534 Smith. — The Temper ature- coefficient of 
units. In fact the only figures which give their hypothesis a semblance of 
support are those in Expts. IV, V, and the later portions of VIII. It 
will be noticed that in these cases the plant has been exposed for some time 
to high intensities of light, and that on being taken back into lower intensi- 
ties its curve is slightly different from that of a fresh plant. It seems clear 
that the plant has been to some extent injuriously affected by the intense 
light. This supposition is supported by the fact that in Expt. VIII, 
where the plant is twice exposed to a light intensity of 16 units, the bubble 
emission gradually falls off through the experiment, so that the last row of 
figures is throughout the smallest of the whole series. In order to avoid 
this source of error Curve B, Fig. 2 , has been drawn, representing the readings 
after ignoring all those taken after the plant has been once exposed to t 6 
units of light. This curve is a typical limiting factor curve and cannot 'pos- 
sibly be interpreted as supporting the hypothesis of Brown and Heise. It 
is significant of the small amount of support which Brown and Heise can 
obtain from any of the experiments, that Curve B is scarcely more con- 
clusively in favour of the interpretation of Blackman and Smith than is 
Curve A, obtained from all Reinke’s readings whatsoever. In fact Brown 
and Heise practically admit this when they say, ‘ This progressive falling 
off in assimilation per unit of increase of light is very rapid, and it is, there- 
fore, not surprising that with high light intensities increasing the intensity 
does not greatly augment the rate of assimilation ’, and later, ‘ if a direct pro- 
portionality is found in such a case (i.e. in low intensities of light) this is 
due to the selection of a particular range of light intensities ’. In the first 
of these sentences they admit the fact that at higher intensities increase of 
light does not increase assimilation, and in the second that at the lower 
intensities the assimilation is proportional to the light. It only remains for 
it to be noted that the latter is true up to half sunlight and the former from 
unit sunlight to 1 6 units in order to make it clear that the hypothesis of two 
limiting factors is correct, on Brown and Heise’s own admission, through the 
whole range of intensities used in Reinke’s experiments. 
Recently Boyson-Jensen (1918) has given curves showing the effect of 
light upon assimilation. As he points out, these curves are of a dual nature, 
showing first the limiting effect of light and later that of C0 2 supply. Owing, 
however, to the fact that individual variations were not eliminated, the results 
are not sufficiently exact to form a decisive test between the interpretation 
of Brown and Heise and that of Blackman. 
Assimilation and Limiting Factors. 
In a third paper W. H. Brown (1918) has made an attempt to prove 
that the results of Blackman and Smith (1911) on Elodea and Fontinalis can 
be more accurately represented by a so-called optimum curve than by the 
