There is no difference. The plant is Vigna caracalla, previously known as Phaseolus caracalla, NOT two seperate species. There is a lot of misinformation out there on the net suggesting that one is a scented vine, and the other scentless. In truth, scent varies from one plant to another, and also from year to year with the same plant. I've noticed mine is more highly scented with lower humidity (70% instead of 90%).

I also must differ. The question I ask, are they two seperate species, or are they vegetatively propogated individuals with differeing characteristics? I belive they are the same genus and species, but variations within species. Most species have large variation within their total populations, but to actually call them different species is misleading. I would go along with 'cultivar' though... :-)

Banyan, it depends on which definition of species you are referring to as there are several different definitions. Here are several that are commonly used:

"A typological species is a group of organisms in which individuals are members of the species if they sufficiently conform to certain fixed properties. The clusters of variations or phenotypes within specimens (ie: longer and shorter tails) would differentiate the species. This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, we now know that different phenotypes do not always constitute different species (ie: a 4-winged Drosphila born to a 2-winged mother is not a different species). Species named in this manner are called morphospecies.

A morphological species is a population or group of populations that differs morphologically from other populations. For example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the beginning of recorded history. This species concept is much criticised because more recent genetic data reveals that genetically distinct populations may look very similar and, contrarily, large morphological differences sometimes exist between very closely-related populations. Nonetheless, most species known have been described solely from morphology.

The biological species or isolation species concept identifies a species as a set of actually or potentially interbreeding populations. This is generally the most useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations and is thus impractical in instances of allopatric (geographically isolated) populations. The results of breeding experiments done in artificial conditions may or may not reflect what would happen if the same organisms encountered each other in the wild, making it difficult to gauge whether or not the results of such experiments are meaningful in reference to natural populations.

A mate-recognition species is defined as a group of organisms that are known to recognise one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually. Unlike the isolation species concept, it focuses specifically on pre-mating reproductive isolation.

A phylogenetic or evolutionary or Darwinian species is a group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species.

In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement. Given the complexity of life, some have argued that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition. For most vertebrates, this is the biological species concept, and to a lesser extent (or for different purposes) the phylogenetic species concept. Many BSC subspecies are considered species under the PSC; the difference between the BSC and the PSC can be summed up insofar as that the BSC defines a species as a consequence of manifest evolutionary history, while the PSC defines a species as a consequence of manifest evolutionary potential. Thus, a PSC species is "made" as soon as an evolutionary lineage has started to separate, while a BSC species starts to exist only when the lineage separation is complete."

I am using the morphological definition because I think that possibly one of the two forms is a hybrid of the other and some other plant. There are just too many instances in which one of the forms is noted as having a fragrance and the other isn't, as well as the color development in one form versus the consistent color in the other. Also there is a vast difference in what the seeds of Vigna caracalla are sold for as opposed to what the seeds of Phaseolous caracalla are sold for and this shows the ease of seed formation in one versus the other. This data seems to indicate that if the two forms are related, that sufficient evidence exists that they would be considered different species.

Banyan, a couple of other things that came to me this morning--Early )!!! First off, perhaps hybrid is a poor choice of word in my description above. I think a better statement would be that I think the two plants may have shared some biological starting point and that they have diverged at some point in their evolution (makes me think more along the evolutionary definition of species). Hybrid was a bad choice of words since hybrids generally don't grow true from seed from a parent plant.

However, this led me to think of species in terms of a group of plants I am intimately familiar with and studied quite a bit--that being clematis. Species clematis will always develop seed that will come true to their parent plant if they are culturally isolated. I said culturally isolated because if they are pollinated by a bee carrying the pollen of another type of clematis plant, the seeds will not come true to the parent plant but rather give rise to hybrids.

In the case of Vigna caracalla (the fragrant plant with changing colors), I have grown them from seed several times and the plants have always had identical properties (leaf shape, fragrance, color changing properties). Given the convoluted shape of the flower and the fact that the flowers seem to be pollinated by ants, it is not outlandish to think that in this manner Vigna caracalla, even when grown amongst other plants, would be pollinated only by pollen from other Vigna caracalla flowers and suggests that they are a species as I described above about clematis.

I have never grown Phaseolous caracalla (all purple flower with little to no fragrance) from seed but perhaps you or someone else interested in this thread has. If it always grows true from seed (leaf, color, little to no fragrance), then it too is a species and as such has to be a different species from the Vigna caracalla.

Just some musings and thoughts I had early this AM. Look forward to hearing your thoughts on this.

I also wanted to comment on your original statement about fragrance being variable depending on various factors. I agree that is true--it can vary depending on each individual's ability to perceive various scents. It can be dependent on humidity also but the thing that stands out in my mind is that although it is variable, the fragrance of Vigna caracalla is very distinctive in my experiences. My plants were in bloom this year from July until the second week in November when I had to cut them back severely and bring them inside the garage for the winter. The humidity range in the summer months here can range from 70 to 90% (if not higher) and in November can be as low as 30 to 40%. My plants have had a very distinct discernable fragrance at all times and in all humidity ranges I have experienced. The descriptions I have heard of Phaseolous caracalla are that it has little to no fragrance regardless of humidity conditions.

There are apparently two different varieties of this plant, one is heavenly scented, the other has no scent, but with bigger flowers.

My nursery has both varieties and there is no mistaking about which one has a scent and which one doesn't.

This is the difference from what I can see:
The non-scented variety has very small leaves and very big lavender-colored flowers. In my nursery, it is just labelled "vigna carracalla" I have never detected any scent of this one, regardless of season and time of day, or growing condition, nor can anyone else, even though the plant is labelled fragrant on the growers's label.

The scented variety has leaves twice the size of the non-scented, with good sized flowers, but definitely smaller than the non-scented variety.
In my nursery, it is labelled as "Vigna Carracalla 'Thomas Jefferson.'" The flower is truly fragrant day and night, regardless of growing conditions and seasons. The fragrance is a strong warm orange blossom, honey and vanilla. Both varieties mainly blooms for us in summer-early fall.

The two maybe closely related, but definitely not the same plant.

The nursery sales person said that the scented variety is called "Thomas Jefferson" because it was the fragrant variety grown by the late president.

Most people in the trade don't seem to know the difference nor do they care. A lot of the regular vigna carracallas are labelled as fragrant, when they have absolutely no scent. This is because the grower gets the information off some textbook, which never bothered to include both the fragrant and the non-fragrant varieties. And fragrant plants sells. Most people don't bother to return even if what they got isn't what they had in mind.

I knew several people who bought the non-scented variety that was being sold as fragrant and when they complained of no scent, they were told that the fragrance depended on the growing condition and that they just don't have the right growing condition to bring out the fragrance. This is totally incorrect.

They then buy the really fragrant one at this particular nursery and have no problem getting the fragrance, same growing condition.

HTH
FG