MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. 877 
Barkhausia rubra, placed in the dark towards evening and allowed to remain there, 
only half-opened on the following day, and remained partially open during the night. 
On the second day the movements were much feebler. These movements could not be 
attributed to variations of temperature. 
(Ö) The differing sensibility of leaves to variations of temperature on the one hand, and 
to variations in the intensity of light on the other, is evident from Pfeffer's statements. 
The flowers of Crocus and of Tulipa which are so very sensitive to variations of tempera- 
ture, close in consequence of sudden obscurity, and open when exposed to light, with 
an energy which is sufficient to overcome the counteracting effect of a temperature- 
stimulus. Considerable variations of temperature may however reverse the opening or 
closing effected by light or darkness. In Oxalis, Nymphcea alba, Taraxacum, and 
Leontodon hastilis, however, the closing in the evening cannot be prevented by a rise of 
temperature, and a fall of temperature does not arrest the opening in the morning. 
But if these flowers be kept in darkness during the day-time, they may be made to open 
in the evening by a rise of temperature. It is necessary, in order that this opening 
may take place, that a considerable time shall have elapsed since the occurrence of 
the stimulating effect of darkness. 
The conditions which are here stated explain why it is that many flowers growing 
in the open manifest an exact daily periodicity, whereas others open and close at all 
periods of the day in consequence of sudden changes of the weather. 
It must be mentioned, in conclusion, that many flowers close in consequence of an 
excessive intensity of light, as they do when the heat is too great. Thus Oxalis 
'valdi'viana, Calendula, Venidium, and others close when direct sunlight falls upon them. 
However, it is still uncertain whether the light or the heat is the active cause. 
(e) It is scarcely possible at present to formulate a brief general explanation of the 
phenomena exhibited by leaves and flowers which are sensitive to the action of meteoric 
influences, for we do not fully understand the internal changes effected by these stimuli 
nor the mutual relations of the two antagonistic halves of the organ. It is important to 
discover whether a variation of temperature or of light acts in each case upon both the 
antagonistic halves in contrary ways, or if one side only is sensitive undergoing changes 
which affect the other side indirectly, as occurs in the case of the sensitive motile organs 
of Mimoseae and probably also in the case of twining tendrils. 
As a general result, it is to be remembered that each variation of temperature and 
of light acts the more energetically as a stimulus the longer (within certain limits) the 
time which has elapsed since the action of the last stimulus in the contrary direction. 
The matter may be put thus : a difference, which disappears at a later period, is pro- 
duced between the upper and the lower side of a growing organ by each stimulus due to 
temperature or to light ; the more nearly this difference has disappeared the more easy 
it is for a fresh stimulus to produce an effect, that is to produce again a difference 
between the upper and the lower surface, or, in other words, to cause a movement or 
a curvature. 
