Effects of heat and chemical agents on plant excitability and protoplasm

In Insectivorous Plants (1875) Darwin tested salts by immersing leaves in measured solutions. For nitrate of quinine he used four leaves, each in thirty minims of one part salt to 437 of water. After 6 hours there was hardly a trace of inflection. After 22 hours three leaves were moderately inflected and the fourth slightly. He concluded that the salt induces well-marked inflection, though rather slowly¹. He also found that diluted alcohol, which he judged hardly poisonous, plainly retarded the later action of a phosphate solution. He then summarised in a table how leaves that had not inflected in other solutions responded to the phosphate².

In The Power of Movement in Plants (1880) Darwin described a second effect of ammonia salts. Hairs on the calyces of a clover were placed in a weak solution of carbonate of ammonia, 7 grains to 1 ounce of water. Their protoplasmic contents immediately became aggregated and afterwards showed the usual slow movements. Darwin took this as showing that the hairs can absorb³. On a leaf, a drop of a solution of carbonate of ammonia, 2 grains to 1 ounce, blackened the glands and induced inflection of many tentacles. The tentacles later re-expanded and the glands regained their colour⁴.

Bayliss (1915) reported electrical work on nerve that used solutions of electrolytes. He cited Macdonald (1900), who measured the demarcation current, the potential difference between a cut end and the normal surface, in such solutions. It followed the Nernst formula for concentration batteries. Bayliss counted this as confirming a membrane origin for the electromotive force of nerve⁵. Macdonald's measurements concerned nerve, not plants.

Child (1915) noted that in the simplest organisms order is scarcely apparent. He said it can be hard to decide whether an aggregation of protoplasmic substance is an individual or merely an aggregation⁶. Meyer, Ansorge and Nakagaki (2017) described the slime mould plasmodium as an aggregate of protoplasm with a network of tubes. They separated its protoplasm into a gel phase forming the tube walls and a sol phase flowing inside⁷. Darwin's clumping and slow movements of protoplasm and these later descriptions use the same word for different observations.