A Pumpkin Lesson
by Michael Rossman
When Halloween comes, I ask my students to save their carved pumpkins, in their back yards or gardens. A week later, I bring two rotten ones to class, so that we can feast our minds on decay. For even the youngest classes, it's a spectacular introduction to the subject, and we take some time going over the actual meanings of these terms. By rot and decay, we don't mean that a thing is dead, or smelly, or icky-repulsive -- for something may be any of these yet not be rotten. We mean that microscopic organisms are at work eating it, breaking down its cells and tissues, changing its chemicals -- which accounts for the different smells, the changes in its appearance and strength.
On one pumpkin, macro-scopic agents are also at work, for its ruined walls are covered with thick fuzzy growths of mold -- here and there in white patches, but mostly black. I set up the projection microscope, take two samples of mold, put them on a slide with a drop of water on each, and project their images on the whiteboard. The white sample shows us the branching filaments (hyphae) that form the mold itself. We see that each thin hypha is almost transparent, so the white appearance of their tangled mass (called the mycelium) is not a color but a play of light, like the whiteness of clouds.
When I switch to the black sample, the reason for the difference shows vividly, for thick spheres appear at the tips of the hyphae. As we watch, we can see one spore-ball coming apart, as the tiny black spores drift off into the surrounding water -- much as dandelion seeds do, revealing the smaller, light-colored center to which they were attached. The spore-balls are large enough to see with a hand-lens. On the intact mold on the pumpkin, they cluster so thickly that the mold's as dark as soot. A light breath sends their dust-sized spores swirling into the air around us.
Since the rainy season's here and mushrooms will soon be popping up, we review what we know about these higher fungi and why they differ from molds. I sketch the ground, with the thin threads of a fungus's hyphae going every which way underneath, feeding on the decaying leaves mixed in the soil. Here's where the real action is, not in the mushroom that later sticks up from the surface. But what good would it do the fungus, to grow spore-balls like the mold does on its hyphae tips, if these were underground? The spores couldn't go anywhere. So I sketch the hyphae coming together into a mat, into a knob sticking up from the surface, growing rapidly to a mushroom profile with gills held high above the ground. A detail sketch shows spores forming in a small cavity on the gill, dropping out and down when they're ripe, to be caught and spread by the breeze.
Enough, back to the fuzzy pumpkin. Looking more closely, we see a few patches of different color, grey and blue-grey. The children identify them as different molds, with differently-colored spores. I promise a minor prize next week, for whoever brings in the pumpkin with the most interesting molds. Then I show them a small, fresh cut on my thumb, before plunging it into the mold. "Yew, ick," says a newcomer, not yet used to my ways. "So am I in much danger of infection?" I ask. Those who know me know I'm fishing for a no. But it takes a while to lead them to deduce why -- for the mold is specialized to feed upon the pumpkin's chemicals, mainly carbohydrates, whereas I am mostly animal proteins and fats, offering it no nourishment.
Stepping back from the moldy ruin, I tell them how I spent the afternoon before Halloween, going round to commercial pumpkin patches with my young son and a plastic bag. At each lot, some pumpkins start to rot early, can't be sold, are discarded as garbage. I rolled up my sleeves and scooped out the seeds, we filled half a garbage-bag. Took them home, rinsed them off, spread them on cookie-tins, sprinkled lightly with salt, toasted them for half an hour at 350 degrees -- and presto, munchies to last all winter. Suppose you found a pumpkin even this rotten -- I ask them, of our mold-garden -- would it be okay to take its seeds, roast and eat them?
They puzzle the matter, and decide that it would, provided the mold washed off and the seeds themselves weren't rotten. So why would it turn out okay? They are quick to decide that the seeds have a protective coat, that keeps them from decay. But it takes more discussion to bring out the subtle rightness of the reason. For how are pumpkin seeds meant to be released into the world, anyway, but by the pumpkin's rotting? They're designed by evolution to resist the flesh's decay, preserving the life within. In this and other ways, pumpkins are like other members of the squash family, from melons and cucumber to the big Hubbards and zucchini -- all of their seeds are edible, can be roasted, though only the large are worthwhile.
Of the two pumpkins before us, one still stands high despite its covering mold. The other has hardly any mold on its surface, but lies slumped almost into slush, in a big puddle of water tinged light-brown, held within a garbage bag. We wonder why they are different, and decide that different kinds of fungi may be at work on them, or that the pumpkins themselves might be of different strains, or both. Focusing now on the second pumpkin's lessons, we start with the smell. They pass the garbage bag around to savor it closely. It reminds them of rising dough, wine, last year's experiments fermenting apple juice. We identify the distinctive chemical odor as alcohol. How did it get there? From last year, we know: yeast organisms turned sugar into alcohol. But if you chew a piece of fresh pumpkin, it doesn't taste sweet. So where was the sugar in the pumpkin?
We go back to basics, review the most complicated and important chemical equation I give them:
6 H2O + 6 CO2 + [energy] ---> C6H12O6 + 6 O2
Photo-synthesis means using light to put together; here sun-energy is used to put water and carbon dioxide together to form a molecule of sugar plus waste oxygen. Animals run the equation backward, burning (oxidizing) the sugar to get the energy back, to run their own chemical processes. So do plants at night, and in daytime too; but during sunny hours they synthesize much more sugar than they burn.
So why don't plants taste sweet then, except for their small flowers/fruits, if they're always making sugar? Because they use the sugar molecules to build two kinds of larger molecules -- one for constructive purpose, the other for storage. Both kinds are polymers. Poly means many; and a polymer is a big molecule formed by putting together many copies of the same smaller molecule. Some children know poly-ethylene, the soft plastic used for food containers; almost all know poly-ester, the plastic clothing fiber made from chains of ester molecules. Just so, each green plant hooks its sugar molecules into polymers. Put them together one way and we get cellulose, the molecule that builds the strong cell-walls of plants; put them together another way and we get starch, stored in grains inside the cell-walls.
