This required two days, after which time a new tracing was begun (Fig. 124). In A we see the complex circumnutating course pursued from 11.30 A.M. Aug. 26th to 7 A.M. on the 30th. The pot was then moved a very little to the right, and the tracing (B) was continued without interruption from 7 A.M. Aug. 30th to after 6 P.M. Sept. 8th. It should be observed that on most of these days, only a single dot was made each morning at the same hour. Whenever the flower was observed carefully, as on Aug. 30th and Sept. 5th and 6th, it was found to be circumnutating over a small space. At last, on Sept. 7th, it began to bend downwards, and continued to do so until after 6 P.M. on the 8th, and indeed until the morning of the 9th, when its movements could no longer be traced on the vertical glass. It was carefully observed during the whole of the 8th, and by [page 279] 10.30 P.M. it had descended to a point lower down by two-thirds of the length of the figure as here given; but from want of space the tracing has been copied in B, only to a little after 6 P.M. On the morning of the 9th the flower was withered, and the sub-peduncle now stood at an angle of 57o beneath the horizon. If the flower had been fertilised it would have withered much sooner, and have moved much more quickly. We thus see that the sub-peduncle oscillated up and down, or circumnutated, during its whole downward epinastic course.
The sub-peduncles of the fertilised and withered flowers of Oxalis carnosa likewise bend downwards through epinasty, as will be shown in a future chapter; and their downward course is strongly zigzag, indicating circumnutation.]
The number of instances in which various organs move through epinasty or hyponasty, often in combination with other forces, for the most diversified purposes, seems to be inexhaustibly great; and from the several cases which have been here given, we may safely infer that such movements are due to modified circumnutation. [page 280]
CHAPTER VI.
MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP OF COTYLEDONS.
Preliminary sketch of the sleep or nyctitropic movements of leaves-- Presence of pulvini--The lessening of radiation the final cause of nyctitropic movements--Manner of trying experiments on leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons of Mimosa--Concluding remarks on radiation from leaves--Small differences in the conditions make a great difference in the result - Description of the nyctitropic position and movements of the cotyledons of various plants-- List of species--Concluding remarks--Independence of the nyctitropic movements of the leaves and cotyledons of the same species--Reasons for believing that the movements have been acquired for a special purpose.
The so-called sleep of leaves is so conspicuous a phenomenon that it was observed as early as the time of Pliny;* and since Linnaeus published his famous Essay, 'Somnus Plantarum,' it has been the subject of several memoirs. Many flowers close at night, and these are likewise said to sleep; but we are not here concerned with their movements, for although effected by the same mechanism as in the case of young leaves, namely, unequal growth on the opposite sides (as first proved by Pfeffer), yet they differ essentially in being excited chiefly by changes of temperature instead of light; and in being effected, as far as we can judge, for a different purpose. Hardly any one supposes that there is any real analogy
* Pfeffer has given a clear and interesting sketch of the history of this subject in his 'Die Periodischen Bewegungen der Blattorgane,' 1875, P. 163. [page 281]
between the sleep of animals and that of plants,* whether of leaves or flowers. It seems therefore, advisable to give a distinct name to the so-called sleep-movements of plants. These have also generally been confounded, under the term "periodic," with the slight daily rise and fall of leaves, as described in the fourth chapter; and this makes it all the more desirable to give some distinct name to sleep-movements.