Zotz et al. examine the velamen radicum. You may be familiar with the velamen radicum from orchids, but in their paper they find The velamen radicum is common among terrestrial monocotyledons.
If you’re not familiar with it, the velamen radicum is a spongy root epidermis. It can have multiple layers of dead cells when mature and overlies an exodermis, which itself can have a barrier of dead cells. It’s a bit of a puzzle, or at least it would be if people puzzled about it. Most of the time they seem to assume it’s something orchids have to protect the roots of epiphytes or reduce water loss.
The advantage of this explanation is that it explains how orchid epiphytic roots are protected. The problem Zotz et al. find is that it doesn’t work so when when you look at what plants have a velamen radicum.
They provide an extensive list and analysis what has this layer. They find that most of the plants are orchids, but far from all. They state: More than a quarter of the 76 families of monocots have a velamen, and 13 of these 23 families are entirely terrestrial. They have constructed a phylogenetic tree of monocots and found that the velamen radicum is something that has either been invented or lost many times.
Taking an evolutionary perspective means that simply looking at orchids is not enough. Why would a terrestrial plant need something that seems to be an adaptation to exposed roots? Interestingly Zotz et al. argue that the velamen radicum could be a preadaptation to epiphytism. Once a plant has it, it becomes much more feasible to colonise trees than developing it in situ.
The literature cited in the paper date from as early as 1888, with Zotz and colleagues noting that Goebel had already identified there was a problem with associating the velamen radicum purely with epiphytes, They add a lot more references to confirm there is something significant to investigate in the roots.
This paper is part of the Special Issue on Morphology and Adaptation. It is FREE access for a limited period to the end of January 2018. It will then be free access from November 2018.
Zotz, G., Schickenberg, N., & Albach, D. (2017). The velamen radicum is common among terrestrial monocotyledons. Annals of Botany, 120(5), 625–632. https://doi.org/10.1093/aob/mcx097