From J. I. Rogers to Francis Darwin   25 March 1878

Dear Sir,

Referring to your lecture on the “Analogies between Animal & plant life” and to the use of the movements of the Sensitive plant not having been accounted for—¹ my attention was directed to this subject by D^r Darwins book on climbing plants,² & from some very rough experiments, I was led to the conclusion that the movements of the Sensitive plant were protective agains⁠⟨⁠t⁠⟩⁠ insect depredations. I am, to my great loss, not a botanist & therefore what follows may be well known to you in which case I must apologize for taking up your time.

On raising some sensitive plants in an in⁠⟨⁠n⁠⟩⁠er con⁠⟨⁠ser⁠⟩⁠vatory case together with some other seeds, I found that all the plants from the latter were much injured by woodlice while the Sensitive plants were untouched. On putting caterpillars & slugs on to the stem of the full grown Sensitive plants, I found that the down pointing thorns caused them such annoyance that they could hardly be forced to climb up. On placing them on the leaves or branches, the movements ⁠⟨⁠caus⁠⟩⁠ed the caterpillars much ⁠⟨⁠in⁠⟩⁠convenience & apparently frightened them, so that they generally dropped off as soon as they could. If not they remained quite still & if left on at night they had disappeared by morning leaving the plant untouched. Sometimes t⁠⟨⁠he⁠⟩⁠ caterpillar got on to the flower which is apparently not sensitive & then began to browse at ease. When the caterpillar remained on the branchlet & the leaves closed up, they presented a hard edge with hairs or thorns so as to increase the difficulty of eating them.³ A bee alighting on the leaves & finding them yield would naturally settle on the flower—thus promoting fertilization.

An Indian friend told me that on brush⁠⟨⁠ing⁠⟩⁠ through an inviting looking green patch of Sensitive plants they suddenly move & the leaves disappear, leaving exposed the horribly sharp thorns which clothe the join⁠⟨⁠ts⁠⟩⁠ of the stems. This may be protective as a⁠⟨⁠g⁠⟩⁠ainst g⁠⟨⁠ra⁠⟩⁠zing animals; for the thorns like those of furze or brambles in waste places, would be unattractive, & the disappearance of the leaves would divert the animals’ attention.

Perhaps the leaf motions of the Indian “Telegraph plant” (botanically called Desmodium gyrans I believe)⁴ may shoot off insects.

Believe me to be dear Sir | Yours faithfly | J Innes Rogers

⁠⟨⁠Fra⁠⟩⁠ncis Darwin Esq.