58 
illumination. This was one of the reasons .why phototropy was 
regarded as a thing apart. 
Braauw once more defends the old theory of Dr CANpoLLE and 
regards unilateral as a special case of omnilateral illumination; the 
former would thus have no specific action. If omnilateral illumination 
gives a photogrowth reaction uniform in all directions, unilateral 
illumination will only differ in giving an unequal growth reaction 
on the posterior and anterior sides. Here it is not the difference of 
light, but the light itself, as energy, which influences the longitudinal 
growth of every cell, but since the front is differently lighted from 
the back, the two sides will show unequal changes in growth. The 
result of this inequality is a curvature towards the source of light 
or away from it; this is the phototropic curvature. By these consid- 
erations phototropy has been saved from its isolation and has become 
susceptible of deeper and more exact analysis. A further important 
point is, that BLAAUW ascribes an influence to the posterior side also, 
which quite corresponds to that of the anterior. It is not the anterior 
nor the posterior side alone which actively causes the curvature; 
both are concerned: it is the difference in the change of growth of 
the two sides which makes the plant curve phototropically. 
The possibility that unilateral illumination might be a special case 
of ommnilateral, i.e. that every longitudinal strip of the plant might 
receive a photo-growth induction independently of the rest of the 
circumference and might execute a photo-growth reaction, suggested 
an investigation of the question, whether this could throw any light 
on various phototropic phenomena described in the literature. 1 found 
that the photo-growth reaction gives us the means of explaining 
satisfactorily many apparently contradictory phenomena. Since photo- 
tropism has been most fully examined in Avena sativa and since 
Voer moreover found a photo-growth reaction here, I have made 
a study of the literature on this plant. While I was engaged in 
working up these considerations for a preliminary publication, a 
paper by BremrkampP') appeared, which gave me no reason to change 
my opinion; I hope to return to this more fully in a later paper. 
Voer illuminated coleoptiles of Avena from above with various 
intensities during periods of various length, but on this account his 
energy numbers are not comparable with horizontal light. By the 
first method the plants receive much less light, since the absorbent 
surface is much smaller. With horizontal illumination we need apply 
1) G. E. B. Bremexamp. Eine Theorie des Phototropismus. Recueil des Travaux 
bot. néerlandais. Vol. XV. 1918. 
