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° C. 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, Hieracium ruulgatum, Scorzonera 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 ^ulipa begins, although previously the opening had become more com- 
plete with every rise of temperature. These phenomena are rendered intelligible by 
Sect. 19. 
Pfeffer mentions, after Crocus and 7uUpa, the flowers of Adonis 'vernalis, Ornithogalum 
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 Nymphcea 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) Ihe action of Light. Sudden obscurity suffices to cause the closure of open 
flowers. Evident closing was observed in Caletidula officinalis, Leontodon hastilis, and 
in Venidium calendulaceum, when the flowers, which had fully opened in diffuse 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. 
It appears from Pfeffer'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 Tolpis harbata. The ligulate 
peripheral flowers of BelUs 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 
