The Plant Self 95“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitysteel-like thorns sharp enough to puncture automobile tires, earning
the name of “the puncture tree.” The exceedingly painful “Wait-a-bit
Acacia” (Acacia brevispica) has opposing hooked thorns which cling to
passers-by and force them to cry out “wait a bit” or “wait a minute”
before disentangling themselves, thus the name. Tall trees like Umbrella
Acacia are a favorite food of giraffe, and they protect themselves with
thorns on the outer branches where the animals can reach, but the inner
branches remain soft and green. Perhaps one of the most “ingenious”
ways of defense is adapted by the Whistling Acacia (Acacia depranolo-
bium). The tree grows hollow galls the size of golf balls. They also pro-
duce flower-like structures (no seeds) that make nectar so sweet that it
invites ants to take residence in the galls. The ants’ vicious sting drives
away any herbivore intruders. The name “whistling” comes from the
sound produced when the wind makes its way through the holes of the
galls. Lastly, Acacia robusta (common name Stinking Acacia) is rela-
tively harmless, but it emits an unpleasant odor that drives away any
herbivore that gets close to it.
Plants also protect themselves by spreading out their progeny geo-
graphically as far as possible, thus compensating for their lack of mobility.
Seeds can be carried by wind, water, and animals (through their fur or
digestive tract) to faraway places. Seeds are desiccated embryos that can
stand adverse conditions and stay dormant for as long as hundreds of
years, only to germinate at the right time in the right place when condi-
tions are favorable. Seeds are well adapted to their environment — some
will germinate only after a period of freezing, in order to ensure germi-
nation in the spring time and not earlier; others will germinate only after
a period of intense heat, in order to ensure germination after a mild
forest fire, when abundant fertilizer is available.
Other than biotic stress, plants must face physical (abiotic) hard-
ship as well. Since they cannot run away from adverse environment,
they have to develop ways to tolerate and to acclimatize. Physical stress
includes extremes of temperature, extremes of humidity, ultraviolet irra-
diation, nutrition deficiencies, and oxidative stress such as ozone in the