150 AISTATOMY AND PHYSIOLOGY OF 



mg up ; in the Composite tlie capitules shutting up, &c. Here 

 ako the times of sleeping and waking have been reversed by 

 artificial illumination (Meyen, '' Phy si ologie" in, 495). 



Undoubted as it is that the movejnents of sleeping and waking, 

 both in leaves and in flowers, aie dependant upon the influence 

 of light, yet they do not always occur in such a way that the 

 waking takes place in the morning when the daylight has reached 

 a certain definite degree, and the sleep begins in the evening 

 when the twilight has brought the light down to the same de- 

 gree, but the waking frequently precedes the dawn of day by 

 several hours (e.g^ in the leaves of Mimosa pudica), while the 

 sleep commences when there is still tolerably strong light. This 

 condition presents itself in a still more striking degree in many 

 flowers, than in leaves. As a general rule, the openness of the 

 flowers is indeed regulated by the light, so that the majority open 

 in the morning from six to seven, and close in the evening at 

 from six to seven, but in many flowers the opening takes place at 

 the very commencement of dawn, while they go to sleep even 

 before noon, or at least early in the afternoon ; on the other hand, 

 some plants require a longer illumination by the sun to cause 

 their opening, so that the flowers of various plants open gradu- 

 ally at the difijsrent hours of the morning until noon, on which 

 peculiarities Linnseus founded his " flower-clock."" These varia- 

 tions may be partly independent of light, and may be caused by 

 the circumstance that every species of plant requires a certain 

 degree of temperature to open its flowers. {Ses Fritzsche, " Sitz, 

 heTicht der Acad, zu Wien" Jan. 10, 1850.) In the flowei-s of 

 many plants we meet with the remarkable deviation that they 

 open first in the evening, reach their full expansion at midnight, 

 and close again in the morning, a phenomenon which, so far as we 

 know, has no parallel in the leaves ; perhaps this phenomenon is 

 analogous to the circumstance that the tendrils of Vitis turn 

 away from the light 



Ohserv, Since the far simpler movements which the influence of light 

 produces in stems and roots have not yet been adequately explained, we 

 can much less expect that tlie experiments to elucidate the movements of 

 leaves should have been more happy. The fall development of the cel- 

 lular tissue in the articulations of the leaves renders it much easier to 

 demonstrate in them, than in the axial organs of plants, that the motions 

 of plants are caused by curvature of the parenchymatous tissue, and not 

 by contraction of the spiral vessels or elongated cells (as was assumed by 

 Malpighi and all physiologists up to Link's time, evidently misled by an 

 erroneous idea of analog}^ between the movements of plants and those of 

 animals depending upon contraction of the muscular fibres). To demon- 

 strate this is merely required the easily performed experiment of cutting 

 away the cellular tissue, -without injuring the vascular bundle, ia the 

 articulation of any leaf possessing a distinct thickening there ; this openi- 

 tian lames the leaf. Our whole knowledge is cs&entially confined to this 



