pudica, apparently the whole of the short sub-petioles of the leaflets have been converted into pulvini. With pulvinated leaves (i.e. those provided with a pulvinus) their periodical movements depend, according to Pfeffer,* on the cells of the pulvinus alternately expanding more quickly on one side than on the other; whereas the similar movements of leaves not provided with pulvini, depend on their growth being alternately more rapid on one side than on the other.** As long as a leaf provided with a pulvinus is young and continues to grow, its movement depends on both these causes combined;*** and if the view now held by many botanists be sound, namely, that growth is always preceded by the expansion of the growing cells, then the difference between the movements induced by the aid of pulvini and
Fig. 63. Oxalis rosea: longitudinal section of a pulvinus on the summit of the petiole of a cotyledon, drawn with the camera lucida, magnified 75 times: p, p, petiole; f, fibro-vascular bundle: b, b, commencement of blade of cotyledon.
* 'Die Periodische Bewegungen der Blattorgane,' 1875.
** Batalin, 'Flora,' Oct. 1st, 1873
*** Pfeffer, ibid. p. 5. [page 114]
without such aid, is reduced to the expansion of the cells not being followed by growth in the first case, and being so followed in the second case.
Dots were made with Indian ink along the midrib of both pulvinated cotyledons of a rather old seedling of Oxalis Valdiviana; their distances were repeatedly measured with an eye-piece micrometer during 8¾ days, and they did not exhibit the least trace of increase. It is therefore almost certain that the pulvinus itself was not then growing. Nevertheless, during this whole time and for ten days afterwards, these cotyledons rose vertically every night. In the case of some seedlings raised from seeds purchased under the name of Oxalis floribunda, the cotyledons continued for a long time to move vertically down at night, and the movement apparently depended exclusively on the pulvini, for their petioles were of nearly the same length in young, and in old seedlings which had produced true leaves. With some species of Cassia, on the other hand, it was obvious without any measurement that the pulvinated cotyledons continued to increase greatly in length during some weeks; so that here the expansion of the cells of the pulvini and the growth of the petiole were probably combined in causing their prolonged periodic movements. It was equally evident that the cotyledons of many plants, not provided with pulvini, increased rapidly in length; and their periodic movements no doubt were exclusively due to growth.
In accordance with the view that the periodic movements of all cotyledons depend primarily on the expansion of the cells, whether or not followed by growth, we can understand the fact that there is but little difference in the kind or form of movement in the two sets of cases. This may be seen by com- [page 115] paring the diagrams given in the last chapter. Thus the movements of the cotyledons of Brassica oleracea and of Ipomoea caerulea, which are not provided with pulvini, are as complex as those of Oxalis and Cassia which are thus provided. The pulvinated cotyledons of some individuals of Mimosa pudica and Lotus Jacobaeus made only a single oscillation, whilst those of other individuals moved twice up and down in the course of 24 hours; so it was occasionally with the cotyledons of Cucurbita ovifera, which are destitute of a pulvinus. The movements of pulvinated cotyledons are generally larger in extent than those without a pulvinus; nevertheless some of the latter moved through an angle of 90o. There is, however, one important difference in the two sets of cases; the nocturnal movements of cotyledons without pulvini, for instance, those in the Cruciferae, Cucurbitaceae, Githago, and Beta, never last even for a week, to any conspicuous degree. Pulvinated cotyledons, on the other hand, continue to rise at night for a much longer period, even for more than a month, as we shall now show. But the period no doubt depends largely on the temperature to which the seedlings are exposed and their consequent rate of development.
[Oxalis Valdiviana.—Some cotyledons which had lately opened and were horizontal on March 6th at noon, stood at night vertically up; on the 13th the first true leaf was formed, and was embraced at night by the cotyledons; on April 9th, after an interval of 35 days, six leaves were developed, and yet the cotyledons rose almost vertically at night. The cotyledons of another seedling, which when first observed had already produced a leaf, stood vertically at night and continued to do so for 11 additional days. After 16 days from the first observation two leaves were developed, and the cotyledons were still greatly raised at night. After 21 days the cotyledons during the day were deflected beneath the horizon, but at night were raised 45o [page 116] above it. 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.
