bryophytes. And it includes a phylum that contains
monilophytes – like ferns. 2. Gymnosperms – these include several phyla: one phylum that has Ginkgo,
one phylum that has cycads, one phylum that has gnetophytes, and one phylum that has conifers.
The word gymnosperm means “naked seed” as they don’t have their seeds enclosed like the next
group we’ll get to– instead, many gymnosperms have cones. Gymnosperms have neither flowers nor fruit.
3. Angiosperms – these are the flowering plants. They have flowers. They have fruit, though not
necessarily an edible fruit. This is a huge group- it’s estimated that around 90% of all plants are
angiosperms. Within the angiosperm group, many angiosperms – but not all - can be categorized
as a monocot or eudicot. You’ll see some of the characteristics we included of these
two groups in our illustrations here but just know, there are some exceptions. Overall, just
trying to really emphasize: the angiosperm group is the largest, most diverse group of plants.
Even though every plant species is unique, let’s talk about general plant structure. The structure
of plants is critical for plants to be able to perform photosynthesis. Photosynthesis is the
ability to make sugar---the plant’s own food source---from sunlight, and it requires three
main “ingredients:” water, carbon dioxide, and light. To get all three of these things,
the plant needs specific structures and adaptations, which we’re going to walk through.
First – water. We mentioned already how a plant can obtain it in different ways depending on
whether they’re vascular or nonvascular. Next, carbon dioxide. Many plants have these fascinating
little openings—pores really---called “stomata” or singular, stoma. Stomata are typically – but
not always- found on the bottom of leaves. Stomata have a major role in gas exchange,
because gases can travel through them. And a gas plants need directly for photosynthesis
is carbon dioxide. But don’t forget when it comes to gases: plants also do cellular
respiration and to do that, they also need the gas oxygen. However, plants generally
produce more oxygen in photosynthesis than they need in cellular respiration.
Now while gases can flow in through these stomata, there’s still a little problem. Plants can’t keep
those stomata open all the time. When they’re open, water can escape. And remember, plants need
water too for photosynthesis. So the plant has to determine whether to open or close its stomata,
and it does this with the help of guard cells. If guard cells have the stomata open, the plant gets
the gases it needs but it can lose water. If guard cells have the stomata closed, it gets to save its
water but then it limits gas exchange. At night, many plants – with a few exceptions- close their
stomata so that they can conserve their water as they aren’t doing photosynthesis at night.
Last on the list that plants need to capture is light. Plant cells have organelles called
chloroplasts. These amazing organelles are not found in animal cells. They are
the site of photosynthesis so they help capture light energy for the process of
photosynthesis. Plants also have pigments: chlorophyll is a major one that can absorb
certain wavelengths of light like red or blue light – but plants can contain other
pigments too which can expand the different wavelengths of light that they absorb.
Leaf structure is really important when it comes to capturing light energy. The size and shape of
leaves can be different based on where different species are found. Plants that live in areas where
they must conserve water might have thin leaves so that they don’t have much surface area to lose
water. Think about pine trees with their skinny pine needle leaves…or think about the plants that
live in the desert. Many of these plants also have especially waxy leaf cuticles to protect against
water loss through the leaves. Plants that have lots of access to water---but maybe are shaded by
a lot of taller plants in a jungle---might have really broad, wide leaves so that they can soak
up as much sun as possible. It’s hard to live in someone else’s shadow. Since there is plenty of
water, these plants may not struggle with water loss concern and can have more leaf surface area.
Now we talked about general plant structure, but obviously there are plants all over the
world with unique structures and adaptions. Before we end our video,
we want to conclude with some especially fascinating ones.
Plants, and other organisms, use nitrogen in the building of proteins. So what happens if a plant
lives where the soil is low in nitrogen? You may have heard of carnivorous plants like the Venus
fly trap or the pitcher plant. These plants still do photosynthesis to make their sugar,
but carnivorous plants also have the ability to digest insects typically by using special enzymes
in a juice they secrete. Insects are a great way to supplement this nitrogen need. If you ever
look at the ingredients in plant fertilizer, you will find many fertilizers are high in nitrogen.
You can also have too much of a usually good thing. What about plants that live where it’s
TOO wet? Many types of mangrove trees live in waterlogged soil. These plants can have
specialized roots that help them deal with the low oxygen and potentially high salt concentrations in
a variety of ways – see our recommended reading. Plants can even turn into thieves to get the
nutrients they need. You’ve heard of mistletoe? Is it terribly ironic that they can be parasitic
as they can use their specialized roots to steal nutrients and water from a host plant?
There are a ton of plant structures and adaptations we encourage you to explore. And don’t
forget about the value of learning about plants. After all, plants are essential for life---they
are producers which means that they are the main support for food webs. Many medications that we
have today are derived from plants. And while plants aren’t the only photosynthetic organisms,
plants are definitely known for their photosynthesis which produces oxygen for
us to breathe by the photosynthesis that they do. Explore more about plants with our Shorts
on plant tropisms and plant hormones and our entire video on plant reproduction and
the structures involved! Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.