of Carbon Dioxide in ULva latissima, L. 43 
after darkening for some time. In the latter case the Alga 
was a dull, fairly opaque, dark green. On illumination of 
even a day it became much lighter in colour as well as 
more transparent— a brighter green generally. Harvey 1 has 
made the same remark about specimens of Ulva dredged 
from deep water, when £ the colour is of a very dark and even 
bluish green reflecting glaucous tints when under water.’ 
Enter omorpha intestinalis , Link., required an even longer 
period of darkening. In the spring not less than two months 
was found necessary, and even then one could not always be 
sure that every trace of starch had been got rid of. It was 
also found to be much more difficult to judge accurately the 
amount of starch in this Alga as compared with Ulva , and for 
these reasons it was early abandoned. 
The method of testing for starch used was Sachs’s iodine 
reaction 2 . Ulva when in a healthy state decolorized easily, 
after being placed in boiling water for a minute or two, and 
then allowed to stand for a day in methylated spirit. A 
sufficient strength of watery alcoholic solution of iodine was 
used without perceptibly colouring the thallus yellow. In 
a few cases Schimper’s chloral hydrate method was used as 
a confirmatory test. Sachs’s iodine method with this Alga 
is, however, sufficiently delicate, and this plant had the great 
advantage that, by this method, the distribution of the starch 
over a considerable area of thallus could be estimated at a 
glance. The drawback with regard to Ulva consists in the 
extremely small size of the cells even under the ordinary high 
power (J inch objective) of the microscope. Experiments 
with plasmolysis were not attempted, and they would be 
very difficult if not impossible for this reason. As might 
be expected, the appearance of the chloroplast was, as a 
rule, no guide to the condition, satisfactory or otherwise, of 
the Alga. 
When the Alga had been rendered starch-free, it was 
exposed to light continuously in a cool greenhouse, which 
1 Harvey (’46), vol. 4 , under Ulva latissima. 
2 Sachs (’ 88 ), p. 1 . 
