Heterotheca’s Heterocarpy

Heterotheca subaxillaris

Golden-Aster, Camphor Weed

Asteraceae

This blazing 90 degree morning John and George* investigated red widow spiders, pine tree tip dieback, loblolly bay in fancy bloom, and mosses in Seabranch State Park near Stuart, Florida.  We botanize there often.

Feral hogs have stirred up patches of soil, with a consequence I always find interesting: pokeweed babies rising from the disturbed earth.   Pokeweeds are known for their heterocarpy, that is, differential circumstances for distribution and germination within a single species.   Many plants make mixed offspring in terms of how far the seeds or fruits will travel, or with seeds having mixed germination requirements and timing.  Reportedly in pokeweed some seeds are prone to sprout soon after release. Other sleep in the earth for decades until some hog stirs things up, even in a deep shaded woods unfit for pokeweed residence.  Some time ago in this blog we covered sea rocket, where half the fruit remains on the mother plant while the other half breaks free to go colonize new beaches.

Heterotheca subaxillaris (by John Bradford)

The heterocarpic flower in pretty bloom this week is named for its hetercarpy: Heterotheca means “different containers,” in reference to its two types of fruits. Sometimes called Golden-Asters, Heterotheca subaxillaris is a common local bright sunny yellow-flowered weed on bright sunny dry sands. This species has extreme tolerance for drought and heat. Heterothecas (grandiflora) are so tough they have become invasive pests on Mars-like volcanic lava fields in Hawaii.

The leaves are fuzzy and smelly, giving today’s plant the name camphor-weed. I like the fragrance, but how many people know what camphor smells like, or even what it is? I just Googled camphor so I arrogantly know much about it for the next hour or two. Fact is, Heterotheca is a one-plant chemistry lab with a wide array of pharmaceuticals.  The “family” of fragrances Heterotheca brings to mind are wormwood, marigolds, and sunflower leaves.  (They are all related, and the similarity may come from lactone sesquiterpenes, but who cares?)

The obvious function of the stinky, sticky, chemical-laden, glandular hairy covering is to deter herbivory. Nothing would want to crawl upon or eat  camphor-weeds!   And there may be a secondary advantage to the hairs—protection from sun and drying.  Look at the death valley habitat in the photo above.   Plant hairs insulate the leaf surfaces from drying wind, and they block sun, maybe even reflecting solar radiation.   I don’t know if this is true of Heterotheca, but some botanists have suggested that glandular hairs might make a “sunscreen” that can spread and protect the foliar surfaces.  Even better, in other fuzzy species of similar habitats the hairs produce water-retentive compounds to create a moisturizing gel when the rains come.   It would be fun to look into some of this in Heterotheca which is so hairy and so oddly happy in a solar oven. The structure of the hairs helps define the genus.  A mutant hairless Heterotheca would probably wither unprotected.

As for the heterocarpy, in a flower head, most of the “seeds” (achenes) have parachutes to blow away to colonize a distant disturbed sand pit.   Some of the seeds, however, have no parachute, and recolonize their home neighborhood.

Heterotheca achenes showing heterocarpy: one has a parachute; one is bare. One flies far away; one keeps the home fires burning.

*Sorta let the blog slide.  Lost my mojo when our awesome international blog friend Mary Hart passed away in the U.K.  But John and George have kept up the botany—John has been working on the Seabranch site linked above, with some help from George.  And George has been developing a companion site to an introductory botany course, with a lot of John’s photos.