Please help Cuscutology: for collectors and curators
Digital Atlas of Cuscuta (Convolvulaceae)
Most of this page— including part of the title—has been taken ad literam from Dan Austin’s paper: Austin, D. 1979. Comments on Cuscuta—For collectors and curators. Bull. Torr. Bot. Club 106: 227-228 [Available from J-STOR]. The suggestions that Dan made almost 30 years ago are still valid nowadays, and they will be at least in the foreseeable future. I just added the comments in blue, and the notes on collecting samples for DNA extraction and cytological studies.
1) Please do collect!
I am sure dodders don’t want to be collected, who would? So far they have been enjoyed a pretty undisturbed life because not many people like to touch them. But this is a poorly known genus from all the points of view. Even if you don’t stumble upon a new species (which by the way, is more likely for Cuscuta than for many other angiosperms), if you collect in the extreme southern U.S.A., Mexico, Central and South America, chances are you will have a new floristic record. Besides, these plants are usually like giant calamari; you can prepare a few servings (collections) from just one “tentacle”.
2) Collect mature specimens
“If the plants are not in full flower, do not waste time collecting them. At the very least, specimens must have fully developed flowers to be determined. Preferably, there should also be fruits on the material. Even with some experience, it may be impossible to select between the last two or three choices in the keys without fruits. Normally, do not try to identify Cuscuta with a hand lens; it takes a dissecting microscope for most species”.
Mihai: I totally agree with Dan. I often sigh and moan when in front of such tease specimens. Sometimes the flowers are immature but old enough to see that is a fantastic collection, perhaps even something new, but alas, it impossible to say anything for sure, and I can’t justify a trip to Brazil (just an example), to check what that really is.
3) Be gentle please!
“Once determined that a plant can be collected, it is helpful to set some material aside or press it so that not all flowers or fruits are squashed. Although in most plants, and Cuscuta is no exception, the original form may be determined by boiling up flowers, it is much easier to press Cuscuta "slightly”. By slightly I mean by adding host plants, wads of newsprint, or whatever is handy, so that some of the reproductive structures dry without being direct pressed. Drying in this manner allows the original shape to be at least partially retained. Cuscuta takes to such handling admirably. If spirits (e.g. FAA, ethanol, vodka, cheap rum, after-shave lotion) are available in the field, it is good to preserve flowers and fruits”.
Mihai: I can’t be so cruel to ask people to sacrifice the field-fuel, but flowers put in any sort of alcohol preserve beautifully.
4) Collecting material for DNA studies
In addition to the herbarium vouchers you could make someone happy and collect dodders in silica gel. Take some inflorescence & stem fragments and put them in double zip-lock bags or Falcon tubes (15ml or 50 ml). Add enough silica gel (in theory about 10 grams of silica for each gram of vegetal tissue) to dry the tissue (that should become brittle) in about 12 hours. After that, make sure that the samples don’t re hydrate from the atmospheric humidity.
5) Collecting floral buds for chromosome studies.
Very few New World species have their chromosome numbers known, and since most dodders are polyploids it is absolutely essential to fill this gap.
Most of the hands-on information comes from the experience of Miguel Garcia who kindly shared it with me. What do you need? 1) Hermetic vials, 2) the fixative solution (Carnoy’s), which is ethanol: acetic acid 3:1, and 3) ethanol 70%. When in the field, collect very young floral buds, open them with the tweezers (to allow the fixative to penetrate the anthers) and place them in the vials with fixative. The anthers are transparent-greenish at the time pollen mother cells happily undergo meiosis. This is why it is important to collect flower buds at different stages of development, from very tiny to a little bigger, to make sure that at least in some anthers meiosis is still taking place. Please note that in most species, anthers dehisce and release the pollen before the flowers open, therefore, for the chromosomes this moment is probably too late. Keep in the floral buds in the fixative for 48h, and then move them in the ethanol 70% solution. Whenever possible, keep them in the freezer at -20C.
6) Please, don’t forget the host
“If possible, obtain enough of the host plant for determination. In several cases it is possible to narrow choices down to a few simply by looking at the host (e.g. Cuscuta polygonorum, C. corylii). Other Cuscuta attack a wide variety of hosts, and this will be of little concern. Also, simply because a plant is growing on a particular a host does not preclude the possibility that it may be one of several species. For example, both C. polygonorum, C. obtusiflora often grow on the same host”.
Mihai: We know little about the host specificity and preference of many Cuscuta species. Race-formation has been described in Cuscuta, but in some situations even speciation may have occurred as a result of coevolution with certain hosts. Also, in the case of the species believed to be extinct (e.g. C. jepsonii), or very rare, knowing the host may help to infer the habitat where such species can be searched for in the future.
Better place most of the collection in a packet
“Once Cuscuta is collected, it is usually handled as a "typical" plant and glued to a herbarium sheet. When it is, every visit to the cabinet will leave a small pile of debris of miscellaneous Cuscuta fragments on the floor. In some loans I recently received, almost as much debris was in the box as on the sheets. This loss is tragic to many sheets, because it may render them so depauperate that future students may not be able to verify past identifications.”
“To offset this problem, I suggest putting most of the specimens in a packet. The size may vary according to the specimen. If there is part of the Cuscuta attached to the host, mount that in the normal manner; herbarium curators like to have some plant visibly glued to the sheet. Then place the remainder of the Cuscuta in a package which is glued to the same sheet. Anything in the package that gets broken off in handling and shipping will remain with the specimen to which it belongs”.
Mihai: Yuncker used this solution to keep the small samples he removed from various herbaria. Stored in this way, the tiny fragments became independent, valuable collections, most of them kept today by NY, but some at B and WLU.