75 
Sense Organs for Gravity and Light. 
regulating the movements of the leaf in regard to light. What 
evidence is there that this is more than an ingenious hypothesis? 
In the first place there is no doubt about the existence of the spot 
of light. It is only necessary to remove by a superficial section the 
upper epidermis from an appropriate leaf, 1 and place the specimen 
under the microscope with the epidermal surface downwards. It 
is then to be illuminated from below, when a bright area surrounded 
by a dark border is clearly visible in each cell. On one occasion 
Haberlandt chanced on epidermic cells whose focal length was such 
that they cast a sharp image of an object standing near on to the 
basal wall of each cell. This remarkable specimen was photographed 
and is reproduced in Haberlandt’s book. It is not suggested that it 
is of service to the plant that images of neighbouring objects should 
be thrown with this degree of sharpness onto the basal wall. The 
case is rather of interest as showing how good a lens may he made 
out of a vegetable cell. Before going on to describe some of the 
more interesting forms of ocelli it will be convenient to give an 
experiment of Haberlandt’s which to my mind is very convincing 
of the truth of his theory. The experiment in question is simple 
enough, namely the submergence of the whole plant in water. If 
Haberlandt is right the submerged leaves should lose their power 
of orientation in regard to light for the obvious reason that submer¬ 
gence must prevent the epidermic cells functioning as lenses, since 
the water is of practically the same refractive index as the cell-sap. 
This is what Haberlandt found to be the case, viz., that the 
submerged leaves were apparently blinded and were at any rate 
unable to orientate themselves. It may be objected that the leaves 
were unable to move under water and that their behaviour had 
nothing to do with the “blinding” action of the water. This 
objection has, however, been met by Haberlandt in a manner which 
for simplicity’s sake has been here omitted. Haberlandt’s 
experiments with the leaves of Tropceolum are of some interest. 
They were found not to lose their powers of orientation when 
immersed, and this is what might have been expected, because they 
are well known to remain dry under water—protected by the layer 
of air entangled in their “bloom” of waxy particles. But if this 
coating of air is removed (by rapidly washing the surface in diluted 
alcohol) the Tropceolum leaf is wetted by water like any other leaf, 
and now ceases to respond to light. 
I am inclined to accept Haberlandt’s extension of his theory 
1 For instance a leaf of Begonia discolor. 
