At the moment parasitic plant expert Sasa Stefanovic is visiting our herbarium to study the genus Cuscuta (Convolvulaceae; but unfortunately I do not have good picture of it). Today he gave a seminar at the ANU, and I noted with interest what terminology he used to distinguish the two main groups of parasitic plants.
The first group are plants that have haustoria, organs that they use to attack the phloem of other plants and draw water, nutrients and energy from them. This adaptation occurs across several groups of eudicots but interestingly not in the monocots.
The second group are plants that parasitise on fungi. An example is the strange European orchid Neottia nidus-avis depicted above. They have clearly evolved from ancestors that used mutually beneficial mycorrhiza, trading sugar against otherwise hard to obtain nutrients, but then turned the relationship into pure exploitation. This adaptation is found in Asterids, Monocots and one truly bizarre New Caledonian conifer, but apparently not in Rosids. In the past, people often believed that this second group was saprophytic, and one can even now see books making that mistake. In reality, there are no saprophytic plants; they are all either photosynthetic or parasitic.
Now the interesting thing is that according to Sasa Stefanovic, the community of parasitic plant researchers calls only the first group parasites, whereas the second group is called mycotrophic or heterotrophic. I must admit I find this a bit strange, as they are clearly both parasitic, only on different groups of organisms, and both heterotrophic. What is more, the people who are still stuck with the impression that the second group is not parasitic would not have their confusion cleared up if they heard these two terms used in this way.
But well, if that is what the community has decided, that is that. Not my area. At any rate it was interesting to learn how these two forms of parasitism are distributed phylogenetically.
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