76 The Development of Oxygen by Plants. [February, 
4’2 cubic centimetres, and contained 5 per cent of carbonic 
acid, or not more than is found during the action of ordinary 
sunlight : the remaining 95 per cent consisted of oxygen, 
with an undetermined admixture of nitrogen. Hence Reinke 
infers that the liberation of oxygen observed does not well 
agree with the theory of a physical mode of action of the 
chlorophyll. 
If we start with the assumption that the activity of the 
chlorophyll in the decomposition of carbonic acid is 
chemical, this chemical function of a given mass of 
chlorophyll cannot — under the most favourable conditions — 
liberate more than a certain absolute quantity of oxygen. 
This exactly agrees with the result of the above experi- 
ments. It appears that the maximum liberation of 
oxygen from Elodea occurs at an intensity of light some- 
times a little higher, sometimes a little lower, and 
sometimes coinciding with that of normal sunlight. A 
further increase of the intensity remained ineffective, but 
no injurious effects upon the liberation of oxygen were 
perceived until the chlorophyll was destroyed by the 
oxidising action. 
It has been known with certainty since the end of the 
last century that the liberation of oxygen by plants is a 
fundtion of chlorophyll. The diredt proof has only lately 
been given by Engelmann that oxygen is given off exclu- 
sively by the chlorophyll granules, and that these alone aCt 
as organs of the decomposition of carbonic acid in the cell. 
A further step is thus taken rendering it probable that no 
substance other than chlorophyll under the impulsion of 
sunlight chemically mediates the decomposition of carbonic 
acid. We know that this process takes place in the 
chlorophyll granules only, and that consequently some 
substance present in these granules must be the effective 
agent. Hence it seems most natural to ascribe this fundtion 
to the chlorophyll itself. 
It is not sought to deny that, in addition to this fundtion, 
chlorophyll may play an important part in protecting the 
cell from light. Along with the principle of the division of 
labour in Nature we encounter also the principle of the 
cumulation of functions. That such a function of protec- 
tion from light may have profound significance for a plant 
is shown, e.g., by the investigations of Berthold. Many 
sea-weeds escape the direct action of sunlight by their 
locality, whilst others equip themselves with the most 
manifold and curious protective arrangements. Wherein 
lies the injurious effect of direct irradiation, which is 
