H. reverchonii var. serpyllifolia, internode variation along a winter stem:
10 internodes between the yellow bars & shown along the bottom.
23 Jan. 2013, Hays. Co.,

by Bob Harms 

Cold season stems exhibit much greater variation in trichome density, length and orientation, in part because starting in very late fall plants produce both types of stems, and most cold-season shoots produce varying vestiture as they elongate (as in the above image). In comparison with warm season stems all 3 taxa have denser vestiture and longer trichomes, these accompanied by very short hairs that curve downward — similar to the summer hairs (note the small hairs in the yellow boxes below). When dense, the vestiture may appear canescent (noted by Irving only for H. reverchonii).

Winter stem pubescence provides a contrast between H. drummondii and both H. reverchonii taxa:

• H. drummondii trichomes of all sizes are curved downward, although most not as strongly bent as with summer hairs;
  H. reverchonii long trichomes are typically spreading with randomly curved orientation;
• The longest H. drummondii trichomes never exceed 0.5 mm; those of H. reverchonii commonly exceed 1 mm.

H. drummondii

40x. 18 Nov 2013	Four sections of the same stem. 4 Dec. 2013. Scale 3 mm.

100x. 18 Nov 2013	20 March 2013	20 March 2013

H. reverchonii var. serpyllifolia

9 Feb. 2013

100x. 10 Nov. 2013	40x. 10 Nov. 2013

H. reverchonii var. reverchonii

4 Dec. 2013

24 Oct. 2013	15 Nov. 2013