MECHANICS OF GROWTH. 
various species of Impatiens, in Polygonum Convolvulus, and in Sida NapcEa. The 
amplitude of these curvatures which follow the alternation of day and night appears to 
increase and diminish with the increase and the diminution of the rapidity of growth. 
* In young leaves,' says Batalin, ' the curvatures are the greatest, and in Chenopodium 
and Stellar la, for example, they go so far that at night the young leaves close 
to form very large buds : the older leaves curve but little, the oldest not at all/ 
Among floral organs it is more especially the movements of the petals and of 
the tubes of gamopetalous flowers which have attracted attention; no movements 
which could be included in this category are known to occur in stamens or styles. 
The movement consists in the curvature of the petal or of the segment of a corolla 
outwards at certain times of the day and inwards at others, in such a way that in the 
ordinary course the flower opens and shuts once in a day. The opening occurs 
usually in the morning or during the day when the intensity of light and the tem- 
perature are increasing, the closing towards evening when light and heat are diminish- 
ing, although the contrary is sometimes the case. In the Compositae the movements 
of the individual flowers eff'ect not so much their own opening and closing as that of 
the whole capitulum. 
Among the very numerous instances of motile flowers the following have been 
more particularly investigated: Crocus, Tulipa, Colchicum, Ornithogalum, Anemone^ 
Ranunculus, Nymphcza, Malope, many Compositae, especially Taraxacum, Leontodon^ 
ScOrzonera, Hieracium, Calendula, Venidium, Bellis, &c. 
As in the case of leaves so here there are no special motile organs, but certain 
parts of the corolla continue to grow for a considerable time and are stimulated by 
meteoric influences to a more rapid growth either of their upper or their under 
surface (or internal and external), which efl'ects the opening or closing of the 
flower. The region of curvature usually lies in the basal half of petals, but in Oxalis 
rosea in the upper half. Among the Compositse with ligulate corollas there are 
some in which the motile zone lies immediately above the tube at the base of the 
ligulate corolla { Venidium, Bellis, Calendula), whilst in others, such as Taraxacum, 
Leontodon, &c., the tube itself undergoes curvature ; in both cases the centre of the 
capitulum is the centre of the movements which take place inwards or outwards 
along radii proceeding from it. 
AH the movements which are now under consideration agree in this respect, 
that the plane of curvature of the organ (leaf, petal, tube) coincides with its median 
plane, which is also the plane of symmetry, since it is the two sides of the organ 
which are diff'erent, that is the anterior and the posterior, which produce the curva- 
tures in consequence of the diff'erent way in which they react to the influence 
exercised upon them by variations of light and of temperature. 
The following paragraphs give an account of the mechanism of the movements 
produced by variations of light and of temperature, taken from the exhaustive 
researches of Pfeffer with some additions from Batalin. 
1. The movements which are now under consideration differ from the periodical 
movements and the movements due to stimulation which are manifested by leaves 
possessing motile organs, in that they only take place so long as growth continues, 
and that they cease with it. 
2. They are effected in consequence of a more vigorous growth of the internal 
