Early research into Exocarpos bidwillii root parasitism and consideration of recent observations. Duncan Cunningham.
THE PARASITE: root connections.
The following researchers conducted some foundational insights into the anatomy and workings of haustoria of Santalaceae including Exocarpos bidwillii. I have provided extracts from each of the main papers in the relevant, academic journal.
Philipson, W. R. 1958. Some Observations on Root-Parasitism in New Zealand. Transactions of the Royal Society of New Zealand Vol. 87, July, 1959.
Access and read here: https://paperspast.natlib.govt.nz/periodicals/TPRSNZ1959-87.2.4.2
“The small shrub Exocarpus (sic) bidwillii is frequent at mid-altitudes on the mountains of the South Island of New Zealand. It attracts attention by its curious appearance: the densely branched erect twigs are reddish-brown and the leaves are reduced to minute scales. The root system is woody and usually runs underground for considerable distances, often among boulders. Sometimes, therefore, it has proved difficult, or impossible, to demonstrate haustoria. However, these have been observed in a sufficient number of cases to warrant the assumption that they are general or even invariable. The haustoria, when they are seen, are found in abundance, and may form such an intricate plexus of connections with several roots that the relations between parasite and host are difficult to interpret. This complexity is due to the finer roots of Exocarpus producing a close succession of haustoria when they are appropriately situated in relation to a host root.”
Fineran, B. A., January 5, 1963.
Root Parasitism in Santalaceae. Nature No. 4862
Access and download pdf here: FINERAN, B. Root Parasitism in Santalaceae. Nature 197, 95 (1963). https://doi.org/10.1038/197095a0
“The Santalaceae are a family of some 250 species of mostly root semi-parasites. Although parasitism has been known in some species of this family, the existence of parasitism in the New Zealand representatives, Exocarpus (sic) bidwillii Hook. f. and Mida salicifolia A. Cunn., was not definitely known until recently. Since then the parasitism of E. bidwillii has been examined in greater detail. The investigation has revealed many features not previously known concerning the parasitism of the plant and of the structure and development of its haustorium.”
“Apart from ascertaining the nature of parasitism in E. bidwillii, the results of this work also suggest that some of the earlier work on santalaceous parasites was incomplete and that the conclusions and comparisons drawn may not be valid. This applies particularly to what is described as the mature haustorium. Where penetration is reported to be only superficial, as was originally described for E. bidwillii, further investigation might show that the haustorium investigated is merely a stage of development. Since young haustoria are usually more numerous and conspicuous than old, an insufficient
examination of the root system has probably been responsible for immature haustoria being described instead of the mature organ. The assumption that most santalaceous haustoria are short-lived organs developing little secondary tissue could also be re-evaluated. Most santalaceous parasites are woody plants, and since their nutrition is apparently dependent in part on parasitism it seems peculiar that this demand should be satisfied by short-lived organs which seldom become intimately associated with host tissue for any length of time. Such a condition would scarcely appear to be successful parasitism.”
Fineran, B. A., August 24, 1963.
Parasitism in Exocarpus(sic) bidwillii Hook.f. Trans. Roy. Soc. of NZ No. 8.
Access and read in text here: https://paperspast.natlib.govt.nz/periodicals/TRSBOT19630824.2.2
“The degree of dependence of E. bidwillii on parasitism is not yet precisely known. Detailed physiological experiments could not be carried out in the time available for the investigation on account of the slow growth of the plant and its haustoria. Nevertheless, some general idea of its demands can be deduced from morphological features and from observations on transplants into cultivation.
In the seedling, haustoria are present but are still rather undifferentiated, hence at this stage they probably contribute little to absorption. Root hairs are abundant, and through them presumably absorption takes place in the normal manner. However, as the parasite grows and haustoria becomes more developed, a greater proportion of the necessary nutrients will be contributed by parasitism.
No doubt, structures involved in normal absorption might then be expected to be reduced. For example, some workers have attributed the reduction of root hairs in adult roots of semiparasites to the increasing importance of parasitism compared with normal absorption from the soil. This hypothesis does not seem to apply to E. bidwillii however, for root hairs are equally as abundant on roots of mature plants as on seedlings - a condition which suggests that normal absorption is still important in the mature plant. If this is the case, the question arises whether E. bidwillii is an obligate parasite or a facultative one in which parasitism is an accessory means of nutrition. Transplants shed some light on this matter. Exocarpus plants removed with a sod containing a host plant usually survive. One specimen transplanted with a small bush of Corokia cotoneaster still flourishes after two years in cultivation. On the other hand, plants taken without a host usually die after a few months. This suggests that normal absorption is insufficient to meet the plant’s demands in the absence of a host. From these results it seems that E. bidwillii is an obligate parasite depending on host contacts in addition to normal absorption for its nutrients from the soil.”
THE GUEST: Dubito, ergo cogito...
These papers have provided useful information about the workings of the root attachment system in which parasitism is implied but practically nothing about the nature of the above ground clues about the relationships with the hosts. The advent of iNaturalist has provided a sufficient body of observations to gain insights into the health of both host and "guest" which may lead us to consider that the term "parasite" is misleading. If future research supports current speculation of symbiotic relationships (including mutualism, or commensalism), then we will need a) a new language to describe the relationship and b) a root parasite-wide investigation into to their relationships with their hosts. Casual observations of Nelson Ultramafic flora where Euphrasia monroi is abundant and healthy, may indicate a more beneficial relationship rather than a typically unhealthy state associated with parasitism.
Current examples of Exocarpos bidwillii among flourishing, potential hosts are in the Flickr album dedicated to Exocarpos bidwillii, "Takana": https://www.flickr.com/photos/52377268@N02/albums/72177720299706749/with/53186983763/
