1108 Proceedings of Royal Society of Edinburgh. [sess. 
Surirella biseriata , Melosira varians, and M. arenaria , may also 
be considered as bottom forms ; they do not belong to the true 
plankton, but once broken off from their place in the bottom 
along the shores, they are capable of floating some time 
and perhaps of multiplying themselves while in a floating con- 
dition. 
True plankton Diatoms are only the following : Asterionella 
formosa , Fragilaria crotonensis , Cyclotella comta, Melosira italica 
and islandica, and the two Rhizosolenice. Of these the two 
Melosirce and Asterionella occur all the year round in the plankton, 
while the other may be found only during shorter or longer periods 
of the year ; the Rhizosolenice have resting spores, and it is 
evident that they form their spores when past the flowering 
period ; the spores sink to the bottom and there they rest until 
the next period. The same periodicity may occur with the 
Fragilaria crotonensis and the Cyclotella , but we have here no 
morphological sign to help us to decide when the period of flowering 
is ended. 
Among the Chlorophyceae the Desmids have no distinct period 
in which they disappear from the plankton ; Sphcerocystis and 
Oocystis, on the contrary, act as the Rhizosolenice. Such perio- 
dicity is to be found also in the Peridinium aciculiferum and 
the Mallomonas, both having resting spores. Although it thus be- 
comes evident that some of the plankton forms lack periodicity, 
inasmuch as they at no time of the year wholly disappear from 
the plankton, they still evidence periodicity in another way, viz., 
they attain at a given period their richest flowering or so-called 
max. ; before this period they become more and more numer- 
ous, and afterwards steadily decrease in number until they 
reach a minimum from which they then again commence to increase 
in number. This periodicity is not much different from the perio- 
dicity mentioned first ; if we suppose the minimum like zero, and 
if this minimum extends over some time, we have the first-men- 
tioned periodicity : the difference is consequently but gradual, 
except in the cases where a formation of resting spores completes 
the max. The causes which produce a periodicity in each 
species are partly inner causes, partly causes based upon physical 
and chemical conditions of the surrounding matter. We do not 
