North American species of Lactarius / L. R. Hesler, Alexander H. Smith.

Appendix III

597

No members of this subgenus that we have studied have the isolated warts observed in L. deceptivus, but many species have spores
which lack a reticulum of any sort or at most have only a broken
reticulum as shown for L. argillaceifolius var. argillaceifolius and
L. mackinawensis. In L. subpalustris, the elements of the ornamentation vary from warts to ridges with few connectives, but the ridges
and connectives may be relatively high-approaching the winged condition. Increasingly reticulate spores are characteristic of L. hibbardae
and L. uvidus where more of the meshes are complete and we consider
such spores to have a partial reticulum. The most distinct reticulum
occurs on the spores of members of stirps Mucidus in which the spores
have a partial or more commonly a complete reticulum, with few or no
isolated warts and/or short ridges. The meshes are angular with the
ridges and connectives varying somewhat in width and height but not
becoming distinctly winged as in subgenus Plinthogalus. This type of
ornamentation approaches that of L. volemus and L. corrugis of subgenus Lactifluus.
The second line of spore ornamentation types can be traced from
the basic spore-type of this subgenus to spores showing a tendency to
have the ridges oriented around (parallel +) to the short axis of the
spore. This tendency can be observed in L. griseus which may have
almost wing-like connectives generally oriented around the short axis,
and only a few complete meshes. In L. circellatus var. borealis, the
spores appear somewhat zebroid; the ridges more or less encircle the
spore and connectives are scarce and low so the total effect is one of
horizontal bands. Again, the plage area is often diffusely ornamented.
L. pyrogalus, of the North American species studied, has the most extreme spores of the zebroid type. The ornamentation is crowded transversely with little unornamented surface as viewed with the SEM. The
ridges or plates are more or less parallel to one another. The plates
are crescent shaped and unequal in height with an uneven surface and
undulating margin. The plates are often thicker near the outer edge,
but may be moderately thick toward the base becoming thinner near
the center. They appear to fan out from the plage area and may extend
approximately half way around the spore. Toward the spore apex, the
plates may completely encircle the spore or pass over the apex. Occasionally cylindric to conic warts may be observed. The arrangement
of these ridges suggests the pattern of stripes on a zebra. No reticulum
is present. This is an unusual and an easily recognized type of ornamentation. With the light microscope and mounted in Melzer's reagent, the
ornamentation appears mostly as coarse lines encircling the spore.
L. arachnisporus Heim and Perreau (1973) also has spores of the zebroid type as shown by their SEM photomicrographs.