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

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North American Species of Lactarius

cuticle in Lactarius is found in only a few species in section Thejogali
of Russularia, and to some extent in the Plinthogali. It intergrades with
an arrangement we term the "cellular type" in which the outermost
cells of the cuticle form a layer of vesiculose cells.
The "cellular" type of layer, as seen in tangential sections of the
pileus, may originate from a simple cutis by the hyphae becoming 10-20
yu broad and having numerous cross walls to produce ~ isodiametric
cells in a layer 1-3 cells deep. This type is unusual in Lactarius, and
is found in the section Thejogali in a few species. The cellular type of
cuticle, however, can originate as a combination of this with the hymeniform type. In the Plinthogali a cellular layer may originate by a different process. A trichoderm is produced, the elements originating from
tramal hyphae and forming a dense turf. The lower 1-3 cells of the trichodermal element inflate to a + globular shape and are packed tightly
together by mutual pressure producing a distinct layer. The ultimate
and penultimate cells may be elongated and produce a turf. It appears
that in some of the Russularia the trichodermal elements of 3-5 cells
in extent are oriented ~ in an upright position but are packed tightly
together. All of the cells in the element enlarge greatly, some being
vesiculose and some only nearly so. As the pileus expands the elements
adhere to each other laterally until the force of expansion separates
them into groups which in sections appear as "heaps" or "mounds"
mentioned in the descriptions. In other species it appears that a thick
cellular cuticle is broken up by the forces of expansion also causing the
appearance of heaps or mounds of cells especially on sections of old
pilei. Pilei showing this feature have a continuous cuticle when young
but in age it is broken up into a pattern described as rimose-areolate.
Avoid the assumption that in any given species only one type of
cuticular development will take place. Both upright clavate cells and
radial hyphae with short wide cells may, as in Psathyrella (Smith 1972),
contribute to a cuticle of the cellular type in a single species as already stated. Many species will show varying degrees of differentiation from one type to another, but in spite of this the diversity in the
pileus cuticle in this genus it is basic to a workable systematic arrangement of the species.
Let us now return to the ixocuticle. In this type, slime is present
in the layer, the hyphae become relatively widely separated, and some
hyphal ends may project into the slime layer if the later is well developed. One highly developed type of ixocuticle is the ixotrichoderm (or
ixotrichodermium). This structure is realized if the projecting hyphal
ends elongate considerably, secrete more slime, and become so numerous as to resemble in the aggregate the arrangement of grass leaves in
a dense turf. The elements in an ixotrichoderm are often narrow (2-4