How a seed knows when to grow

AA seed lying in the soil has one decision to make and only one chance to make it. Germinate at the right moment and it becomes a plant; germinate a month early, into a late frost, or a month late, into a closing canopy, and it dies without leaving anything behind. Since a seed has no brain, no eyes and no way of looking at a calendar, the question of how it gets this right is less trivial than it sounds.

BThe first thing a seed does is refuse. Most seeds are shed in a state called dormancy, in which they will not germinate even in conditions that are perfectly suitable. This looks wasteful and is the opposite. A seed that sprouted the moment it was warm and wet would sprout during a mild week in autumn, which is the single most reliable way to die, so dormancy is a lock whose purpose is to prevent a response to the wrong signal.

CThe lock is opened by time spent in particular conditions rather than by any single event. In many temperate species the requirement is a long period of cold and damp, a process called stratification. This is an elegant piece of bookkeeping: a seed cannot detect that winter has ended, but it can count how much winter it has had, and a seed that has received eight weeks of cold can be fairly confident that what follows is spring rather than a warm spell in November.

DTemperature is not the only thing counted. Many small seeds also require light, and specifically a particular quality of light. Leaves absorb red wavelengths and let far-red pass, so light that has travelled through a canopy is depleted in red. A seed that measures the ratio between the two is in effect measuring whether anything is growing above it. Buried seeds get no light at all, which is why a patch of bare soil turned over in spring produces a flush of seedlings from seeds that have been waiting, in some cases, for decades.

EA third signal is fire, and the mechanisms are surprisingly varied. Some species have seed coats so hard that water cannot enter until heat cracks them. Others respond not to heat but to chemicals in smoke, which can be washed into the soil by the first rain after a fire. The second group is the more precise of the two, because smoke reaches seeds that were never hot enough to be scarred, and a seed that germinates after a fire enters a landscape briefly free of competition.

FWhat is striking is that no species relies on one signal. A seed that required only cold would be fooled by a cold summer; one that required only light would be fooled by a fallen branch. Real species combine requirements, and the combination is specific enough that two closely related plants growing side by side may germinate in different years. Seen this way, a seed is not waiting. It is running a short list of conditions and refusing until all of them are met, which is a form of caution that has outlasted every animal that ever ate one.