202 MISSOURI BOTANICAL GARDEN. 
the COz had been absorbed and that oxygen had been thrown 
off instead. During the interval masses of bubbles appeared 
in this jar just as in the control and in previous experiments, 
evidently bubbles of oxygen, as shown by the lighted match, 
and showing that vigorous photosynthesis had been going on 
in the jar, making use of the excess COs. 
March 1. This jar, “B,” was again exhausted and refilled 
as before, causing the color to disappear rapidly as the COz 
was absorbed, thus changing the normal carbonate to 
bicarbonate. After a few days the color reappeared, bubbles 
formed, showing continued photosynthesis as before. 
March 14. A lighted match indicated, as before, that the 
COz had been absorbed and replaced by oxygen. 
As further control for comparison, a jar half-full of tap 
water, which was alkaline to phenolphthalein, was half ex- 
hausted and refilled to atmospheric pressure with COz, which 
quickly dispelled the color just as when the plants were ~ 
present. But the color did not reappear when placed in sun- 
light, as it did when the plants were present. This seemed 
to leave no further doubt that the alkalinity and color changes 
were due to the photosynthetic activity of the plants, thus 
appropriating the COs. 
February 27. Another experiment like the above was set 
up in which Hydrodictyon was substituted for Pithophora, 
using only tap water, and the same amount of CO: as before. 
The color faded in about 20 minutes and did not reappear 
again when placed, like the other, in the south window for 
over two weeks. 
March 16. The Hydrodictyon seemed colorless and dead. 
March 16. An attempt to revive with oxygen was a failure. 
Apparently this concentration of CO: is too great for Hydro- 
dictyon, which is not so resistant as Pithophora. 
Schtitzenberger (’73), on the basis of some work with 
Elodea, states that for water plants 5% to 10% of complete 
saturation with COz is the optimum. I have not found any 
account of work done on alge with high per cents of carbon- 
dioxide. 
