A friend's father, who possessed a kind of countrified Steven-Wright humor, once remarked on the ability of a thermos bottle to keep hot things hot and cold things cold. "How does it know?", he asked. My friend's father was joking, of course, but I have another question that is far more interesting (if not as funny): how do seeds know when to start growing?
As a standup comedian might say, think about it. If a temperate zone plant produces seeds in late fall, and they germinate right away, all those little seedlings are going to be wiped out by the first hard frost. If a desert plant produces seeds that mature in the dry season, and they germinate right away, those little seedlings will die for lack of water. You get the idea – seeds that germinate at the wrong time or under the wrong conditions won't be able to fulfill their function of perpetuating their parent species.
Granted that seeds don't actually "know" anything, what mechanisms have plants evolved to ensure that their seeds germinate at the optimum time? For the surprising and complicated answers to that question, I am indebted for my own limited understanding in large part to the work of the late Norman C. Deno, formerly Emeritus Professor of Chemistry at Penn State. Professor Deno had a side job, which was developing an extensive catalog of the conditions under which seeds of different plant species germinate, and developing a theoretical understanding of the underlying processes. He self-published "Seed Germination Theory and Practice" in 1993, and subsequently published two supplements that eventually covered hundreds of plant genera. A projected book tentatively titled "Seed Storage Theory and Practice" was, to the best of my knowledge, never written.
Deno's first principle of seed germination is this: "Every species of plant has one or more mechanisms for delaying germination until the seed is dispersed." This is, for some of us, a reversal of the way we have characteristically thought about germination, which is perceived to be triggered by some event – longer periods of daylight, or rising temperatures. Deno starts with the assumption that all seeds have delay mechanisms programmed into them, and that germination begins when those obstructing mechanisms – physical or chemical – are removed.
Some of the factors involved in removing inhibitory mechanisms are obvious: temperature, light, and humidity. Others are a little more obscure – digestive processes, symbiotic relationships with fungi and other organisms, other chemical interactions. To illustrate this last category, some fruits contain chemicals that inhibit gemination, and only when the seed is finally free of its surrounding fruit will it be able to germinate. Even the obvious factors may require certain sequences of events – low temperatures followed by higher temperatures, for example, or a period of low humidity followed by an abundance of moisture – in order to remove the virtual STOP sign from the germination process.
Professor Deno's work is now available for free download, and I should warn you that it's not light reading. But all of us interested in the plant world owe him a tremendous debt. He has helped us understand the ingenious and mysterious processes of germination, and has also given us an immense body of practical data for successfully germinating our own plants.
Image credit: Microphotograph of seeds, Rob Kesseler