To elongate or not to elongate

The goldenrods are sending up their stems in spring.  Or not.

This is a growing stem of early goldenrod, Solidago juncea, and it might get to be a meter tall.

This is also early goldenrod, but it won’t get tall.  It will remain a rosette, a cluster of leaves close to the ground.  The leaves are large, sometimes quite large, but the plant remains short.

This is another species, probably S. arguta, sharp-leaved goldenrod (I’ll check my identification later in the season).  Sometimes they grow even taller than early goldenrod.

But sometimes they remain a rosette.

Goldenrods are perennials, but even so, a plant that produces a tall stem in one year might be a rosette the next year, and vice versa.  Or the plant will be the same form this year as last.  I don’t know what triggers a switch, or triggers a repeat, but the growth form is one or the other each year, not in between.  

Other goldenrods nearly always produce all tall stems, almost never any rosettes.  These clusters of goldenrods (probably S. rugosa and S. canadensis) are all growing tall.  Next year, they will grow tall again.  Some plants might not grow tall enough to flower, but they will have an elongated stem.

In the key for the identification of goldenrods in Gray’s Manual of Botany, Merritt Fernald split major groups of goldenrods according to how their leaves and stems grow.  On some, the basal leaves are the largest, and the leaves decrease in size dramatically going up the stem – if there is an elongated stem – or remain as a rosette.  

In contrast, the leaves on other goldenrods are similar in size up the stem, and the plants hardly ever form rosettes of leaves.  

What’s going on in each growth form?  Defining a few terms will help:

What’s a node?  That’s a location on a stem where one or more leaves are attached.

What’s an internode?  The stem between nodes.

What’s a rosette?  A stem that didn’t elongate its internodes. 

What makes a stem tall?  One or more elongated internodes.

Somewhere in the ancestry of goldenrods, one or more lineages went all in on elongated internodes, and others retained internode plasticity, able to elongate them or not.  I don’t know the underlying mechanisms of internode development, but I am pretty sure hormones are involved, probably gibberellins.  In some, they are always on (expressed).  In others, they can be turned on or off, apparently early in the growth of any one stem.

I think this difference is impressive, one more aspect of the genus Solidago that intrigues me.

This is the only website I’ve yet found that points out this developmental contrast.  From my perspective, its inclusion speaks well of the authors of the website.