876 MECHANICS OF GROWTH, 



and then in a warm atmosphere, it is easy to observe that an increase of temperature 

 causes a movement of opening, and a decrease a movement of closing. Pfeffer succeeded 

 in making a Crocus flower open and close eight times in one day. The opening is more 

 complete if the flower has remained closed for a considerable time, and -vice 'versa. 

 Particularly sensitive Crocus flowers will open or close in as few as eight minutes in 

 consequence of a variation of temperature amounting to 5° C. : a variation of from 

 12° to 22° C. will cause it in three minutes. Immersion in warm or cold water has 

 the same effect. It is possible to ascertain, by appropriate arrangements, that Crocus 

 flowers are sensitive to a variation of 0*5° C. Flowers of Tulips are not so sensitive, 

 but they will react to a variation of 2° G, A reversed variation of temperature is not 

 immediately followed by a reversal of the movement ; this continues for some time before 

 it exhibits the effect of the more recent stimulus. The lower limit at which variations of 

 temperature will still induce movements lies, for the Crocus, above 8° C, according to 

 Pfeffer. The flowers of Leontodon hastilis, Hier actum 'vulgatum, Scor%onera hispanica^ and 

 of Oxalis rosea, open between 8° and 10° C, whilst between 1° and 3° C. they remain 

 closed in the light. 



When a certain maximum of temperature is exceeded, a partial closure of the flowers 

 of Crocus and of Tulipa begins, although previously the opening had become more com- 

 plete with every rise of temperature. These phenomena are rendered intelligible by 

 Sect. 19. 



Pfeff'er mentions, after Crocus and lulipa, the flowers of Adonis vernalis, Ornithogalurn 

 umbellatum, and Colchicum autumnale, as being very sensitive to variations of temperature ; 

 and in a less degree those of Ficaria ranunculoides, Anemone nemorosa, and Malope trifida, 

 all of which perform movements at any time of the day in consequence of variations 

 of temperature, the more energetically the longer the period since the last movement. 

 This is very evident in Nymphoea alba, Oxalis rosea and valdi'viana, Mesembryanthemum 

 tricolorum and echinatum, and in all motile flowers of Compositae. When these have 

 closed in the evening a rise of temperature from 10° to 28° C. produces scarcely 

 any opening; in the morning, on the contrary, a rise of temperature causes them to 

 open even in the dark. 



(y) The action of Light. Sudden obscurity suffices to cause the closure of open 

 flowers. Evident closing was observed in Calendula officinalis, Leontodon hastilis, and 

 in Venidium calendulaceum, when the flowers, which had fully opened in diff'use daylight, 

 were placed in darkness from eleven to twelve o'clock in the morning ; the temperature 

 varied between 19° and 20° C. In the afternoon, after longer exposure to light, the 

 closing consequent upon sudden obscurity is more marked. It appears also in the 

 case of Compositae and of Oxalis, that a sudden increase of the intensity of light 

 causes a more vigorous opening if the flowers have been previously kept in darkness 

 for a considerable time. The motile organ reacts the more vigorously to variations 

 in the intensity of light, as also to variations of temperature, the longer the time since 

 the occurrence of the last movement due to a stimulus acting in the opposite direction. 



According to Pfeffer, it is only darkness which accelerates growth ; but a consi- 

 derable time must elapse before the acceleration is perceptible in one (the inner) of 

 the two antagonistic masses of tissue. At the same time this side becomes compressed 

 by the other. 



1 1 appears from Pfeff'er's manuscript that as in the case of leaves so in flowers which 

 are motile and which are sensitive to variations in the intensity of light, a persistent effect 

 may be observed of such a kind that the daily periodicity of movement brought about by 

 the alternation of day and night will continue to be manifested for some time in 

 continued darkness. An instance of this is afforded by lolpis barbata. The ligulate 

 peripheral flowers of Bellis perennis curve outwards between seven and eight o'clock in 

 the morning in August (at a window facing south), and close between five and six o'clock 

 in the evening. In obscurity, the opening begins one or two hours later, and the closure 

 at night is incomplete. The flowers of Taraxacum officinale, Leontodon hastilis, and of 



