Cells, little rooms—why worry about cells? I thought we were talking about trees, nice big trees. Why these little cells, and how small are they, anyway? Trees are amazing, as is a human child, a mouse, or a blade of grass. We use these names for their finished forms, but many living creatures, including humans, are made of trillions of cells. Yes, we are all cells. Cells functioning in groups are called tissues; humans have over one hundred tissue types, including liver tissue, brain tissue, and so on. Trees are less complex in this regard, but they do contain many tissue types, as the most cursory glance at the difference between leaves, trunks, and roots reveals. The size of cells varies greatly. Some are big, just visible to the human eye, but others are so small that some form of magnification is needed. Indeed, most of the detailed modern understanding of life, cells, and so forth, did not occur until the 1950s and beyond, when powerful microscopes could finally see down into cells and their much smaller internal components, their organelles.
Let's use an example to get us into the zone: a baby's hair. It would be approximately 50 microns in diameter, 50 millionths of a metre. Some cells are much smaller, and some much larger, but the diameter of a baby hair serves as a good example of cell size.
All plant cells start off as parenchyma cells. The three tiny organs in the seed are all parenchyma: the meristematic mother cells of the SAM, RAM, and the VC, and the cork cambium, all are parenchyma cells. That's the shoot apical meristem, the root apical meristem, the vascular cambium, and the cork cambium, also known as the phellogen, all parenchyma cells. After differentiation and growth, they are modified, often reinforced with cellulose and lignin, and are then called by other names, with many different functions. Parenchyma can be long-lived, sometimes over 100 years, as living axial and radial parenchyma cells deep in a living trunk, still alive, connected to the symplast, storing energy, moving water and oxygen as needed. Cells with a single layer of reinforcement inside the initial cell wall are called collenchyma. These cells are used where some strength is needed that can't be supplied by parenchyma cells, for example in the leaf blade and its stem, called the petiole. The leaf cells are parenchyma, mostly chloroplasts, where photosynthesis occurs, guard cells, the stomata, and pavement cells, the outer cuticle or leaf epidermis. The petiole is given extra strength by the addition of collenchyma cells, which help hold up the leaf, allowing it to face the sun.
Sclerenchyma cells are woody cells. They have, in addition to a cellulose layer, a layer of lignin, which is very strong, and in their billions, are all that hold a tree trunk up. Vessel elements, tracheids of conifers, fibers, are all sclerenchyma cells. Many are dead at maturity, vessels and tracheids especially, as they need to be hollow for optimal water movement. Another form of sclerenchyma cells are called sclereids. These are the tough, woody cells that form the protective layer around peach seeds and others.
No matter where or in what tissue, all plant cells are one of three distinct types: parenchyma, collenchyma, or sclerenchyma. The differences are defined by the cell’s function. Parenchyma are the fundamental plant cells, and all cells begin as parenchyma and are differentiated later as needed.
Plant organs—leaves, stems, and roots—are made of tissues designed for specific functions. Tissues can be simple, made of one type of cell, or complex, made up of at least two different types of cells. A leaf reinforced with collenchyma cells is a complex tissue.
Parenchyma cells perform a variety of functions such as photosynthesis, nutrient incorporation, respiration, storage, and secretion. When needed, parenchyma cells can also transform into all other needed cell types. Parenchyma cells are concentrated in ground tissues, in the pith of new stems, and in the cortex of roots. Parenchyma cells can be highly specialized, such as chlorenchyma (chloroplasts), which perform photosynthesis, or they can simply be ground tissue. Parenchyma cells alone retain the ability to perform mitosis. Parenchyma cells are found in all primary and secondary meristems, the SAM, RAM, vascular cambium, and the cork cambium.
Collenchyma cells, with their inner cellulose reinforcements, are used when some extra support is needed without the rigidity of sclerenchyma cells. They are found in leaf petioles and in elongating (growing) new stems where support is needed, but where the inflexibility of sclerenchyma cells would be too much. Collenchyma cells also supply support along major veins in leaves.
Sclerenchyma cells are used primarily for strength, protection, and transport. Highly variable sclerenchyma cells fall into three main groups: fibers, sclereids (e.g., stone cells in peaches), and water conduction cells. Sclerenchyma cells are rigid and do not depend on turgor pressure; they rely on the secondary inner cell wall component lignin for physical support. They are usually dead at maturity. Water transport cells, vessels, and tracheids are a major part of the xylem tissue. Indigestible to insects and other animals, sclerenchyma cells create a formidable barrier to their attacks. Fibers are long, narrow cells that are dead at maturity, not usually involved in water conduction. Fibers are found within vascular tissues, xylem and phloem, supplying additional support.
