July 28, 2019

Case study: Cirsium wheeleri

Collected 7-27-2019 near the summit of Mt. Lemmon on "radio ridge" at about at 9000 feet. Purple.

Begin on page 830 in "Arizona Flora" 

Corollas distinctly bilabiate? No.
Flowers all perfect with strap shaped corollas? No.
Rays present? No.
Pappus evident? Yes!
Pappus of awns? No -- we have capillary bristles.
This takes us to group E -- note that when it began asking about the pappus, it was necessary to take a razor blade and chop the flower in half. It is guarded by an big imbricate bunch of bracts. Also note that you would have to "know" that the colored threadlike corollas are all disk flowers Looking at these under the microscope, there are 5 purple corolla tips and a stigma (not split in two like so many Asteraceae) also colored purple.

Now for the group E key on page 834. 

Receptacle densely bristy? Yes!
Leaf margins bristly? Yes.
Pappus bristles barbellate? No, they are plumose.
Phyllaries narrow and spine tipped? Yes, indeed.
This takes us to Cirsium (123). The key is on page 951. Note that the number "123" in the key is handy. Unlike the Jepson Guide which alphabetizes the Genus names in the Asteraceae, the Arizona Flora presents them in some random order, perhaps the order of their appearance in the key.

Now for the Cirsium key on page 951. 

Plant dioecious from creeping rootstock? No.
Corollas greenish yellow? No.
Phyllaries tomentose? No. (this would be C. Neomexicana)
Prickles on phyllaries very small (less than 2mm)? No.
Inner phyllaries with elongate reddish tips? No.
Phyllaries stiffly hispidulous on the margin? No.
Heads very large (4-6 cm)? No.
Heads normally more than one and only tinged with purple? No.
Phyllaries broad (ovate or lance-ovate)? Yes.  C. Wheeleri
Note that many keys use the first few questions to dispose of truly unique and unusual possibilities, then settle down to a "divide an conquer" approach for the remaining suspects. But this strategy can alternate.

Comments

This was quite straightforward, with the interesting lesson that C. neomexicanum has tomentose phyllaries. Shreeve and Wiggins call the plant "densely arachnoid-lanate", especially in youth. This is a nice way to distinguish these two purple thistles.

The group E key first asks if the receptacle is bristly, and indeed it is. When individual flowers with their pappus bristles are removed, a forest of long bristles remain that are attached to the receptacle.

Some other genera that could be candidates for thistles appear in the group E key. Cardus, Silybum, Cynara, Arctium, Centaurea (not Centaura), and Cnicus. These probably comprise the "tribe" Cynareae in Arizona.

Have any comments? Questions? Drop me a line!

Tom's Plant pages / [email protected]