98 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”compounds (called VOC) are currently known. These distress signals
serve as warning of approaching danger to distant branches of the same
tree and to other trees in the vicinity as well. VOCs stimulate receptors
in the receiver plants, leading to gene activation and a series of bio-
chemical and metabolic changes, the outcome of which is to increase the
resistance to stress before it actually arrives.
It was initially noticed that when a sagebrush plant and a tobacco
plant are located next to each other, clipping of the former releases vola-
tile substances that activate the defense readiness of the latter, reducing
its susceptibility to herbivory.^3 In a subsequent study using a series of
Lima bean plants, it was shown that the receiving plant not only increases
its resistance but also releases the same messages, creating relay warn-
ing along the line.^4 What is relevant to self-recognition is that the dan-
ger cues are more effective in eliciting a response between two related
plants.^5 Since each plant upon injury emits a mix of chemicals, the com-
position of which is characteristic of the species, it is possible that plants
can decipher the signal and tell whether it comes from a related source.
This ability increases biological fitness, as related plants share common
natural enemies. Some of the VOCs, such as methyl jasmonate, methyl
salicylate, and ethylene, are plant hormones.^6
The broadcasting of stress signals through the air is a one-way com-
munication and benefits the receivers rather than the emitters. The sit-
uation has been compared to kin selection in animals, as the outcome of
the defense process is the preservation of the common gene.
5.4 Racing to the Top
Most plants absorb blue-green and red light for photosynthesis. In a
dense community, each plant competes with neighboring plants for
usable light in a behavior called “shade avoidance syndrome” (SAS). Red
light (wave length 655–665 nm) is absorbed by chlorophyll and far-red
light (wave length 725–735 nm) is reflected by the leaves. Therefore, a
low ratio of red/far-red represents shading which is to be avoided. This