MOVEMENTS OF THE PARTS OF PLANTS. 551 



A. Movements which evidently depend on external influences, 

 as 



a. Periodical. 



In many plants it is observed that the foliar organs, the leaves 

 of the stalk as well as of the flower, assume a different direction 

 during night from what they do in the day, and these phenomena 

 are frequently produced by the brightness or the cloudiness of 

 the sky. This, since the time of Linnaeus, has been called the 

 sleep of plants. In general, it may perhaps be assumed as a rule 

 that the parts of plants during the absence of light resume as 

 nearly as possible the position which they occupied in the bud, 

 and this the more accurately the younger and more tender the leaf. 

 The deviations arising in this respect from day and night are 

 slighter in older and tougher leaves ; they disappear entirely in per- 

 ennial and leathery leaves. The very compounded leaves of the 

 Leguminosce and Oxalidacea exhibit these phenomena in the most 

 striking manner. 



Similar movements may be observed in some flower stalks, which 

 are curved during night in such a manner that the flower is turned 

 towards the ground ; for instance, Euphorbia sp., Ranunculus poly- 

 anthemos, Draba verna, Verbascum blattaria. 



In opposition to this, there are some few flower leaves which 

 deviate from their normal position in the bud during night, and 

 again return to it in the day; for instance, Mesembryanthemum 

 noctiflorum. 



The movements here mentioned, especially of the first kind, are so 

 remarkable in some plants that even Pliny observed them (N. H. viii. 

 35.) But Linnaeus first of all traced them more accurately, and pub- 

 lished an elaborate account of them. (Somnus plantarum. Upsaliae, 1755, 

 Amosnit. Acad. vol. iv. p. 133.) The number of observations has subse- 

 quently increased, and every one may by individual researches confirm 

 the matter. I am of opinion that it is based upon the same cause as the 

 phenomena which will be spoken of under b. The anatomy of the parts 

 in which the movement takes place should be examined in a larger series 

 of plants, and the state of the cellular tissue, especially in the day, should 

 be accurately compared with the state which it exhibits at night ; exact 

 measurements should also be made of it. The movements are observed 

 most frequently and most strikingly in that region where the petiole of 

 the leaf passes into the stalk, and where the petiolules pass into the com- 

 mon petiole of the leaf, particularly when that swelling of the cellular 

 tissue called the pulvinus is very considerable. But experiments in 

 which the pulvinus has been carefully stripped off seem to prove that 

 the cause of this motion is not seated in this part, as Dutrochet supposed. 



With regard to the facts of the present paragraph, I have no observa- 

 tions of my own to offer, and therefore merely communicate the most 

 essential of these facts. I must refer to Meyen's Physiologic (vol. iii. 

 pp. 473 562.) and to Dassen's works (the principal work quoted by 

 Meyen*) for details, and more especially respecting the results of ex- 



* Natuurkundige Verhandelingcn van de Hollandschc Maatschappij der 

 schappen te Harlem II. Deel. Te Harlem 1835, pp. 309-346. and: Tijdschrift voor 

 natuurlijke Gcschiedenis en Phys. 1837, vol. iv. pp. 106131. 



