61 
We are here concerned with a conduction of the photo-growth 
reaction; the magnitude of the growth retardation in the lower 
zones will likewise depend on the energy applied. 
The curve, which represents the dependence of the growth 
retardation (ordinate) in respect of the energy applied in 5 mins. 
(abscissa), we call: growth retardation curve. From the numbers of 
table I we cannot very well construct this curve, as there are too 
few data. There is however every reason for assuming that there 
is an ascending portion which can bring about positive curvatures 
and a descending one producing negative ones. Hence somewhere 
there is a maximum; its existence and position will be determined 
later by another method. Since the rule of products will apply also 
to the photo growth reaction within certain limits, there belongs to 
every amount of energy (x) a definite retardation of growth (y) after 
a certain time (here 1'/, hour). 
Now in unilateral illumination the front receives the full energy *) ; 
the back receives much less. If we are on the ascending part of 
the curve, the retardation of growth on the anterior side (vq) is 
greater than that on the posterior side (y,) and only a positive 
curvature will oeeur. Now 20 M.C.S. is the practical threshold 
value for a readily visible curvature; on applying this amount of 
energy therefore, there will occur a difference of growth retardation 
between the front and back, which is just sufficient to produce a 
curvature which can be readily detected macroscopically. The 
threshold value of 20 M.C.S. is however only applicable to plants 
which have been grown in the dark, and have received a quantity 
of light energy — 0. It is different with plants which have had an 
omnilateral fore-illumination. This can be done by rotating the 
plants before the source of light, so that successively all sides are 
illuminated; in this way all parts of the circumference receive an 
equal amount of light. If subsequently there is a unilateral illumination, 
the threshold value for a positive curvature is found to be much 
higher. 1 will now consider some experiments of Arisz?) on the 
socalled “disposition (German “Stimmung’’) to light” in oats and in 
the first place the omnilateral fore-illumination with less than 2000 
M.C.S. applied within 3 minutes with an intensity below 25 M.C. 
The unilateral after-illumination always took place immediately after 
the omnilateral. 
I find that all the changes, produced by the omnilateral 
1) Reflection etc. is here left out of account. 
2) Arisz le. Tables 24 and 25. See also Arisz: Adjustment to light in oats. 
Proceedings Kon. Ak. v. Wet. Amsterdam. 1913. 
