Sunday, May 17, 2015

Early Meadow-Rue - Thalictrum dioicum - Pigamon dioique

Thalictrum dioicum (early meadow-rue) is native to North America (range map here), with a listed global conservation status of secure [1], unlisted in the US [2], and secure in Canada [3] except in Manitoba where its conservation status is unknown [3]. Note that, as with many of the USDA maps, the Canadian range is listed as more limited than the corresponding Plants of Canada database map.

This has happened a few times now with range maps I've looked at recently, so I'm going to take a moment to talk about my stance on disagreements between USDA listings and Plants of Canada database listings. For Canadian range information I will put my faith in the Canadian government source. So if you're in Manitoba with T. dioicum in front of you, scratching your head wondering if you're looking at evidence of the introduction or spread of T. dioicum based on that USDA map-- I doubt it, since the Canadian listing includes Manitoba in the native range of the plant. Ideally I would only draw the data from one source, but the Plants of Canada database maps don't show any information about US distribution at all. Since plants don't care about human borders, it is still informative to determine the rest of the range of the plant. So, two sources it is. In all cases where the two sources disagree about the Canadian range of a plant, I will assume that the data from the Canadian government is more accurate. I won't mention this again in my blog posts.

Thalictrum dioicum in bloom (male)
This plant is a perennial and a spring ephemeral. I noticed that it had started to emerge on May 2. Here is a picture of what the young, emerging plant looks like:

Thalictrum dioicum sprouting - note the furled leaves and the emerging flower stalk, with tightly closed buds
This species is wind-pollinated [4], a fact which shows very clearly in the structure of the flowers. Like most other anemophilous (wind-pollinated) plants in the angiosperm lineages, the display parts of the flower (petals, sepals) are either absent or much reduced; T. dioicum has 4 small sepals but no petals [4]. There is no scent, and no nectar [4].

Thalictrum dioicum male inflorescence - notice the sepals, which are spread up and almost flat above the dangling anthers - there are four on each flower, but they are small and unassuming ; note the absence of petals
T. dioicum is a dioecious species [4], meaning that each individual is either male or female but never both. It is easy to distinguish between the two when they are blooming, as the male flowers are staminate (contain stamens), and the female flowers are pistillate (contain pistils).

The two sexes are easy to distinguish. The males have visibly pendulous anthers which are quite numerous. This configuration makes it easy for pollen to become airborne whenever there is a wind gust.

Thalictrum dioicum - staminate (male) flowers - note the dangling stamens
The pistillate (female) flowers, meanwhile, look quite different. They are composed of pistils, and because reproductive success relies on the pistils sifting pollen from the air, they fan out a bit relative each other and are visibly thick and fuzzy - these characteristics increase the flowers' capacity to sift pollen from passing air currents and therefore improves the odds of fertilization.

Thalictrum dioicum - pistillate (female) flowers - note the fanning pistils
In my meanderings, I noticed what appeared to be two quite distinctive colourings for T. dioicum's staminate (male) inflorescences. The above-pictured individual had a yellowy-green-brown cast, while the individual below has a distinctively red-brown colour:

Red-tinted Thalictrum dioicum staminate inflorescence
The red tint also is visible, in these photographs, on all parts of the flower: the darker sepals, the red-tinted filament (thin part on which the anther sits) and the anther (pollen-producing portion, thicker part).

Close-up of the red-brown individual shows that the colour difference is present in the sepals (green but with purplish veins), filaments (visibly red-brown), and anthers (red-brown tint as well as yellowish portions)
In my various readings, I haven't found any indication that there is any documented polymorphism (polymorphism: distinctive groupings for shape or colouring) in this species, not even in the more extensive reports, like this one produced by the US Forest Service. So, I hazarded the guess that the difference in colouring between these two individuals is related to other factors, eg the soil composition or age of the flower. I set out to test the age of the flower hypothesis by going to re-photograph the two individuals pictured here (fortunately I remembered where I photographed them!). 

However, when I returned I found that neither individual showed any colour change over time, and both were arriving at senescence.

So there was no change in colour even as they senesced. Curiouser and curiouser. Therefore the difference is likely not related to the age of the flower. Is this polymorphism? If it is, I would have to wonder why; polymorphism is usually believed to be pollinator-mediated, and for obvious reasons pollinator perception doesn't really come into play for a wind-pollinated plant. 

There is also the possibility that individual b is a different species of Thalictrum, of which there are several, eg Thalictrum occidentaleThalictrum fendleri, Thalictrum venulosum; however, these species have quite distinctive-looking females and I only saw Thalicturm dioicum females on the mountain. An interesting conundrum. If you have a positive ID or an explanation, I would be very glad to hear from you.

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