Francis Darwin. 
124 
geotropism. The flower buds are at first in line with the scape and 
emerge in a vertical position. They afterwards bend over towards 
the light, but their final position is determined by gravity, not by 
light. Even in darkness the tube of the corolla becomes hori¬ 
zontal. If the flower-scape is now placed obliquely so that the 
flower-tube points obliquely upwards or downwards, it regains the 
horizontal by an increase or diminution of the angle it makes with 
the scape. The flower of Narcissus differs from diageotropic 
stolons in not being in equilibrium when rotated through 180°. 
Thus if the flower-scape is made to point vertically downwards the 
flower does not remain horizontal, but gradually unbends until it 
too points vertically downwards. 1 Some curious cases of a change 
from upright to more or less horizontal growth have been recorded 
as caused by external conditions. Thus Vochting 2 has shown that 
the shoots of Minmlus Tilingii becomes plagiotropic under the 
influence of cold, and a similar result of low temperature has been 
observed in other plants. 
In the same way the air of a laboratory contaminated with 
lighting-gas has been found to produce horizontal growth in Pisum 
seedlings. Neljubow 3 found that in laboratory air, purified by 
being passed over red hot copper oxide, the plants grow normally. 
Also that pure air purposely contaminated with Acetylene or with 
Ethylene (both of which occur in gas) produces horizontal growth. 
Diaheliotropism . 
If a seedling whose leaves are diaheliotropic is illuminated from 
The arrows show the direction of the light, 
above figure the leaves are horizontal, i.e., at right angles to the 
line of light. If the plant is now exposed to oblique light from one 
side figure, the leaves adapt themselves to the new stimulus by 
simple flexion of the petioles, one leaf bending downwards, the other 
1 Vochting, loc. cit. 
2 Vochting, D. Bot. Gcs. XVI., p. 37. 
3 Neljubow, Beihefte Bot. Centr. X., p. 128. 
