The leaves as they grow older generally sink more and more downwards. The
movement of an oldish leaf, the glands of which were still secreting
freely, was traced for 24 h., during which time it continued to sink a
little in a slightly zigzag line. On the following morning, at 7 A.M., a
drop of a solution of carbonate of ammonia (2 gr. to 1 oz. of water) was
placed on the disc, and this blackened the glands and induced inflection of
many of the tentacles. The weight of the drop caused the leaf at first to
sink a little; but immediately afterwards it began to rise in a somewhat
zigzag course, and continued to do so till 3 P.M. It then circumnutated
about the same spot on a very small scale for 21 h.; and during the next 21
h. it sank in a zigzag line to nearly the same level which it had held when
the ammonia was first administered. By this time the tentacles had
re-expanded, and the glands had recovered their proper colour. We thus
learn that an old leaf
[page 239]
circumnutates on a small scale, at least whilst absorbing carbonate of
ammonia; for it is probable that this absorption may stimulate growth and
thus re-excite circumnutation.
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