After 24 days from the first observation (begun after a true leaf had been developed) the cotyledons ceased to rise at night.

Oxalis (Biophytum) sensitiva.--The cotyledons of several seedlings, 45 days after their first expansion, stood nearly vertical at night, and closely embraced either one or two true leaves which by this time had been formed. These seedlings had been kept in a very warm house, and their development had been rapid.

Oxalis corniculata.--The cotyledons do not stand vertical at night, but generally rise to an angle of about 45o above the horizon. They continued thus to act for 23 days after their first expansion, by which time two leaves had been formed; even after 29 days they still rose moderately above their horizontal or downwardly deflected diurnal position.

Mimosa pudica.--The cotyledons were expanded for the first time on Nov. 2nd, and stood vertical at night. On the 15th the first leaf was formed, and at night the cotyledons were vertical. On the 28th they behaved in the same manner. On Dec. 15th, that is after 44 days, the cotyledons were still considerably raised at night; but those of another seedling, only one day older, were raised very little.

Mimosa albida.--A seedling was observed during only 12 days, by which time a leaf had been formed, and the cotyledons were then quite vertical at night.

Trifolium subterraneum.--A seedling, 8 days old, had its cotyledons horizontal at 10.30 A.M. and vertical at 9.15 P.M. After an interval of two months, by which time the first and second true leaves had been developed, the cotyledons still performed the same movement. They had now increased greatly in size, and had become oval; and their petioles were actually .8 of an inch in length!

Trifolium strictum.--After 17 days the cotyledons still rose at night, but were not afterwards observed.

Lotus Jacoboeus.--The cotyledons of some seedlings having well-developed leaves rose to an angle of about 45o at night; and even after 3 or 4 whorls of leaves had been formed, the cotyledons rose at night considerably above their diurnal horizontal position.

Cassia mimosoides.--The cotyledons of this Indian species, 14 days after their first expansion, and when a leaf had been formed, stood during the day horizontal, and at night vertical.

Cassia sp? (a large S. Brazilian tree raised from seeds sent us [page 117] by F. Müller).--The cotyledons, after 16 days from their first expansion, had increased greatly in size with two leaves just formed. They stood horizontally during the day and vertically at night, but were not afterwards observed.

Cassia neglecta (likewise a S. Brazilian species).--A seedling, 34 days after the first expansion of its cotyledons, was between 3 and 4 inches in height, with 3 well-developed leaves; and the cotyledons, which during the day were nearly horizontal, at night stood vertical, closely embracing the young stem. The cotyledons of another seedling of the same age, 5 inches in height, with 4 well-developed leaves, behaved at night in exactly the same manner.]

It is known* that there is no difference in structure between the upper and lower halves of the pulvini of leaves, sufficient to account for their upward or downward movements. In this respect cotyledons offer an unusually good opportunity for comparing the structure of the two halves; for the cotyledons of Oxalis Valdiviana rise vertically at night, whilst those of O. rosea sink vertically; yet when sections of their pulvini were made, no clear difference could be detected between the corresponding halves of this organ in the two species which move so differently. With O. rosea, however, there were rather more cells in the lower than in the upper half, but this was likewise the case in one specimen of O. Valdiviana. the cotyledons of both species (3 ½ mm. in length) were examined in the morning whilst extended horizontally, and the upper surface of the pulvinus of O. rosea was then wrinkled transversely, showing that it was in a state of compression, and this might have been expected, as the cotyledons sink at night; with O.

Charles Darwin

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