http://www.piam.com/mms_garden/officinalis.html has a description of
the species epithets 'officinalis', 'officinale' and 'officinarum',
which were assigned (mostly by Linnaeus, I presume) to plants that
were used medicinally. Of the 62 species listed on that Web site, 3
are annual, one is annual and biennial, and one is annual, biennial,
or perennial. One is biennial, and two are biennial and
perennial. And the majority (54) are perennial, with many of those
being shrubs or trees.
I also got some interesting responses that weren't posted to the
list. Here's a summary below. Thanks to all of those who
responded. There are some annuals with significant secondary
compounds (e.g., marijuana, although it can apparently be perennial),
so there isn't a simple relationship between life history (annual vs.
perennial) and secondary plant chemistry.
David Inouye
I would have thought that because the leaves of annuals spend a
greater proportion of their time as softer, new growth, they are more
susceptible to being eaten and so would have also have to invest in
such compounds?
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Here's a paper you may want to read. You may find many useful
references therein:
Stepp, J.R. and D.E. Moerman. 2001. The importance of weeds in
ethnopharmacology. Journal of Ethnopharmacology 75: 19-23.
Below is an excerpt from the Discussion:
there are low molecular weight qualitative defense compounds such
as alkaloids, cardiac glycosides or terpenoids. These compounds are
toxic and highly biologically active (Feeny, 1976; Coley et al.,
1985). Ephemeral, successional or r -selected species, meaning
species that are opportunistic, rapidly colonize an area, and are
short-lived (all common characteristics of weeds) tend to rely on
these sorts of toxic chemical defenses (Rhoades and Cates, 1976; Abe
and Higashi, 1991). Fast-growing species tend to invest in
qualitative defense compounds rather than quantitative compounds
(Coley et al., 1985). Also, a positive correlation has been
demonstrated between short-living plants and a reliance on
qualitative compounds (Coley et al., 1985). Further support for the
relationship between lifespan and type of defense is provided by the
fact that all over the world, alkaloids are twice as likely to appear
in annuals than perennials (Levin, 1976). One example of such an
annual with alkaloid defenses is Catharanthus roseus (L.) G. Don.
This plant provides two important chemotherapy drugs used in cancer
treatment, vincristine and vinblastine, and is also considered a weed
(Holm et al., 1979). Leaf life is also an important predictor of
defense strategy (Coley, 1988). Plants with long-lived leaves rely on
immobile defenses while plants with short-lived leaves invest in
toxic compounds. Leaves of weeds are short-lived. There is also
evidence that weeds increase allelochemical production under
environmental stresses, such as disturbance (Putnam, 1985). Taken as
a whole, these data indicate that weedy plant species will rely on
bioactive low molecular
weight defense compounds against herbivory.
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There are plenty of annual medicinal plants, though I guess it also
depends on what definition of medicinal you're using (folk medicine
or standardized trials?). A list from a quick flip through an herb book I
have around:
Bidens spp. (Asteraceae)
Calendula officinalis (Asteraceae)
Eschscholzia californica (Papaveraceae)
Stellaria media (Caryophyllaceae)
Galium spp. (Rubiaceae)
Verbena spp (Verbenaceae)
Glandularia spp (Verbenaceae)
Some of these blur the border between annual and perennial, depending
on climate. I think that particular book has about twice as many
perennials as annuals, but who can say if that's a random sample?
Besides that, there's Papaver somniferum and Cannabis sativa which
produce plenty of secondary compounds (to be used medicinally or otherwise).
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This may or may not be helpful to you, but here are some pages out of
Michael Moore's "Medicinal Plants of the Desert and Canyon West." I
do like the way in which he categorizes them here by use.
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In addition to the poppies, the obvious omission (to me) from the
public replies are the cannabis and salvia plants. Whether they are
medicinal or not seems to be more a political discussion than a
scientific one, although I am unsure as to whether or not they are
perennials in southerly climes.
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I've been teaching Medical Botany for many years and I think you have
hit upon an intriguing question. However, as others have already
pointed out, it is much more complex than perhaps first
thought. First, I can also see the reasoning for thinking that
perennial plants may be more likely to be medicinal (i.e. produce
secondary compounds) and in fact, I can think of many, many
examples. But on the flip side, one could also argue that secondary
compounds may be more likely to evolve in annual plants because of
their quicker generation time. Second, there are hundreds and
hundreds of medicinal plants all around the world, and their uses are
exceedingly varied; personally, I do not know of any single,
comprehensive source that lists everything. Keep in mind too that
most of the world does not practice "Western medicine" and many of
theses different cultures in other areas of the world have their own
medicinal species that reflect what grows in the area and also oral
traditions. And most importantly, the nomenclature and taxonomy of
many of these species is often unclear (especially common names -
don't get me started on that). I assume there that we are also
talking about medicinal uses as they pertain to humans but there are
many documented cases of other animals using plants medicinally. I
think an important point already raised is that the perennial life
history can vary within the range of a given species or across
related taxa, which complicates things.
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*Cannabis sativa*
