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Show 116 PUL VINI OF CO'rYLEDONS. CHAP. II. above it. After 24 days from the first observation (bcgun.after a true leaf had been developed) the cotyledons ceased to nso nt night. Oxali.~ (Biophytu:rn) sensitiva.-Th~cotyledons of scvora~ secd-l'nO's 45 days after their first expansiOn, stood nearly vertical at ~igl1t, and closely embraced either one or t~o true leaves which by this time had been formed. These seedlmgs had boon .kept in a very warm house, and their development had boon ra~1u. OxaUs cornic11latn.-The cotyledons do not stand vertical at niO'ht but generally rise to an anglo of about 45o above the bo~lz~n. They continued thus to act for 23 days after their first expansion, by which time two leaves had been f?rmed_; even after 29 days they still rose moderately abovo thell' honzontal or downwardly deflected diurnal position. Mimosa pudica.-The cotyledons wore expanded for the first time on Nov. 2nd, and stood vertical at night. On tho 15th tho first leaf was formed, and at night the cotyledons wore vertical. On the 28th they behaved in the same manner. On Dec. 15th, that is after 44 days, the cotyledons wore still considerably raised at night; but those of another seedling, only one day older were raised very little. Mi'mosa albida.-A seedling was observed dming only 12 days, by which time a loaf had been formed, and the cotyledons were then quite vertical at night. . Trifolium surde~·ranr-um .-A seedling, 8 days old., haclltscotylcdons horizontal at 10.30 A.M. and vertical at 9.15 P.M. After an interval of two months, by which time tho first and second true loaves 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 anmch in length! . . t Trifol'iu.m strictum.-Aftcr 17 clays tho cotyledons still I0SO a night but were not afterwards observed. Lotus Jacobrous.-The cotyle<lons of some seedlings having well-developed leaves rose to an angle of about 45° at night; and even after 3 or 4 whorls of leaves had been formed, the~tyledons rose at night considerably above their diurnal hori· zontal position. . Cassia mimosoides.-The cotyledons of this Indian speCies, 14 days after their first expansion, and when a leaf ha~ been formed, stood during the day horizontal, and at night vertiCal. Cassia sp ? (a large S. Brazilian tree raised from seeds sent us CHAP. II. PUL VINI OF COTYLEDONS. 117 by F. Mtillor).-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 wore not aftcrward.s observed. Cassia neglecta (likewise a S. Brazilian spccies).-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 wore nearly horizontal, at night stood vertical, closely ombracjng tho young stem. Tho cotyledons of another seedling of the same ago, 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 unusally good opportunity for comparing the structure of the two halves ; for the cotyledons of Oxalis Vald-imana rise vertically at night, whilst those of 0. rosea sink vertically; yet when sections of their pulviui were made, no clear difference could be detected between the corresponding halves of this organ in the two species which move so differently. With 0. 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 0. Valdiviana. The cotyledons of both species (3! mm. in length) were examined in the morning whilst extended horizontally, and the upper surface of the pulvinus of 0. 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 0 . Va.ldiviana it was tho lower surface which was wrinkled, and its cotyledons rise at night. Trifolium is a natural genus, and the leaves of all * Pfeffer, 'Die Periou. Bewegungen,' 1875, p. 157. |