CHAP. II. PULVINI OF COTYLEDONS. 117 



by F. Miiller). 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 neghcta (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 cotyle- 

 dons 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 down- 

 ward movements. In this respect cotyledons offer an 

 unusally good opportunity for comparing the structure 

 of the two halves ; for the cotyledons of Oxalis Valdi- 

 viana rise vertically at night, whilst those of 0. 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 0. rosea, however, 

 there were rather more cells in the lower than in the 

 upper half, but this was likewise the case in one speci- 

 men of 0. Valdiviana. The cotyledons of both species 

 (3J 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. Valdiviana it was the lower surface 

 which was wrinkled, and its cotyledons rise at night. 



Trifolium is a natural genus, and the leaves of a]l 



* Pfeffer, ' Die Period. Bewegungen,' 1875, p. 157. 



