Stem Cells: How To Build and Maintain Bodies, Including Plants

Until recently, I didn’t know much about stem cells except that they produced other kinds of cells and that the medical research on them was controversial. In the context of the history of life, it turns out, their importance is as fundamental as you can get.

It took more than a billion years for the first cell with a nucleus to come together. Since then, the only reliable source for a new cell has been another cell. Every cell is an offspring. True for plants as well as animals.

An embryonic stem cell (Wikipedia)

An embryonic stem cell

But while cells are specialized for one task or another, they are not always very good at dividing and reproducing. Muscle cells, blood cells, and nerve cells don’t reproduce at all. Other cells in the body divide only under some circumstances or only a limited number of times.

But reproduction is the stem cell’s specialty. When it divides, it produce another stem cell, ready for the next round, along with a muscle cell or blood cell or nerve cell or a cell of another organ. It looks the part for such flexibility—blob-like, unstructured, not committed until needed.

Stem cells are stationed throughout the body, small groups of them in each organ, like local hospitals on call to repair the sick and damaged. They are a profound piece of bodily engineering, a design for the long-term, like a futuristic car that carries little 3-D printers throughout the engine and chassis to create new parts and replace the old parts automatically.

In human embryos, in contrast to adults, stem cells literally build the body. When an embryo is only a few days old, its stem cells begin to form all—all—of the specialized cells needed in a body, some 200 of them.

In this root tip, the number 1 marks the relatively unstructured stem cells in the meristem. (Wikipedia)

In this root tip, the number 1 marks the relatively unstructured stem cells in the meristem.

Plants have stem cells too. Located near the tips of the roots and stems in a layer called the meristem, plant stem cells divide into both specialized cells for the plant and additional stem cells. Stem cells are, in other words, the place where a plant grows.

One of the wonders of any living thing is the sheer variety of its parts, the inventory of its tubes, organs, fluids, surfaces, protrusions, electric circuits and rigid pieces. As we pause to appreciate this profusion, sing the praises of the smudgy cell that creates and repairs them all.

The Most Amazing Thing About Life

“The most impressive aspect of the living world is its diversity. No two individuals in sexually reproducing populations are the same, nor are any two populations, species, or higher taxa [categories of organisms]. Wherever one looks in nature, one finds uniqueness.” So wrote Ernst Mayr in This is Biology, published in 1997.

Grains of sand under an electron microscope (wikipedia)

Grains of sand 

This was, to an extent, a new idea to me. Clearly each species is different from the next. But I had not fully absorbed the notion that every single organism, if it reproduces in pairs, is different from every other in its species. Every individual grass plant, every tree, every insect, every ant is as different from the one next to it as two human beings are. Why? As Mayr explains, diversity ensures that some individuals will fit the environment; as the environment weeds out some versions, others will survive.

But what about the diversity in the non-biological, inanimate world? “Nature” includes not only living things but also rocks, water, air, light and other forces and materials. Aren’t they unique in their own ways? Snowflakes are famously singular. Clouds are constantly changing. So is the surface of the ocean. Air flows and spins. I’m pretty sure I’ve never seen two rocks that are identical. It’s a good bet that every asteroid, planet and star is different from the others in some ways. Looking out over the dessert, the ocean, or the skies, we certainly see diversity in shape, motion, color and light.

Diversity and fertility in grass (


Still, I think the diversity of living things does “impress” us, as Mayr wrote, in a distinct way. The variety of the organisms in a species is more individualistic, unique, than the motions, colors and contours of the elements around us. And it captures our attention partly because of the sheer power of fertility. New life is always thrusting itself in front of us—in a sister’s baby, in a new puppy, among the trees at the back of the backyard, in the horde of ants and bees and birds of spring. In Origin of Species, Darwin wrote, “There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.”

Diversity is the music, but fertility is the amplifier. It’s the combination that gets our attention. And we are not remote observers. We are both notes in the music and members of the orchestra.

No Pain, No Sympathy

Which living things merit our sympathy? Our pets? Certainly. What about human embryos? And plants? Is it consciousness or complexity or being human or the capacity for pain that makes an individual life worth our empathy?

The debate over animal rights has reinforced one ancient measure of an organism’s worth. An organism that shows signs of experiencing pain is said to be entitled to live without that pain. These sentient beings include cats and dogs and most of the animals that we eat or use for clothing or experiment on. Insects, with their minimal nervous systems, are on the margin of the sentience line.

Below the line are the simplest animals such as sponges and jellyfish, along with plants and single-celled bacteria and other microbes. These don’t experience suffering and have no awareness of a kind that we would recognize. They may have value collectively as part of the environment, but a single jellyfish or an individual plant has no standing for human commiseration.

diseased tree (

Although I understand its role in the cause of animal rights, I find this sentience distinction unsatisfying. If an organism appears to be “feeling” a disease or injury, we give its condition more weight and urgency than we would if the organism—a plant, for example—were one that did not consciously experience such a condition. A tree may be attacked by insects or suffer other life-threatening conditions, but if we are accustomed to imagining the terror of a pig going to slaughter, we probably won’t empathize much with the stress on the tree as chemical messages sweep through it.

When we feel compassion for another being that is in dire straits, what are we feeling sympathy for, exactly? Is it for the possible loss of an individual life? Is it for the dangerous condition that is attacking the organism? Or is it for the pain that the organism is enduring? I think it is the last, if only because the evident pain in an animal reminds us of our own capacity for suffereing. Perhaps we should remind ourselves that non-sentient beings, though without visible torment, may struggle, crave, and weaken as relentlessly as sentient ones.

The uniqueness and persistence of life over 3.8 billion years endows all living things, both with and without awareness, with value. I’m not recommending the ludicrous extreme of not killing plants; we, like all animals, can not survive without eating other living things, either plant or animal. But I am suggesting that we heighten our awareness of the sentience distinction that cleaves the living world in two and can diminish our connection with non-sentient life.