475 
Wager ,—The Perception of Light in Plants. 
as it is under the normal conditions, and if so marked a difference is 
required in the one case, it is reasonable to suppose that it would be equally 
required in the other. From experiments which he has recently made, how- 
ever, upon the amount of light differentiation required for heliotropic 
response in plants, Haberlandt 1 comes to the conclusion that it is sufficient 
to bring about a response. 
Gius 2 concludes from experiments which he has made upon various 
seedlings, and upon leaves of dia-heliotropic plants submerged in water, that 
it is not the perception but the power of movement which is inhibited. The 
response to light is markedly later and slower than in air. If, however, the 
submergence in water is limited to the period required for the perception to 
take place, there is no delay in the motor response. The explanation 
offered is that the difference in turgor of the cells required to bring about 
the movement is prevented by the surrounding water. This explanation 
cannot apply when the surface only of the leaf-blade is covered with water. 
But it is possible that even here the water may produce some change in the 
leaf, even if only a very slight one, detrimental to the perception of light or 
to the motor reaction. It is quite possible that leaves may vary in their 
power to resist the detrimental action of water, as indeed is seen in water 
plants and those grown in marshy places, as compared with plants normally 
accustomed to drier conditions, and which, when submerged in water, soon 
begin to decay. 
Haberlandt 3 shows that even when covered with water the highly 
curved or pyramidal epidermal cells still maintain the power to some extent 
of light differentiation. In cells in which the outer wall is less curved, but 
still moderately strongly curved, as in Tropaeolum Lobbianum , the whole 
basal wall is equally illuminated when covered with water, but in oblique 
light a distinct differentiation can be observed. Under such conditions the 
orientation of the leaf would still take place when submerged in water, but 
not so perfectly or so quickly as in air. 
Haberlandt describes (loc. cit., 1909) some interesting experiments in 
which separate parts of the upper surface of a single leaf were illuminated 
from opposite sides by equal light, one half of the leaf being covered with 
a layer of water, the other half dry. With almost equal illumination from 
opposite sides of corresponding parts of the leaf, and with the wetted part of 
the leaf 2*2 to 4-8 times as large as the dry part, the inclination of the lamina 
towards the light was always in the direction of the dry part of the leaf, if 
the two halves of the leaf were of equal size and the wetted half twice as 
strongly illuminated as the dry half, the leaf still turned in the direction 
of the dry half of the leaf, that is, towards the weaker light. The experi- 
1 Zur Physiologie der Lichtsinnesorgane der Laubblatter. Jahrb. f. wiss. Bot., xlvi, 377, 1909. 
2 Ueber den Einfluss submerser Kultur auf Heliotropismus und die Lichtlage. Sitzungsber. 
d. k. Akad. Wien, cxvi, 1907. 
3 Zur Physiologie, &c. 
