Keying Out a Mustard Part 1

On my recent Nature Quiz I asked about the identification of a plant that grows in my neighborhood. I was pretty confident about its genus already, but getting down to the species level can be a little tricky sometimes. I was planning on being a wimp and just giving the answer at the genus level that I was comfortable with, but Kitty's comment inspired me to go and try to key it out.

I ran in to some trouble. Kitty kindly agreed to provide some analysis/commentary which is in italics throughout the post.

I used Flora of the Pacific Northwest (Hitchcock & Cronquist) with a bit of help from Plant Identification Terminology An Illustrated Glossary (Harris & Harris). Rather than jump to the genus I believed it was, I started at the Cruciferae (mustard) family level.

Trouble right away. The first fork of the key was:

1a Fruit stipitate (stipe as least 1mm) (Stipe means stalk supporting a structure)
1b Fruit Sessile or subsessile (stipe < 1mm)



I had scanned ahead to see where I wanted to end up and this was taking me down the wrong fork. But what the heck, might as well follow what the plant and key tell me rather than my preconceived notions.

I could clearly see a stalk supporting the fruit that was around half a centimeter long, so I chose 1a.

In this case stipitate refers to a stalk associated with the fruit, not the short stalk that connects the fruit to the main stem. If you look at the drawings in the H & C key, there is a clear join between the stalk subtending the fruit and the stalk that connects to the stem. I took 1b.

Next branch was easy (2b), this plant has siliques (long and slender fruits) rather than silicles (more oval fruits).

This is the correct choice, but you are definitely on the wrong road...

Then another easy choice (3b) since the flowers are white rather than yellow (see the quiz for photo with flowers). This is a nice illustration for a morality tale, one wrong turn... Then more trouble.

4a Basal leaves generally hastate (sort of arrowhead shaped), white petals, glabrous (hairless)
4b Basal leaves not hastate, petals often purplish, pubescent



The petals are clearly white, but the basal leaves are also definitely not hastate. There is not a lot of hair on the plant, but there is some. So neither fit exactly, but 4b seems to fit better which would take us to the genus Thelypodium.

The plant is definitely hairy, and the leaves not hastate, definitely not purple though, this is a clue that things are not good.

From the Thelypodium key

The first branch (1b) is easy because the stem leaves are petiolate (have a stalk), the next branch (5b) the plant is biennial (I believe), and I could go with often glabrous though I'm not sure what it means that the siliques are generally spreading to erect.

Fruit with a variable attitude? I don't like glabrous though

But both options of 6 have big problems:

6a Stem 1-6 dm (okay), sepals and petals purple (wrong), siliques 4-7 cm (wrong), ...
6b Stem 3-25 dm (wrong), often hairy (not exactly) petals 6-20mm (nope), ...

That means I had a problem somewhere along the way. Not surprising since I'm still pretty sure I am right about which genus it is (and even confirmed my guess using the much simpler key in Plants of the Pacific Northwest Coast - though it doesn't even list Thelypodium as an option).

After I got this written up I think I figured out the original problem for myself. In the glossary's illustration the stipe is between the receptacle and the fruiting part, rather than between the fruit and the stem as I had been thinking. On closer examination the fruit seems to be sitting directly on the receptacle (is sessile). I will continue from there in the next part of this series.

Nice job figuring out the original problem.