798 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



that in this case also water escapes from the cells into the intercellular spaces, esi)ecially 

 because he was able to determine that a flaccidity of the tissue results from the irrita- 

 tion. In proportion as water escapes from the parenchymatous cells does the whole 

 tissue contract from the elasticity of the cell-walls, of the stretched central bundle, and 

 of the epidermis. That the contraction is in this case so considerable depends on the 

 greater extensibility of the cell-walls and of the delicate central fibro-vascular bundle 

 in Mimosa, which is stated by Pfeffer to be on the contrary but slightly extensible \ 



2. Mechanism of the changes caused by 'variation in the temperature and in the intensity 

 of the light. 



(a) The Opening and Closing of Flowers. With the exception of a few statements by 

 Dutrochet, and the observation by Hofmeister- that the flowers of the tulip open with 

 an elevation and close with a depression of temperature, scarcely anything is at present 

 known regarding the mechanism of these movements. I can now^ only make a few^ 

 general remarks which Pfeffer has placed at my disposal ; they are based on a series 

 of investigations which is not yet completed. The opening of the flowers is connected, 

 in the crocus, tulip, and dandelion, with an increase in length of the inner side of the 

 perianth-leaves ; there is no considerable increase of the length of the outer side ; the 

 part where the curvature takes place is always belovv\ 



Under the ordinary conditions of vegetation these movements are not caused by a 

 change in the amount of moisture in the air, since they take place even under water. 

 In Crocus •vermis and luHpa Gesneriana and syh^estris small variations of temperature give 

 rise to remarkable movements, every rise causing the flowers to open, every fall to close. 

 The crocus is especially sensitive even to variations of half a degree C. The opening 

 takes place whether the rise is rapid or slow, and both phenomena may be repeated many 

 times in a short space. As in analogous phenomena, there are here also superior and 

 inferior limits, and a temperature of greatest sensitiveness. The crocus, for instance, 

 only opens above 8° G. ; at a temperature above 28° G. any rise causes the flowers to 

 close. 



When the temperature is constant a sudden change in the amount of light causes 

 movements in the crocus, tulip, and Compositae, an increase making them open, a de- 

 crease making them close ; but slight changes of temperature may reverse the move- 

 ments in the case of the crocus and tulip. Spontaneous periodic movements also occur 

 in the two latter plants, although not considerable in amount ; more so in other species 

 that win be named. 



Ornithogalum umbellatum, Ayiemone nemorosa and ranunculoides, Ranunculus Ficaria, 

 and Malope trifda also manifest similar phenomena resulting from changes of temper- 

 ature at any time of the day, but not so strikingly as in the case of the crocus. 



The phenomena are somewhat different in the dandelion and other Gompositae and in 

 Oxalis rosea; in the evening a considerable rise of temperature (e.g. from 9° to 30^ G.) 

 does not cause the flowers to open, although a slight but scarcely perceptible curvature 

 outwards results. In the morning, on the contrary, an increase of temperature acceler- 

 ates the opening to a very remarkable degree. If flowers of the dandelion are kept in 

 the dark in the day-time at a temperature below 10° G., they scarcely open at all; but 

 in the evening open rapidly and entirely if the temperature is raised ; the next morning 

 they are again closed at the ordinary temperature ; and when this is raised they do not 

 open at all or only very slightly. The flowers of some other Compositae and of Oxalis, 



^ [The most recent experiments of Pfeffer on the stamens of Cynara Scolymus and Centaiirea 

 Jacea show that the filaments are irritable along their whole length. Irritation caused a contraction 

 of from 8 to 22 p c. in length, accompanied with but a very shght increase in thickness, the 

 diminution in mass being caused by an escape of water into the intercellular spaces, which oozes out 

 when the filament is cut through. If the intercellular spaces are filled with water by injection the 

 stamens are no longer irritable. — Ed.] 



2 Hofmeister, Flora, 1862, p. 517; Boyer, Ann. des Sci. nat. 1868, vol. IX. 



