Plants of Viola maviensis show that the Hawaiian violets begin life with prostrate stems, like violets all over the world, but then they change, and the stems become upright, a feature unique to the Hawaiian violets.
RAYLESSNESS [ PDF ]
I didn’t write the first paper on raylessness in woods—Barghoorn did. But what hadn’t been appreciated was that this was a form of paedomorphosis related to change in the growth form of the plant. In the cambium, subdivision of fusiform cambial initials into a files of short cells, which are thus ray initials and give rise to rays, occurs. But what if juvenilism is so pronounced that such subdivisions, which occur in most woods, do not occur, and rays never form? Raylessness results. Any evolutionary significance? Yes, absence of rays can make for greater strength, because the entirety of the wood is devoted to water conduction (vessels) and mechanical strength (libriform fibers). This sacrifice of radial conduction (of photosynthates in ray cells) in favor of mechanical strength (by forming libriform fibers where rays would potentially exist) can only go on for so long. So raylessness does not occur in any “seriously woody” species. Mostly, raylessness occurs in “woody herbs” like prickly phlox (Leptodactylon), Hebe (a well-known garden shrub) or the weed called plantain, Plantago. Some plants that start with rayless wood eventually do produced rays, and thereby do attain a tissue that can conduct photosynthates radially. Plantago arborescens on the Canary Islands is interesting because it forms miniature shrubs up to half a meter tall. The wood is mostly rayless, but if one looks at the most recently formed wood at the base of an old plant, one can find a few rays. There are no rays evident in the two sections of Plantago arborescens shown here. Plantago is an interesting case because the woody species are on islands, and none of them have rays. Probably several different nonwoody ancestral species migrated to islands independently, and so there are several instances of how nonwoody changed to woody within the genus Plantago.
Rayless woods are found in the Hawaiian species of Viola. The flowers of Viola tracheliifolia and V. maviensis look like ordinary violet flowers, and they are closely related to ordinary violets. DNA studies by Ballard showed that the geographical source for Hawaiian violets was northwestern North America. The change from herbaceous to woody has taken place on the Hawaiian Islands, and as my studies of Viola maviensis and the other Hawaiian species of Viola show, one can see the pathway taken in this change. The Hawaiian species of Viola begin with stems that grow horizontally along the ground, like violets all over the world. However, soon the stems of the Hawaiian species turn upwards, a unique evolutionary happening.
For more information on how woodiness evolves on islands, see the section of this website on Secondary Woodiness under the Island Biology heading
Raylessness seems to me a very good indication that a plant group has evolved from herbaceous ancestors. Hebe is a New Zealand shrub familiar in gardens in temperate areas of the world. Recently, Hebe has been shown to belong to the herbaceous genus Veronica, a fact that underlines the rather abrupt development of woodiness from nonwoody ancestors. One annual species of Phacelia (Hydrophyllaceae) is known to have rayless wood—so raylessness can be a useful tactic for a plant that lasts only a year. Some dicotyledons begin with a year or two of rayless wood and then develop rays. Cyrtandra (Gesneriaceae) and Artemisia (Asteraceae, tribe Anthemideae) do this. This hints at herbaceous ancestry for these genera, but also shows a probable advantage of raylessness for providing mechanical strength for a plant early in is development.
There are a few instances of raylessness that may represent a different kind of functioning, in some plants with successive cambia [ PDF ]. In the iceplant and other succulents of the family Aizoaceae, as well as some members of the family Nyctaginaceae (Heimerliodendron), rays are absent. So what substitutes for rays? The stems (and roots) in those particular plants have a peculiar thickening mechanism, successive cambia, that produces concentric cylinders of conducting tissue alternating with cylinders of parenchymatous tissue—like the rings in a beet, which also has successive cambia. The parenchymatous cylinders are intercontinuous with each other as seen in three dimensions, as are phloem strands of the conductive tissue in Heimerliodendron and some Aizoaceae, so radial conduction can occur even though though rays are not present. The arboreal nature of Heimerliodendron may take advantage of the unbroken cylinders of fibers. And the cylinders of soft parenchymatous tissue in such genera of Aizoaceae as Aptenia and Carpobrotus may be related to the sprawling habit of stems in those genera. Stayneria of the Aizoaceae is a shrub that has conductive strands embedded in a rayless background of thick-walled fibers.
In all of the above instances, seeing the plants in the wild permitted interpretation: wood sections on slides carry some, but certainly not all of the information one needs about how woods evolve. Plants evolve in relation to an ecological context; one must see that context to understand wood evolution.