Unique to plants, the plant cell wall, especially the fortified multi-layered cell wall, is a miracle of adaptation. Without the need to grow up into the sunlight and be strong enough to do so, many of the essentials of our lives—wood, tree fruit, spices, and the list goes on—would not exist. Plants would have stayed close to the ground, as all herbaceous plants without secondary growth still do, and the wonder of trees and all they give humankind would not be here. That would take away the dominant worldwide vegetation, some three trillion trees. Two miracle molecules created by plants, cellulose and lignin—a strengthening, glue-like substance manufactured inside cells from phenyl propene—allowed plant cells to be stiff enough to hold up a giant tree trunk. Indeed, the difference between human muscle and fibers in tree trunks is no contest; a small sample of human muscle lies as a blob in a petri dish, while beside it, the smallest wood sliver stands firmly. Plant cell walls have allowed us to make the world we live in.
Reinforced cell walls are found mostly in a group of plants known as tracheophytes with secondary growth, that is, plants with an extensive vasculature, a xylem and a phloem, like most shrubs and trees. The bryophytes, which consist of the hornworts, liverworts, and mosses—all ancient plants—do not have these. Vascular plants depend on hydraulics and cells strong enough to perform growth, expansion, and long-range transport.
The individual plant cell’s contents, its cytoplasm, is firstly bound by an inner layer called the plasma membrane, a lipid bilayer. All biological membranes are semi-permeable, meaning that cells can control what passes through them. The cell membrane also allows compartmentalization, or to some degree, cellular autonomy.
Hydraulics in plants is the role of an inner cell organelle called the vacuole, a semi-permeable water sack. Its actions control turgor pressure, akin to a water balloon in a shoe box. When the balloon is pressing firmly against the inside of the box, the cell wall, we have high turgor pressure. This can easily be seen anytime you water a limp house plant. In growing meristems, high turgor pressure helps growth by stretching expanding cells.
Plant cells can develop high internal pressures; pressures of 200 psi are not uncommon.
Without strong walls, most plant cells would rupture under these conditions. Plant cell walls come in two main types: primary and secondary. All plant cells have primary walls, usually made up of cellulose; secondary walls use the miracle polymer lignin to help fortify their structure. The primary wall provides support during the growth and expansion period, while secondary walls support cells that will need to be extra strong, such as vessel elements, tracheids, fibers, and wood. The vessel elements are dead at maturity. Once growth and wall reinforcement have occurred and the cell is complete, a program of planned cell death occurs. To function optimally, vessel elements need to die, to be hollow, allowing the dead cell to be as efficient as possible in performing liquid transport.
Communication and fluid movement between adjacent living cell primary walls are performed by tiny channels in the cell wall called plasmodesmata and by other openings called pits in secondary walls of non-living vascular tissue. Both plasmodesmata and pits line up with their companions in adjacent cell walls to make fluid movement efficient. Plasmodesmata are very small, 30 to 60 nanometers, that's 30 to 60 billionths of a meter.
The cell itself can be 50 microns across, that’s 50 millionths of a meter. Yes, life does happen on a pretty small scale.
The plasmodesmata in cell walls connect the cytoplasm of all living cells into a great network called the symplast. Average plant cells have between 103 and 105 plasmodesmata per cell. Used for fluid and sugar movement and communication by molecular exchange, proteins, transcription factors, and messenger RNA are all transported through plasmodesmata. The space outside cell walls, the intercellular space, is called the apoplast and is also used for water and molecular movement. It is the space where water from cells can be removed to, especially when concentrating cell contents for extreme cold conditions.
Reinforced secondary walls using lignin are made only where they are needed. Many cells once formed remain with only the primary wall in place (parenchyma). For example, inner leaf cells have no need for secondary wall reinforcement. These extra strong walls would only impede light penetration in chloroplasts, reducing photosynthetic output. Where newly generated cells find themselves will determine how much wall thickening is needed. The leaf midrib and petiole may need some support (collenchyma) whereas new vessel elements, tracheids, and fibers in the trunk will need all the support they can get (sclerenchyma). This is an example of amazing gene expression. No matter how complicated, the system almost always works. Wall thickness costs the cell some internal volume as wall fortification always occurs inside the primary wall.
For cells designated to be wood inside tree trunks, the production of secondary walls is their main function, and it is what their cytoplasm is designed to do. In the strongest multilayer cell walls, lignification has occurred, leading to hardening and a loss of elasticity and flexibility. In tracheal elements, the secondary wall material is not a solid layer but is laid down in rings, semi-circular forms, or strands that allow spaces for the pits to align with adjacent cell pits, forming pit pairs to make upward water movement as easy as possible.
All of this comes from my decades of experience as a Calgary arborist and Calgary tree doctor. I do it for the betterment of our urban forest, a delicate created biome.
