MECHANISM OF THE MOVEMENTS. 799 



when kept for a whole day at a temperature of from 1° to 3° C. remain closed, and ex- 

 hibit the same phenomena as the dandelion if the temperature is raised in the morning 

 and evening. The flowers of the tulip and crocus also open in the morning when the 

 temperature rises after they have been closed during the night more quickly than in the 

 evening, or when they have been made to close again after first opening in the morning 

 by lowering the temperature. While therefore in the crocus and tulip an alteration in 

 the temperature always causes a movement of opening or closing, in the Compositae it 

 only assists the spontaneous movements. 



An alteration in the amount of light also affects the flowers of Compositae, but sudden 

 darkening in the day-time only causes them to close slightly. The flowers of Leontodon 

 hastilis, Scorzonera hispanica, and Hieracium, when placed in the dark for a whole day 

 open spontaneously, but do not close so completely in the evening as when in the light ; 

 on the second day this is more striking. Flowers of Oxalis rosea which unfold in the 

 dark open as fully as in the light, but close less completely. The spontaneous periodic 

 movements pursue their course during the whole time of flowering as in the light. The 

 flowers of the daisy, on the contrary, expand completely in the dark, thsir periodic 

 movements being but very slight. 



The curvature takes place, both in the Cynaraceae and the Cichoriaceae {e.g. Taraxa- 

 cum) in the lower part of the flower or corolla-tube ; the inner side elongates, the length 

 of the outer side not changing. 



We see therefore that light, like warmth, causes an elongation of the parenchymatous 

 tissue of the inner side of the flower, this being directly opposed to the phenomena dis- 

 played by leaves, whose contractile organs exhibit, as Pfefler has shown in the case of 

 Phaseolus, Oxalis, and Trifolium, a contraction of their parenchymatous tissue in the 

 light, and an elongation in the dark. 



The elongation of the inner side of the petals when the flower opens appears intel- 

 ligible only on the supposition of a change in shape of the cells or an increase in their 

 size ; but direct observation has not yet established either alternative. It may however 

 be considered certain that the curvature which causes the opening of flowers takes place 

 also in narrow strips of the petals when the air is damp. It is however obvious that the 

 elongation of the inner side on a rise in temperature cannot be the result of the rarefac- 

 tion of the air contained in the intercellular spaces, for the movements would then 

 take place also when the air is artificially rarefied, which is not the case. Pfeff'er con- 

 siders that the opening of flowers is connected neither with flaccidity nor increase in 

 rigidity of the tissue. 



(b) The opening and closing of lea'ves resulting frotn alternation in the amount of light 

 and temperature {the Sleep of Plants'^). If plants with motile leaves, like Papilionaceae 

 and Oxalideae, after having remained in the light, are suddenly placed in the dark, the 

 leaves after some time take up their nocturnal position, closing upwards or downwards 

 according to the species (Sect. 27). If light is now let in upon the plant in the state of 

 sleep, the leaves again open and assume their diurnal position. Placing them in the shade 

 has the same effect as complete darkness, but not so strongly. 



These facts show that fluctuations in the intensity of the light cause curvatures of the 

 motile parts of plants. If these parts are also irritable to concussion, as in Mimosa and 

 Oxalis acetosella, darkness causes a similar position of the leaves to concussion. But the 

 internal conditions are, as has been mentioned, very different in the two cases ; for the 

 folding up caused by dark is associated with an increase in the rigidity of the part, and 

 therefore with an increase in its turgidity ; while in that caused by irritation there is a 

 decrease of all these, as Briicke was the first to show^ in the case of IMimosa. In the 

 leaves of Phaseolus which are not irritable to concussion Pfeffer also found an increase of 



^ Dutrochet, Mem. pour servir, vol. I, p. 509 — Meyen, Neues Syst, der Pflanz.-Phys. vol. Ill, 

 p. 487. — Sachs, Bot. Zeit. 1857, Nos. 46, 47. — Bert, Recherches sur les mouvements de la sensitive, 

 Paris 1867. — Millardet, Nouvelles lecherches sur la periodicite de la sensitive, Marburg 1869. 



