October 31	Winter leaves — December 4
H. drummondii (click image to enlarge)

by Bob Harms 

Nearly all plant surfaces in the Hedeoma drummondii complex are evenly covered with multicelular uniseriate trichomes, but these hairs vary a great deal depending upon their location, and even upon the seasonal ontogeny of the plant. (Contrast the October and December stem partials of H. drummondii above.) In the images below eight distinct anatomical areas are labeled A—H.

A—C — hairs are retrorsely bent or curled

A: stem, B: peduncle (not visible for H. reverchonii var. serpyllifolia), C: pedicel

D—F — hairs are antrorsely bent or curled

D: petiole, E: leaf surface (upper surface may lack hairs), F: bracteole

G — hairs short & erect, spreading

outer corolla tube

H — proximal hairs retrorsely curved; distal hairs antrorsely curved; spreading hairs generally occur at a point in between, that point varying by taxon

calyx tube

[Click on image for enlargement.]

H. reverchonii var. reverchonii

H. drummondii	H. reverchonii var. serpyllifolia

This pattern is shared by all 3 taxa. One must wonder what is going on here. I would speculate that the trichome orientation impedes insect access to the axillary shoot meristem and the ovary at the base of the calyx. Trichomes of the stem and pedicel are directed down against insects ascending from below. Those of the leaf (including bracteole) and petiole are directed upward (as are the upper calyx trichomes) against insects alighting on the leaves. Possibly more substantial protective mechanisms are the dense annuli within the calyx and the corolla tubes, as well as the calyx teeth.

H. drummondii trichomes (left: 20x; right: 40x)