Taking some off the top (sometimes)

On its way to flowering, a goldenrod plant grows upward.  Sometimes they grow really tall, two meters or more, and sometimes they are fairly short.  Height differs between species and between locations.

The apical meristem adds cells at the top of the growing plant.  These cells become stem, leaf, and eventually, flower, seeds, and fruit.  The apex also inhibits the growth of lateral meristems lower on the plant.  The ideal growth direction is up.  Only when it’s time to produce flowers might multiple stems be allowed to branch out and form the inflorescence (or not – many species have little or no branching to support their flowers).  

But things can go wrong as the stem grows.  If the apex is destroyed by a disease or herbivore or physical damage, then that avenue for growth is gone.  But all is not lost, far from it.  Those abundant lateral meristems, some not far below the apex, can take over once the inhibition of the apical meristem is gone.

For a species that has short branches in its inflorescence, as in Solidago puberula (downy goldenrod), a damaged apex leads to the production of branches far larger than normal, and the earlier the damage occurs, the longer those branches are.  I know this because, many years ago, I removed the apical meristems from downy goldenrod plants at various times during the growing season and watched what happened.  Apex-free plants looked quite different from intact plants.   But as much as they grew, they couldn’t quite produce as many flowers as intact plants.  They never caught up, though they certainly tried.

This June, there are already some stems that have lost apical dominance.  Here are two Solidago arguta stems, one that has an intact central stem growing taller, and one that has lost its apex and started to grow lateral branches.  This species normally produces a wide, branched inflorescence even when the apex remains intact.  We’ll see how different they look by the time they flower.