184 MISSOURI BOTANICAL GARDEN. 
and hence is not deprived of its dissolved gases, as is the case 
in the tropics just mentioned. However, the summer is 
marked by change of filamentous forms, such as Melosira and 
Fragilaria. It is believed that diatoms in general prefer 
colder and better aérated water, although Fritsch mentions 
the well-known cases of diatoms in hot springs, where they 
usually assume filamentous forms. 
In this connection it is interesting to mention H. W. 
Clark’s (’08) report of Melosira in Guatemala, where, he 
says, “One of the most striking features of the Amatitlan 
plankton is the abundance of Melosira, which is found 
abundantly in the bottom of nearly all the hauls, and usually 
makes up the main mass of all the filamentous material. The 
specimens occur in long rigid filaments.” 
West (’09) says: “Diatoms are not a feature of the 
plankton in the Yan Yean reservoir at Victoria, Australia, 
although Melosira granulata occurs abundantly and is never 
absent from the collections.” 
“After all,” says Fritsch (’07), “We know as good as noth- 
ing at present as to the influence of the amount of dissolved 
oxygen in the water, on the various fresh water algal genera.” 
As to the desmids, both Fritsch (07), in Ceylon, and West 
(709), in Yan Yean, Australia, remarked the abundance and 
variety of and tendency toward the filamentous forms. 
West (’09) says that the most striking feature of the plank- 
ton at Yan Yean is the richness of its desmid-flora. 
On the other hand, in the fish-pond near Bristol, Fritsch 
(09) finds a notable lack of desmids, the number of 
individuals being small, and likewise the number of genera. 
This is no doubt due to the fact that water containing car- 
bonated lime, which is known to be unfavorable for an 
abundant development of desmids, drains into the pond. 
This fact is also noted in regard to the absence of desmids 
from the calcareous waters of Kew and the Thames river. 
The desmids, which generally are so abundant and varied in 
the temperate waters, were found by Fritsch (’07) to be even 
more abundant and varied in the waters of Ceylon. The 
great variety of form, he says, may be only the result of a 
