A thundershower drowned out John’s and George’s usual botanicals today, but I’ve been in the wilderness much this week with my Palm Beach State College classes. Walking in a sunny, burned, -browsed slash pine savannah, a special sort of beauty is at its best now: species with basal rosettes: lilac-flowered Elephant’s Foot, white-headed Hat Pins, purple Chaffheads, yellow Pinebarren Milkwort, and more. Think of dandelions. Leaves on the ground arranged like propeller blades, flower(s) in the middle.
Here’s a great case where plant form correlates with habitat. Savannah life favors plants hunkered down with their leaves and buds safely close to the ground.
The world’s greatest botanist concerned with form and function was Christen Raunkiaer (1860-1938) who developed a classification of plant life forms related statistically to ecological circumstances. His system thrives 80 years later. Every botany student memorizes the polysyllabic category names in the “Raunkiaer Life Forms.”
Rosettes fall into the category of “hemicryptophytes,” plants with their vulnerable buds at ground level just like soldiers in battle relatively out of harm’s way. What harm? Grass-fires, hungry herbivores, hot dry winds. Layin’ low is how I survive the brushfires of office politics.
Rosettes have benefits beyond cowardice. Leaves lose water mostly from their undersides. Undersides pressed against the ground with topsides in full sun maximize photosynthesis while minimizing the associated water loss.
All spread out, the leaves cover competitors’ seedlings like mulch fabric while not shading each other.
Rosette leaf bases are direct contact with the roots. No pesky stem in between to slow the exchange of water, minerals, and carbs. When the root is loaded up with food and seasonal conditions are right, a flower stalk rises confidently from the rosette into the lofty realm of pollinators. When the stalk dies down, or burns, or feeds a buffalo, no problem, the vital portions of the plant remain basking on the sand.
If you’re going to lie upon the soil with your leaves spread out, you might as well catch extra nutrition with that sprawled array. I suspect, and have read hints, that rosette leaves can be involved in rootlike absorption, including (rarely) growing “root” hairs on the leaves. To go a hungry step further, several rosette plants are carnivorous, including some bromeliads, butterworts, sundews, venus flytraps, and certain pitcherplants.
New growth comes from buds. If each leaf in a rosette has a bud, the plant is well set up for reproduction. Even if the center plant dies, each of those buds can turn into a chick around the mother hen, or into a rhizome soon to start its own rosette. Rosette plants can colonize space.
The rosette lifestyle has evolved many times in many different plants, so we might wonder if it is “easy” to come by. A rosette plant is merely a species where the stem fails to elongate. The plant stays condensed.
Treatment with the stem-elongating plant hormone gibberellin can cause a rosette plant to stretch out and rise. To become a rosette species, maybe all you need is to cut your gibberellin content, perhaps not involving substantial genetic change. No wonder rosettes are prime examples of convergent evolution—great “idea” and easy to achieve.