The lecturer first described the infinite variations in life reactions
in plants. The same external stimulus, he said apparently produces one
effect in one plant; and precisely opposite in another. Some leaves move
towards light; others are repelled by it. The root bends towards the
centre of the earth, the shoot rises above away from it. Numerous other
"tropic" movements are caused by contact, by electricity, by moisture
and by invisible radiations. These effects appear so extremely diverse
and capricious that some of the leading physiologists were forced to
come to the conclusion that there was no law guiding such movement, but
that the plant decides for itself what should be the effect of external
conditions on it.