94 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”to prevent being over-consumed to extinction. As they cannot turn away
from predators, they evolve strategies to ward off intruders. Some plants
develop bad taste in their leaves or thorns on their twigs. Others produce
compounds poisonous to animals but innocuous to themselves. History
tells us that Socrates was executed with hemlock in 399 B.C.E. Hemlock
contains a potent alkaloid that causes convulsion, paralysis and death,
and was commonly used for execution in ancient Greece. Another his-
torical case was the assassination of the Holy Roman Emperor Charles
IV with the Death Cap mushroom (Amanita phalloides), which is also
responsible for most accidental mushroom poisoning. Among the most
poisonous substances is ricin, a protein from the seeds of castor-oil plant
(Ricinus communis), which is lethal to humans at a dose of 0.5 mg, sized
smaller than a grain of salt. Ironically, some plant toxins are fortuitously
beneficial to humans as medicines if consumed in moderate amounts,
such as the cardiac glycoside digitalis from foxglove. About 25% of all
drugs available now are derived from such plant “poisons.”
Plants tend to control insects that feed on them. Milkweed leaves
are a favorite food for caterpillars of the monarch butterfly. Against this,
milkweed develops three lines of defense. First, the leaves are covered
with a layer of hair, which must be shaved off by the caterpillar. Second,
when bitten, the leaf oozes sticky latex that can drown the insect. Third,
the leaf contains a cardiac glycoside that can kill the caterpillar when
taken in large amounts.
Another plant strategy is to recruit carnivores that prey on the
insects that bite the leaves, through the release of volatile attractants
following injury. Lima beans, when infested with spider mites, emit a
terpenoid that attracts spider-mite predators to the scene. The same
mechanism of defense has been observed in strawberries and other
plants. Some plants alter the taste of their leaves when attacked, render-
ing them less palatable to the herbivores.
The trees in the Serengeti ecosystem of East Africa provide an
excellent example of how plants cope with herbivores. First, they
cover themselves with spines, needles and hooks. Some trees produce