A 4-Letter Word

CELL is a very misunderstood 4-letter word.

In working with students across a range of ages, I have to believe people generally do not give much thought to cells.  We know of red blood cells (RBC) – they can be isolated from the liquid that streams through our veins.  RBC outnumber others; according to Wikipedia: Adult humans have roughly 20–30 trillion red blood cells at any given time, constituting approximately 70% of all cells by number.

But red blood cells are not normal, they are brainless bots (i.e. they are enucleate) that live only 3-4 months and must be constantly replaced. The Huntington’s history of medicine collection documents the long struggle that led to current understanding of bodily fluids and the nature of flesh, from historical explanations about humors through to contemporary detailing of circulatory, skeletal, and muscular systems. It is bloody history, filled with grim tales of cadavers and surgical theaters.

The author, dissecting a pumpkin with a chainsaw

Plants, which are blessedly, inanimate, insensate, and soulless (at least by standard doctrine) are also made of cells. But botanists gleefully dissect and dismantle fruit and flowers, right in the public eye, with no sense of morbid curiosity. This lack of carnal sensibility means plants are wonderful learning models for biologists, just as coconuts were once used for surgeons to practice sawing into a skull without damaging the gray matter inside.

Cross-section of a very thin and small leaf from the grass Sesleria, showing basic cellular structure (Marilaun, 1913)

Plants teach students the lessons of life, because life is a cellular affair. And plants can slip students past that squeam-inducing blood stuff straight to the more solid tissues, the corporeal foundation — useful, because every muscle is made of fibers (which are cells) and the nervous system is made of neurons, each of which is a living cell.

Call it cell-blindness, but most students do not intuit the consequences of cellular life. When pressed to study plant tissue through a microscope, at simple magnifications from 20× to 400×, students seem unprepared to accept the context – the reality that every bit of plant material in their field of view is organized as cells – cells in layers, cells in files, cells in spongy arrangements. Until someone has sliced and diced, stained and strained to study plant tissues, that basic reality is nonsensical.  Plants look so plastic, so very homogeneous.

Field biologists are allowed to ignore that basic truth about cells and anatomy, able to determine a lot about plants through studying complex 3-D shapes, with minimal regard to internal structure, to the millions of specialized cells involved…. But a plant isn’t just the form (the morphology) studied for identification; lives are involved here.

I say “lives” because living cells are there, busily maintaining their own balances, their homeostatic integrity. Some retain potential to change, such that you can harvest a tiny bit of tissue and culture an independent tissue sample, a callus, that you might coax into becoming a new, liberated plant. This can be done with animal cells too, not making new cloned people, but maintaining tissue in sterile containers (in vitro, even though a lot of the containers are plastic.) Henrietta Lacks passed away in 1951, but her cancerous cells constitute a famous “immortal cell line” named HeLa, the first successful strain of human cells successfully maintained.

Most cells cannot be seen without the aid of microscopy, but there are a few extraordinary plants cells you can make out with the naked eye; a single cotton fiber is one cell that can be over 2″ long. (see Cells to Contemplate) An amazing green alga, Valonia, looks like small jellybeans, but each bean is a single multinucleate cell.  

The micro-dimensions of typical cells mean scientific advances had to await technology in optics before much could be known about plant and animal life. I wonder what Robert Hooke first thought when he viewed a thin slice of cork through his new “microscope” and discovered it was compartmentalized, made of bladders, “cells” he called them. It seems he considered the cells to be bubbles of some kind, just as we understand styrofoams today. Hooke’s description of magnified cork in Micrographia, (1665) constitutes the first recorded use of the term in a biological sense.  (See Super Suber)

Hooke, however, didn’t take the concept of plant cells further – indeed there was no reason he should have imagined more. In Hooke’s wake, Nehemiah Grew published a beautiful English atlas of plant structure in 1671, followed in 1679 by Marcello Malphighi’s studies.  Immediately then, scientists knew plants were built of millions of specialized microscopic cells, though people uniformly believed cells were simply bubbles or beads. We have to recall this was over three hundred years ago. Carbon, hydrogen, oxygen, and nitrogen were not known; even the early suggestions of photosynthesis and respiration were yet to emerge. Nearly two centuries would pass before people understood the living nature of those cells.  

Nehemiah Grew, Anatomy of Plants
Grew, Anatomy of Plants
Illustration from Marcello Malpighi, Opera omnia, 1687, borrowed from Romero, 2011

Around 1842, when shared qualities of plant and animal cells became apparent, botanist Matthais Schleiden and zoologist Theodor Schwann popularized the realization that life is a cellular affair, an epiphany we call the “cell theory.” The term “biology” had been introduced earlier in 1802, but only with the cell theory did we come to appreciate that plants and animals are multicellular organisms that share life’s most basic processes, such as metabolism and genetic control.

Plate 1, from Smith’s 1847 translation of Schwann’s Microscopical Researches…., showing cellular structure of plants (from work by Schleiden) and animals. Fig 1 is onion cells.

The fact that life exists because cells exist means the simple 4-letter word “cell” carries overwhelming freight. An oak tree, built of tens of trillions of cells, began as one cell, which evidences a lineage, a continuity of life reaching at least 2 billion years, to the first true nucleate cell. Somewhere in that living continuum there was a cell that is also part of human ancestry. You and I and the oak share some ancestor, unknown and long extinct.

If students can construct a 3-dimensional sense as to the cellular nature of plants through dissection and examination, they can more readily understand their own tissues: skin (epithelial), muscle, connective (blood and bone), and neural. Learning about plants is learning about life. Plants and gardens are a gateway to self-awareness.

Romero, Rafael R, 2011. Marcello Malpighi (1628-16940, Founder of Microanatomy, Int. J. Morpho. 29(2): 399-402. https://scielo.conicyt.cl/pdf/ijmorphol/v29n2/art15.pdf

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