Square-stemmed herb 35cm tall.
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I agree this is purple loosestrife in addition to Chris, the stem is downy and skullcap is hairless.
Perhaps it is the teacher in me, and perhaps I'm doing the readers of iSpot a diservice but I thought I'd just explain your wonderful description Chris; I hope you don't mind.
Zygomorphic means that the shape of the flower is a mirror image in one plane only. By that I mean if you cut the flower down the middle both halfs would be the the same shape but the opposite). In the same way each side of a face is the mirror image of the other - e.g. your ears are in the same place but one is to the left of the face and the other is to the right.
Most flowers are actinomorphic. This means there are more than one place (plane) you could put a mirror to get an exact opposite. If there were six petals then you could put your mirror between any two petals and get a mirror opposite image. This is unlike our faces where the trick only works if the mirror is put in the exact middle along the bridge of your nose. Put the mirror anywhere else on a face and it does not work. Therefore a face is zygomorphic and flowers are often actinomorphic.
Clear as mud no doubt! If you are not sure then try it.
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