We remember the potato-starch experiment. A raw potato doesn't taste sweet at all, because it's mostly starch. But chew a piece, mixing it well with saliva, and in time it starts to taste sweet -- because our saliva contains a special chemical, molecules of an enzyme (amylase) that takes the starch polymer apart, breaking it down to sugar molecules again. Just so with the starch of the pumpkin: one yeast has provided an enzyme to break it down to sugar; and another yeast has changed the sugar to the alcohol we smell. What we have here is a puddle of pumpkin wine. If it sits around longer, some ubiquitous bacteria will turn its alcohol to acetic acid, giving us pumpkin wine vinegar.
But we could chew the cellulose of stiff plant walls all day and still not have its taste turn sweet, since human bodies have no enzyme to break the cellulose polymer back down to its component sugar molecules. In this, we're like every other animal larger than one cell. The grass-chewing cow, the wood-eating termite, the wood-boring beetle larva -- all depend on symbiotic microorganisms (protozoans, bacteria) in their intestines, who do have enzymes that can digest (take apart) cellulose. The larger animals then digest these sugars, and the microorganisms themselves, eating their internal gardens. (An adult cow eats half a pound of protozoans daily.)
So much for sugar. Meanwhile, we're looking at this big puddle. Where did the water come from? From inside the pumpkin, clearly -- but where was it hidden? It doesn't spill from the hollow interior, whose seeds and stringy fibers are much too small to have contained it; nor does it gush from the thick sides when we prick them. Where could it be? We consider another example, pretend we're in a hot desert, where I get so irritated with them for not listening that I have a stroke. It's three weeks till they can come back and get my body, which had weighed 150 lbs. What does it weigh now? Much less, they guess, maybe 75 pounds. What did it lose? Evaporated water. Yet my blood, urine, and saliva together couldn't have been over ten pounds; so where was the other 65 pounds of water in my body? For me and the pumpkin both, it must have been in the cells -- after all, that's what our bodies are built of, right?
But why were the cells full of water in the first place? The answer is so fundamental and simple that it's very hard to guess. We go back to basics, recall the three states of matter, and solutions, and how molecules behave in each. We recall also what a chemical reaction is -- molecules interacting and changing to become other molecules -- and note that all living things depend on chemical reactions, to make the molecules that build their bodies and to change molecules to get the energy to run them. Now we think about two chemicals that might react. If they're in solid form, they can hardly begin, because hardly any of their molecules can come into contact to interact, being bound almost in place. For reactions to occur, the molecules must be able to move around freely, in liquid or gas or in solution, to encounter each other.
In the fluid leaked from the pumpkins, and even more within our own cells, hundreds or thousands of different chemicals are dissolved in water, to participate in the many kinds of reactions necessary to run such complex organisms. So water in cells is necessary simply to let life proceed. Dry the cell, and all the other chemicals remain, but can't interact. I cite the tardigrades, cute, microscopic, bear-shaped animalcules who live in mosses, and dessicate so thoroughly when the mosses dry, as to enter a state of suspended animation; yet their life resumes when water greens the moss and reenters and reanimates their cells.
So how did the water get out of the cells to form the puddle -- given that the fungus has no teeth to tear open the tough cell-walls? We consider the action of a single microscopic strand of fungus, one hypha. Fungus is to green plant as salamander is to reptile. The salamander's skin is permeable to water both ways, it dries to death in a dry room but can drink through its skin (whereas a reptile's skin is impervious, keeping it safe in dry climes but forcing it to drink through its mouth.) Similarly, though most green plants are protected against water-loss, by their bark or waxy cuticle, most fungus hyphae can't survive dryness but must have humid environments, can absorb water from their surroundings.
So here's the leaky fungus filament atop the thick-walled pumpkin cell. From it oozes a bit of water, with some molecules of an enzyme that can digest cellulose, breaking the polymer down to its sugars. The sugar molecules promptly dissolve in the exuded water, which is joined by the rich fluid now leaking from the cell; and the cells of the fungus now re-absorb this amplified soup, which furnishes the fungus with the water and sugar it needs to keep living and growing, plus all manner of other useful chemicals.
This explains why one pumpkin sits slumped like wet clay, and why even the other, that seems to hold its shape better, yields like stiff mud to the poking finger, that once would have bounced off its proud citadel. The cell walls are damaged, no longer hold, so the whole structure built of them loses its strength. I go on to discuss how the strength and tall-standing of grasses comes from cellulose walls, but the greater strength and durability of wood comes from a combination of cellulose and lignin. These last forever if dry, but in damp forest the woody body's components are eaten first one and then the other, or vice- versa, by two different classes of fungi that feed on these different substrates -- leaving the wood powdery and collapsing if lignin's eaten first, brittle and weak if cellulose goes first, until the other kind of fungus comes to finish the recycling.
There is scarcely any end, to the lessons that unfold so naturally from decay. But since our hour together is at an end, I leave them with a homework assignment: Take your used pumpkin, stick it in the backyard, on the dirt, where no one will be bothered by the smell and mistake it for a pile of garbage instead of for what it is, a big heap of food -- and not just for molds and microorganisms. If you leave it there for for several weeks, a multitude of small animals will visit it, drawn by the complex odors of decay spreading on the wind, to eat or leave their offspring, or perhaps to eat those who do. Snails, slugs, beetles, worms, moths drawn by fragrant esters, three kinds of fruitflies, five kinds of spiders to eat them -- who knows what? I promise a small prize, for whoever brings in the most different kinds of visitors to their crumbling golden treasure, and bid them farewell till next week.