396 Dr. F.:F. Blackman and Mr. A, M. Smith. [Dec. 19, 
value of about 0034 :grm. CO. assimilated. per hour (consult lig. 1), but 
this unhealthy plant did not exceed 0:008 grm. 
Such unhealthy plants would naturally be avoided in collecting, but it is 
not only those whose appearance obviously indicates loss of vigour that give 
figures below the normal for assimilation. Plants which have been kept in 
the laboratory shade-greenhouse in tap-water for a few days always show a 
falling-off in assimilation, though their outward appearance remains unchanged. 
For example, on May 26,1905, a quantity of Elodea was collected, some 
experimented upon the same day, and the rest kept in a large basin of tap- 
water in the shade-greenhouse until June 6. The value obtained on 
May 26 was 00216 grm. CO, per hour in 5°7 intensity of light and 
COz-concentration of 0:0426 grm. per cent. (which figures in Table I as a 
normal value), while on June 6 under no less favourable conditions the value 
was only 00179, or about 17 per cent. less. 
Again, Elodea gathered on June 8 which gave full active ascii when 
measured on the same day gave on June 14 in 5°7 light and COs strong 
enough not to be limiting (0°0318 grm. per cent.) only an assimilation value 
of 0°0167 grm. CO2 per hour when at least 0°0240 would be normal. This 
falling off of nearly 30 per cent. was unaccompanied by any microscopic 
sign of change, and was brought on by five days’ stay in a basin in the 
laboratory and 24 hours in the dark in the actual assimilation-chamber* 
before the assimilation value was measured. : / 
It is reasonable to conclude that among a large number of random 
gatherings of healthy-looking Elodea from natural habitats some will be 
depressed below their normal vigour, and therefore it seems more instructive 
to eliminate from the final comparison of assimilation values, those which fall 
considerably below the general level under similar conditions. 
Section LIJ.—THE INTERACTION OF LIMITING FACTORS. 
When the two series of experiments described in the first section were 
carried out in 1905 we were not certain whether it was the light or the 
temperature employed that limited the assimilation of both Elodea and 
¥ontinalis to approximately the same value in all the higher concentrations 
of CO: The temperatures lay between 19° and 23° C., except in two cases, 
and the light intensity was usually 5-7. Inspection of the table would incline 
us to the view that it was the light that was limiting because, in the two 
cases where temperatures of 28° and 30° C. were employed with Elodea, no 
* This long enclosure in the chamber may have been specially deleterious ; it was never 
repeated. 
