RELATION OF ALGAE TO DISSOLVED GASES. 185 
longer evolution, but the predominance of filamentous forms 
in tropical waters, which are so rare in temperate waters, 
indicates an evident adaptation to poorly aérated waters, 
since they are found to be most abundant in small stagnant 
ditches and rock pools. Even the species which elsewhere 
exist as separate simple individuals are here found attempt- 
ing a filamentous form. This is not merely a local condition, 
for in the upland pools (altitude 6,000 feet), where condi- 
tions are semi-temperate, the filamentous forms are just as 
rare as in the waters of temperate regions. He has elsewhere 
suggested that we may again find an explanation for these 
phenomena in the relative aération of the waters, the pres- 
ence of little dissolved oxygen appearing to encourage 
filament formation. This is a suggestion which must be left 
to experiment for verification, but it certainly would explain 
the relative abundance of filamentous desmids in the tropical 
waters. The same theory also will account for filamentous 
diatoms in the hot springs. In a foot-note it is stated that 
this theory is based on Senn’s observations on colonial Pro- 
tococcales. 
Senn (’99) put Celastrwm cells from pure cultures con- 
taining no ccenobia, into culture flasks containing nutrient 
solution. One flask he stirred up well with air so as to 
secure good aération. The other flask was completely ex- 
hausted of air by means of a suction pump. After a week 
samples from the former showed only single cells, no ccenobia 
having been formed, while samples from the latter showed a 
number of eight and sixteen-celled ccenobia, perfectly green 
and with gelatinous conditions well developed. 
In another case he put single cells into glass cylinders 12 
em. high, with 0.25% Knop’s nutrient solution. Boiling 
the solution to sterilize it had driven out all the air. In a 
few weeks this contained ceenobia with gelatinous arms or 
connections developed. Similar cultures in 2% solutions, 
instead of 0.25%, gave similar results, showing that the 
action was not due to concentration of the solution. More- 
over, distilled water gave the same results. The depth of the 
water prevented the diffusion of air to the lower layers of 
