| BEN |
| BOTANICAL ELECTRONIC NEWS |
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| ISSN 1188-603X |
| No. 158 February 22, 1997 | aceska@freenet.victoria.bc.ca Victoria, B.C. |
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Parasites, in general, are organisms that live on or in other individuals and draw their nourishment from their hosts. Most plants, on the other hand, are autotrophic, and can manufacture their own complex nutrients independently from simple naturally occurring substances. But even here in the green (with chlorophyll), self-sustaining plant kingdom, evolution has perhaps been diverted several times to produce a small number of saprophytes and parasites.
The saprophytes are those that survive on a wide variety of complex organic substances without depending on other plants. Their aerial parts are non-green (lacking chlorophyll) and the underground roots become variously modified by showing very irregular branching. Often these irregularly branched roots morphologically resemble corals, and hence they are called coralloid roots, as in some plants such as Pine Sap or Indian-pipe (Monotropa) of the family Monotropaceae, or Coralroot Orchid (Corallorhiza) of the family Orchidaceae. Their roots are usually associated externally and/or internally with fungi, and such an association is called mycorrhiza.
Unlike the saprophytes, parasites depend directly on other plants for their growth, development and reproduction. The parasites can be classified into two types - a) complete or holo-parasites and b) semi- or hemi-parasites. The complete parasites are those that depend on autotrophic plants for their living. They are non-green and cannot photosynthesize. [Broom-rapes (Orobanchaceae), Dodders (Cuscutaceae), and Mistletoes (Loranthaceae) can be given as an example.]
The semiparasites do contain chlorophyll but depend on other living plants for water and other simple nutrients. [A large group of genera in the Scroph family (Scrophulariaceae), including Indian Paintbrush (Castilleja), some member of the Santalwood family (Santalaceae) such as Bastard Toadflax (Comandra), can be given as an example.] Some semiparasites in nature may or may not depend on hosts for their living. The former are called obligate semiparasites; the latter, facultative or circumstantial semiparasites. In all parasites the roots, usually the lateral roots, become modified to form haustoria (the part of the root that penetrate the host), which facilitate the uptake of water and other nutrients from the host plant. The facultative semiparasites, under favourable growth conditions, may not produce haustoria. (In some instances, haustoria of the parasite become attached to its own root, causing some destruction. This phenomenon is called self-parasitism, which is not uncommon in parasites, especially in semiparasites.)
My advisor - Marion Ownbey commented that he had simply planted Poa pratensis in the plots where he was growing Castilleja. At least, that is what I think he said - it was a long time ago ... But the experimental garden did not have shrubs in it. Has Art Guppy tried grass (as a host)?
In my experience, Castillejas do not have restricted host ranges, nor are the hosts necessarily woody plants. I have grown C.levisecta on for about 6 years, from a seed population given me by Mrs. Florence Free, of Seattle Washington. Mrs. Free had obtained seed of this species on Whidbey Island Washington, and had maintained it in the garden for about 20 years when she gave me the seed. She has since had to give up her garden.
Mrs. Free had started both Castilleja levisecta and C. miniata in her garden by rubbing seed directly into mats of the New Zealand composite Raoulia levisecta, growing in her rock garden. Both Castillejas had become self-seeding in this rock garden, among a great variety of exotic and native plants.
I sow Castilleja levisecta in 4-inch pots in mid-winter, and germination is usually complete by mid-March; seed sown outside after the end of February will not germinate. (This and other observations indicate that this is a D-40 germinator in Norman Deno's terminology.) The seed of C.levisecta, and probably of most Castillejas is very long-lived in dry storage. Some of the original batch of seed from Mrs. Free's garden germinated last winter, about 6 1/2 years after harvest. The seed had been stored dry in a basement room at about 15-20 C, without desiccants.
Castilleja levisecta grows on very well to about the 6-leaf stage, after which it is necessary to transfer the seedlings to a host. Acceptable hosts have included Raoulia tenuicaulis, Festuca ovina, Aster alpinus, Potentilla megalantha. While all of these are exotic species, it is clear that C.levisecta can successfully establish root connections with a great range of host plants of diverse taxonomic groups.
I have grown and bloomed for a couple of years several clones of what I identified as C. hispida. These were grown from seed in pots of a soilless mix based on composted fir bark with a substantial amount of pumice added. No "host" plants were present, although weeds frequently appeared. Weeds were removed whenever noticed. Moss also colonized the pots and was pretty much a permanent fixture. I doubt the Castillejas were able to use the moss as a host.
The seedlings made very slow growth for most of their first season, but appeared healthy. The pots were constantly moist and fed with slow release fertilizer and an occasional shot of soluble. They were well, but not extravagantly fertilized. The second season they got the same culture and grew moderately well and bloomed in mid summer.
The following spring a took some cuttings from these plants. When the new shoots were just poking up through the ground I removed half a dozen of these right were they were attached to the crown. These rooted easily under cover in a mix similar to the one growing the mature plants but with more pumice and no fertilizer. Most grew on after being potted up and eventually bloomed.
These Castillejas were around a couple more years and bloomed but never looked like they were happy. They might have done better in the ground. When I've seen this species in the wild it has been growing in a fairly clayey loam with good drainage and little organic material in the soil. I don't think their eventual demise was due to lack of a host.
[Similar observations were reported by L.R. Heckard in 1962, in the article on "Root parasitism in Castilleja" published in the Botanical Gazette, 124: 21-29.]
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