Plants are living organisms, so like any other organism on the planet, we know that they must be made of cells. And more importantly, plants are rather large multicellular organisms, so we should say they are made specifically of eukaryotic cells, just as all multicellular organisms are. As we learned in the biology series, eukaryotic cells have a variety of organelles inside, and most of these organelles are quite similar whether the cells belong to plants or animals. But we can identify plant cells very easily under a microscope because they have a few key differences. These would include the presence of chloroplasts, a central vacuole, and a cell wall, all of which are structures that are absent in animal cells.
If you need to review general information about plant cells, visit my biology tutorial on the subject now, as well as the one on animal cells, if you’re rusty on organelle function and organization. Now, with the basics regarding plant cell structure understood, we need to get more specific. With animals such as ourselves, we have all kinds of different cell types that allow for the wide variety of organs and tissues in our bodies, as we learned in our study of human anatomy and physiology. Plants are precisely the same way, in that not all the cells in a plant are exactly the same. We can verify this just by looking at a tree.
The roots, trunk, branches, and leaves are all made out of cells, but those cells look very different under a microscope and they have very different jobs, just like our own nerve cells look very different from muscle cells, which look very different from skin cells, and so forth. Let’s name the different types of plant cells and describe them now. First, there are the cells that plants use to grow. These are called meristematic cells, and they’re very much like the stem cells we have in our own bodies. What this means is that meristematic cells are undifferentiated, or without a specific job assigned to them when they are first created through mitosis.
So when meristematic cells divide and replicate, they can produce daughter cells belonging to any other kind of plant cell. The important difference between meristematic cells in plants and stem cells in animals is that meristematic cells don’t get used up, so they can continue dividing and helping the plant grow indefinitely. Meristematic cells can be found in the tips of the roots, which are the parts of a plant that grow down into the soil, and in the tips of the shoots, which are the parts of a plant that grow up into the air. This means meristematic cells allow the roots of a plant to grow deeper into the soil and the branches of a plant to grow taller into the air. More specifically, they can be present in the apical or farthest position at the tips of roots and shoots, the lateral or side position within the vascular or transportation tissues of the plant, or in the intercalary position where branches intersect and where leaves attach to branches.
The second type of plant cells, parenchyma cells, do most of the work within a plant, being that they’re a sort of general-use cell. Parenchyma cells are responsible for most of the photosynthesis that occurs, but they also do most of the energy and nutrient storage for the plant, as well as much of the nutrient transport. These cells have thin walls, no specialized structure, and come in a variety of shapes to support their diverse functions. In leaves, parenchyma cells form the two layers of mesophyll where photosynthesis and gas exchange take place. In roots and seeds, parenchyma cells are responsible for storing starch, fat, and water.
Parenchyma cells also make up most of the structure of a fruit, and they create new structures to heal areas where a plant has been wounded. Collenchyma cells are the third type of plant cell, and they’re a kind of back-up system for the plant. These cells can contribute to photosynthesis and nutrient storage. But the most important job for collenchyma cells is providing flexible structure to the plant. Collenchyma are long cells that have thickened cell walls, meaning that when they’re in a group they act to make that part of the plant stiffer. You’ve probably eaten collenchyma cells before. In fact, the “strings” in celery are collenchyma cells. Groups of collenchyma cells can keep leaves from tearing, allow petioles, which are the little stems on the ends of leaves, to bend and flex in the breeze, and essentially give the plant some room to stretch without breaking.
What’s really amazing is that plants which experience a lot of bending due to wind, or even artificial disturbance from a scientific researcher, have collenchyma cells that can be up to twice as thick as normal. Even though collenchyma cells are pretty stiff in their structure, they’re also very flexible and able to grow and change as the plant grows. However, collenchyma cells don’t last forever. In woody plants, the collenchyma cells are only needed to do their job of stiffening and strengthening the plant until the sclerenchyma cells take over. The final type of plant cell, as we just mentioned, are the sclerenchyma, and these are a little different from the first three. Though sclerenchyma also have thickened cell walls like the collenchyma, the real difference is that sclerenchyma cells are dead and found in parts of the plant that are no longer growing.
These sclerenchyma cells provide the most support for the plant by creating woody tissue in stems and trunks. Sclerenchyma cell walls contain lots of cellulose and lignin, which are both complex biopolymers that are difficult to break down, so they last a long time. These structural cells can be arranged in two basic ways. Sclerenchyma fibers are stretched lengthwise in a plant stem and provide most of a plant’s support. These fibers are also what people use to make rope and fabric out of plants, such as flax, jute, and hemp. The other group of sclerenchyma cells are the sclereids, and they’re much more versatile. These sclereid cells are what make up the shells of nuts, the hard coatings of seeds like those found in peaches and plums, and many other hardened structures in a plant. So now we know about the four types of plant cells, those being meristematic, parenchyma, collenchyma, and sclerenchyma. But cells don’t really function individually inside a plant.