of Carbon Dioxide in Ulva latissima, L, 41 
any way for a long period, or for the large amount of material 
which was necessary for the work. Another difficulty, which 
also was no doubt partly due to the difficulty with tempera- 
ture, was the enormous increase of Bacteria during the period 
of darkening. These Bacteria also make their appearance, 
but in a less degree, in cultures exposed to light, as has been 
recorded by Benecke 1 and Oltmanns 2 . In darkness, how- 
ever, this becomes a matter of great difficulty. In less than 
a week, a most evil-smelling scum, white or brick-red in 
colour, and of appreciable thickness, makes its appearance on 
the surface. It is of course impossible to sterilize the sea 
water, and the only thing to be done is to constantly change 
the water at least three times a fortnight, and to skim the 
surface every other day. When the sea water was changed 
the Algae were rinsed thoroughly with tap water and drained 
in a sieve, and then put into a fresh basin of sea water. That 
the formation of the scum was largely due to the temperature, 
was proved by the fact that in June it was much greater than 
in March. It may be pointed out, as indeed Oltmanns, Noll, 
and others have shown, that the changing of the sea water is 
an absolute necessity in other ways, as by so doing a fresh 
supply of nutrient salts is brought within the reach of the 
plant. Another difficulty was found to be very common in 
cultures carried out in the summer months. The Alga 
seemed then to be more liable to lose its chlorophyll than 
in the spring, probably owing to the greater intensity and 
duration of the illumination on exposure in the greenhouse. 
On the whole, winter work, with longer darkening and less 
constant illumination, was found to give better results. In 
consequence of these difficulties I was unable in some cases 
to make as many experiments or to extend the work as far 
as I could have wished. 
It was found to be fairly easy to tell, as a rule, when Ulva 
was in an unhealthy condition. Under adverse circumstances 
the Alga would become full of holes (as may be seen naturally 
1 Benecke (’98). 
3 Oltmanns (’95), 
