21. Grindelia Willd.
(gumweed, gum plant)
Plants annual,
biennial, or perennial herbs, usually with taproots. Stems 1 to several, erect
or ascending, usually with few to several ascending branches toward the tip,
with fine, longitudinal lines or grooves, glabrous or occasionally sparsely
hairy toward the base (variously hairy elsewhere). Basal leaves absent at
flowering, similar to the lower stem leaves, short- to long-petiolate. Stem
leaves sometimes somewhat reduced toward the tip of the stem, sessile, the
blade linear to narrowly oblong, oblong-lanceolate or oblong-obovate, rounded
to bluntly or sharply pointed at the tip, tapered to rounded, truncate, or
shallowly cordate at the usually slightly to strongly clasping or sheathing
base, the margins entire or variously toothed (the lowermost leaves
occasionally appearing somewhat lobed), the surfaces glabrous, with moderate to
dense, impressed glandular dots. Inflorescences appearing as solitary heads or
less commonly loose clusters at the branch tips, occasionally a few heads also
in the axils of the adjacent leaves. Heads often relatively short-stalked, with
few to several short, inconspicuous, linear bracts that grade into the
involucral bracts, radiate or rarely discoid, slightly to strongly
resinous-sticky. Involucre 6–15 mm long, cup-shaped to hemispherical or broadly
urn-shaped, slightly to strongly resinous-sticky. Involucral bracts in 3–9
unequal to subequal, overlapping series, mostly narrowly lanceolate to linear,
thick and yellowish below the midpoint, the tip green and usually with inrolled
margins, loosely ascending to recurved or curled, the surface glabrous, with
sparse to dense, impressed glandular dots. Receptacle flat or slightly convex,
sometimes with low, toothlike ridges around the attachment points of the
florets. Ray florets 14–45 or rarely absent, when present pistillate, the
corolla 8–20 mm long, yellow, not persistent at fruiting. Disc florets numerous
(more than 100), perfect or some of the outer or inner ones sometimes
functionally staminate, the corolla 4–8 mm long, yellow, not persistent at
fruiting. Pappus of the ray and disc florets similar, either of 2–8 awns or
numerous bristles, in either case, these variously slightly shorter than to
longer than the disc corollas, smooth or minutely barbed, off-white to
straw-colored or light brown, not persistent or more or less persistent at
fruiting. Fruits often of 2 types, those of the disc florets somewhat
flattened, those of the ray florets more or less 3- or 4-angled in
cross-section, both types more or less oblong to slightly wedge-shaped in
outline, often more or less truncate at the tip (sometimes with the angles
extended into minute teeth), the surface smooth or finely and inconspicuously
few-nerved, glabrous. About 30–55 species, North America to South America,
introduced in the Old World.
Grindelia was the subject of Julian Steyermark’s
doctoral research, and his dissertation studies resulted in a lengthy series of
papers on various aspects of the ecology, cytology, and systematics of the
genus, including a taxonomic monograph of the North American species
(Steyermark, 1934). A notable feature of gumweed species is their production of
sticky, resinous exudates consisting mostly of complex mixtures of terpenes.
These are produced by glands that are usually sunken into the surface of most
of a plant’s tissues but are especially noticeable on the leaves and heads.
Young heads often have the saucerlike space on top of the developing disc
florets filled with a characteristic shiny layer of milky-white resin. Resin
production in the genus has been studied for possible industrial applications
as a substitute for conifer resins (Hoffmann et al., 1984). In particular, the
Californian G. camporum Greene has been documented to produce resins at
about 10 percent of the dry weight of the aboveground portion of the plant (McLaughlin,
1986; Hoffmann and McLaughlin, 1986), making it feasible for potential future
development as an arid-adapted crop in the southwestern United States. This
same resinous exudate had a long history of use among Native Americans for skin
sores and lesions, to bind together edges of wounds, as an inhalant for
bronchitis and asthma, and taken internally for coughs and a variety of other
ailments (Steyermark, 1963; Moerman, 1998).