BOTANY: OSTERHOUT AND HAAS 
87 
This result is surprising, but it has been confirmed by numerous experiments 
on Ulva, as well as by experiments on Enteromorpha, Spirogyra, Hydrodictyon, 
Potamogeton, and other plants. 5 
It is therefore evident that photosynthesis is a process which shows a 
gradual acceleration until a steady rate is attained. 6 A question of great in- 
terest now presents itself: What is the cause of the initial acceleration and 
why is a steady rate attained after a certain length of time? 
The suggestion which first offers itself is that photosynthesis belongs to the 
class of autocatalytic processes, in which the reaction is catalyzed by one of its 
own products. Such reactions begin slowly but as more of the catalyzing 
substance is produced the reaction goes on at an increasinlgy rapid rate 
until it begins to slow down as the reacting substances are used up. If these 
substances are constantly renewed, the reaction will not slow down but 
continue to go on more and more rapidly. 
In our experiments on photosynthesis the reacting substances are con- 
stantly renewed. 7 The substances entering into the reaction are presumably 
carbon dioxide and water. The concentration of the water reiriains constant, 
while as soon as the concentration of the carbon dioxide has diminished by 
a very small amount it is brought back to the original point by the renewal 
of the sea water. 
If photosynthesis were an autocatalytic reaction, it should, under these 
conditions, continue to increase in speed as time goes on. As a matter of 
fact it soon attains a steady rate. This might be accounted for by sup- 
posing that the concentration of the catalyst cannot exceed a certain amount, 
being limited by its own solubility. But in that case the rate would increase 
more and more rapidly up to a certain point and suddenly become stationary 
when the limit of solubility was reached. 8 This is not the case. The rate in- 
creases rapidly at first then more and more slowly until it finally becomes 
stationary. 
It might be supposed that the speed of the reaction is checked by the ac- 
cumulation of the products of the reaction. In that case, however, the rate 
would not become constant but would gradually diminish to zero. Such in- 
fluence of the products would be possible only in the case of a reversible reac- 
tion and we have no ground for believing that photosynthesis comes under 
this head. 9 
It might also be suggested that the rate becomes constant through the 
operation of a 'limiting factor' such as lack of light, carbon dioxide, or of 
temperature. But it is evident that the effect of such a factor would be 
fully felt at the very start of the reaction and that it could not cause a gradual 
falling off in the increase of speed. 
This puts clearly before us a fundamental difficulty. The fact that the 
rate increases most rapidly at first and then more slowly shows that photo- 
synthesis is not an autocatalytic reaction in the usual sense of the word, for in 
such a reaction 10 the rate would increase slowly at first, then more and more 
