and the FI age llat a, 653 
investigations on the effect of varying external conditions on 
the form and function have made such rapid progress. 
Klebs (’ 96 ) was the pioneer in this as in many other 
important algological departures, and from his work it appeared 
that under certain definite conditions filamentous forms may 
break up into small aggregates or into single cells. 
To Klercker (’ 96 ) we owe an absolute proof of the truth of 
this, by observation of two species of the genus Stichococcus. 
Since the early investigations of Nageli (’ 49 ) a unicellular 
Alga, Stichococcus bacillar is, had been recognized, which had 
the shape and method of vegetative division of a large, thick 
Bacterium. It has, however, one clear green chloroplast 
attached to the side of the cell and leaving the two ends 
colourless. No zoospores, gametes, or resting-cells occur, 
so it clearly found a place in the Pleurococcaceae. A fila¬ 
mentous Alga with rather similar cell-contents had been 
variously named as Ulothrix subtilis or Hormidium subtile , 
and though nothing was known of its methods of repro¬ 
duction, its unbranched filamentous form entitled it to a place 
in the Ulothrichaceae. 
The similarity of the cytological structure suggested some 
affinity between these forms, and Klercker has succeeded in 
showing that each of them can exist in a filamentous state 
and a unicellular ‘coccoid’ state. He proves this by suc¬ 
cessive drawings under the microscope, showing the individual 
cells of the filament separating from one another and drifting 
apart with rounded ends to continue existence and multipli¬ 
cation in the coccoid state (Fig. 14 a and b). The cytological 
characters are retained unchanged through the transition, and 
these two species, N. subtilis and S. bacillaris , are clearly 
thus distinguishable. 
Normally these two species exist, one (S. bacillaris) as 
separate single cells, and the other ( 5 . subtilis) in the form 
of filaments, so that we have a genus with one species in one 
main group (Confervoideae), and the other in another (Proto- 
coccoideae). Nothing could better exemplify the unnatural¬ 
ness of the distinction. 
