]
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 ? 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. Valdiviana it was the lower surface which was wrinkled, and
its cotyledons rise at night.
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