Even
in prehistoric times, men were using plants as medicines. It is likely that an early
caveman might accidentally have found that eating willow bark alleviated his
headache. This discovery would have been passed on to others, and a great
wealth of information about the properties of various plants could have been
accumulated in this way, developing into the herbals of the middle ages. Many
of the published remedies were of doubtful value, but with experimentation many
useful medicines 
were
also found.
Most cultures have developed their own pharmacopoeias based
on the plants found in their localities. Probably one of the earliest
discoveries was the use of salicin, the glycoside of salicyl alcohol, found in
willow bark cited above. This has strong anti-inflammatory and pain relieving
activity, mediated by inhibition of the production of prostaglandins, and is
now the basis of our conventional drug known as aspirin, acetylsalicylic acid.
The word salicin refers to the willow genus Salix, while the word
aspirin is derived from the genus Spiraea, Meadow Sweet, another plant
that contains salicin. A closely related compound, methylsalicylate, known as
wintergreen, used for the treatment of rheumatism, is found in many higher 
plants
(e.g. Birch) where it is a signalling volatile.
It is only recently that the cause of the toxicity of the
leaves of the Yew tree has been established. Ingestion of even small quantities
of the leaves or of the seed are sufficient to cause death in humans from heart
failure. Several toxins are present in the leaves, most notably Taxol, and
consequently few herbivores will eat it, and it is host to very few insects,
and this is probably the key to its success in surviving since the age of the
dinosaurs. Taxol blocks a cell's
ability to break down the mitotic spindle during cell 
division and with the spindle still in place, cells
are unable to divide into daughter cells. Taxol was found to be effective for
the treatment of various tumours, and especially for ovarian cancer. At first the source of the drug was the bark
of the North American Yew but it has now been found that a similar drug
(Taxotere) can be produced from the leaves of the English Yew. Consequently,
the trimmings from our English Yew trees are now in great demand by the
pharmaceutical industry. Since there are considerable technical difficulties in
synthesising these complex taxanes, there are suggestions that many more yews
should be planted to provide a source in 
the future.
2,5-Dihydroxymethyl 3,4-dihydroxypyrrolidine (DHMDHP), an alkaloid in the Bluebell is a powerful bacterial glycosidase inhibitor. This has considerable potential for the treatment of tuberculosis and leprosy, but it also makes the plant poisonous to cattle.
Extracts of St John’s Wort (Hypericum
perforatum) have antidepressant activity. The multiple constituents
apparently function in several different ways. Some evidence supports its
effect as a selective serotonin reuptake inhibitor (SSRI). It may also inhibit catechol-O-methyltransferase,
an enzyme capable of destroying biological amines. Still another mechanism
seems to suppress interleukin-6 release, 
affecting
mood through neurohormonal pathways. Valerian (Valeriana officinalis)
has a tranquillizing effect, used for the treatment of sleep disorders. The
active constituents appear to be complex. Other plants having useful properties
include Dandelion (Taraxacum officinale) that can act as a diuretic, and
Nettles (Urtica dioica, roots) that can alleviate prostatic hyperplasia.
The mechanism for these activities is unknown. The complex alkaloid
galanthamine found in Narcissus and Snowdrop, is used in the treatment of
Alzheimer’s disease. Acylated iridoids in Figwort (Scrophularia nodosa)
stimulate proliferation of fibroblasts, explaining the wound-healing properties
of
the poultice made from the seeds of this plant. Evening Primrose (Oenothera
biennis) contains gamma linolenic acid in the seed oil, which is a
precursor of prostaglandins, and is particularly useful in the treatment of
skin disease. The white colour of the bark of Silver Birch
is due to betulin (up to 30 % in the cells). This may have anti-viral activity
and it may also antagonise certain malignant cell lines.
This is only a small selection of the pharmacologically active plants found in the UK, but there are many others. Just because we do not at present understand the mechanism for these activities does not diminish their efficacy. However, understanding the mechanisms could lead to the synthesis of even better drugs. Throughout the world, many other highly significant and useful remedies have been found, and many others are awaiting discovery. Several drug companies are working on this subject, known as pharmacognosy, following up the leads suggested by the folklore of various ethnic groups. This is yet another good reason to conserve the biodiversity of our planet.
The structure of Taxol is reproduced from Phytochemistry 47, 817, J. Soto and L. Castedo; Taxoids from European Yew, Taxus baccata. 1998 with permission from Elsevier.
The structure of Betulin is reproduced from the Merck Index (14th edition) with permission.
Terry Smith 14th May 2007