Topics in Depth
The Theme of Plant Tissues in Plant Biology
No, we’re not talking about Kleenex or Puffs or other handy products during cold season, but the tissues that make up plant bodies. A tissue is a group of cells with a common function, structure or both.
Some of the key tissues in plants are:
- vascular tissues
- dermal tissues
- ground tissues
In animals, vascular tissue transports blood throughout the body, delivering oxygen, removing waste, creating Olympic athletes, and so on. In plants the tissues that transport things around the plant are called xylem and phloem. Want to hear a funny plant joke? Of course you do. If a plant became a musician, he might play the xyle-phloem. OK, maybe "funny" was a bit of an overstatement.
Non-vascular plants such as mosses don’t have vascular tissue, but in all other plants xylem transports water around the plant. Xylem cells are like zombies in that they are dead when functional. They don’t, however, eat brains. At least, not that we know of. Dead cells, which don’t have organelles filling them up, have a greater capacity for transporting water.
There are two types of cells that make up the xylem: tracheids and vessel elements. Both of these cell types are dead when they are used in the xylem. Using dead cells, which don’t have organelles filling them up, allows more capacity for transporting water.
Tracheids are long, narrow cells whose ends overlap. They have small holes between their ends, which allow water to move vertically between the cells. They also have small holes called pits on their sides, which allow water to move laterally. Tracheids look like flutes standing on one end.
Vessel elements are shorter, wider cells. If a tracheid is a footlong hot dog, vessel elements would be more like little breakfast sausages; shorter and wider. When they are stacked on top of each other, the walls between them dissolve and water can move through them. Only woody angiosperms (flowering plants) have both vessel elements and tracheids; other vascular plants have only tracheids.
Phloem transports nutrients, most of which are products of photosynthesis. The main type of phloem cell is called a sieve element. Sieve elements are different than tracheids and vessel elements because they are alive when functional. But how could they possibly have room to transport all those nutrients if they are living cells filled with organelles? Unlike Dr. Who’s TARDIS, these cells aren’t magically bigger on the inside. Sieve cells rely on companion cells.
Sieve elements are just cells, and if they are going to transport anything then they have to work together. The ends of the sieve element cells have little holes in them; where two sieve elements meet is called a sieve plate. Liquid can pass from one sieve element to the next through the sieve plate. A sieve plate is very similar to the colander you might use to drain the water from pasta after it is done cooking—except that a colander is round and a sieve plate isn’t.
Xylem and phloem make up the vascular tissue. The vascular tissue is arranged in bundles, and is easy to see in a cross-section of a stem:
Another important tissue is the dermal tissue, which is the outer protective layer. Plants don’t have skin exactly, but they do have an epidermis. (Did anyone else just picture a plant covered in human skin? No? Just us? Right then, moving on.) The epidermis is the outermost layer of tissue, the tissue that has to meet the cruel world out there. The epidermis is the tissue that encounters the rain, the heat, the pollution, and all the other elements.
In non-woody plants, the epidermis is the main security guard against the big scary world, but leaves and stems also get protection from a waxy coating on top of the epidermis called the cuticle. In woody plants, the epidermis gets replaced by a protective tissue called the periderm as stems and roots get older.
Because the dermal tissue is the outermost layer of cells, it is the plant’s first layer of defense and is important for protecting the plant. One way the epidermis protects the plant is by growing little hairs, or trichomes. Before you start imagining plants that look like they’re related to Cousin Itt, keep in mind that these hairs are super-fine. Not superfreaky. You may have never noticed the hairs before, but take a look at the next plant you pass. The little stem that connects the leaf to the rest of the stem or branch often has tiny hairs on it.
Trichomes have a few important functions, but one thing they do especially well is prevent insects from eating plants. Just as some people prefer nectarines to peaches (no fuzz), most insects would prefer trichome-less plants. No one wants a mouthful of hair when they’re eating. Trichomes can also help reflect sunlight, acting as a natural sunscreen for the plant. As it does with people, too much sun can harm plants because it damages cells and increases water loss.
Roots have epidermal hairs too. Underground, the hairs do not provide as much protection from insects as they would aboveground. Instead, the root hairs increase the surface area of the root and help take up more water. Plants are in a constant struggle to keep themselves hydrated, like a marathoner racing on a hot day. Bring on the Gatorade.
Tissues that make up part of the plant but are not vascular tissue or epidermis are called ground tissue. Ground tissue can be found inside and outside the ring of vascular tissue in the plant stem. Tissue that is on the inside of the vascular tissue ring is called pith, and tissue on the outside of the vascular tissue is called cortex.
Ground tissue is made up of three main cell types:
Parenchyma cells are the most common cells you encounter inside a plant. They have thin walls, are alive when functional, and are all around pretty awesome. (But beware! You might run into this term again, used differently, in animal biology. Don’t say we didn’t warn you.)
These cells are involved in some A-list activities in the plant, including being the site of all that photosynthesis plants are doing. But wait, you say, you’ve already read the section on photosynthesis, and you know that it happens in chloroplasts? That’s correct, but chloroplasts are organelles, and have to live inside cells. Those cells are parenchyma.
Though photosynthesis usually happens in the leaf, parenchyma cells can be found all over the plant body. They often function as storage cells, serving as both attic and garage for extra starch. Parenchyma stores starch in roots, stems and even fruit.
One other cool thing about parenchyma cells is their ability to divide and differentiate into different cell types, given the right conditions. Because of this property, parenchyma cells get involved in wound repair. We wonder when they had time for a first-aid class, with all the other things they’re doing.
Collenchyma cells have thicker walls than parenchyma and are often involved in support. Since plants don’t have bones, they have to figure out some other way of standing up straight.
Collenchyma cells like hanging out with each other so much that they often form strands, like the strings on a stalk of celery. Although they’re strong, collenchyma cells are flexible too. Some cells have lignin in them, which is a polymer that makes the cells sturdier. Collenchyma cells do not have lignin in their cell walls, so they can be nice and bendy.
A sclerenchyma cell, in addition to having a long name, has a tough job to do. It has to give its life to a greater cause: the support and protection of the plant body. Sclerenchyma cells get their name from "scleros," which is Greek for hard. These cells are hard because they have lignin in their walls (unlike Mr. Collenchyma up there) and are usually dead when functional.
Sclerenchyma cells come in two types:
Fibers are important for structure and support, acting like a skeleton for a plant body. These cells are usually grouped in threads and are long cells. Some fibers don’t have lignin in their cell walls, which makes them soft and commercially attractive. Hemp and flax fibers are both used commercially.
Sclerids are usually used for protective purposes in addition to being support cells. They have thick walls and are perfect for making up seed coats and nutshells.
Don’t be freaked out by all these cells that are dead but still working in the plant. Humans use dead cells too—our hair and nails are made up of dead cells, and many people spend more time prettying up their dead cells than their living ones with gel, mousse, hair dye and nail polish.
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