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culi, of a pea-green colour, which are most frequently seen 
united to one another in larger bundles. This union of the pri- 
mary fasciculi into secondary ones is not permanent, and under 
certain circumstances very imperfectly understood, but, in some 
cases, depending perhaps on meteorological conditions, the se- 
condary fasciculi become broken up into primary ones, or, at 
least, into less complicated bundles, and the plant, which had 
previously lain upon the surface of the pond in an extensive 
stratum, becomes nearly uniformly diffused through the water. 
A return of the former conditions will again cause the union 
of the simpler fasciculi into more complex ones ; and the re- 
accumulation of the plant in masses on the surface. 
The primary fasciculi are composed of straight filaments, 
which are about saath of an inch in diameter, and possess the 
three kinds of cells characteristic of the Nostochinex, namely, 
the ordinary cells, the heterocysts, and the sporangia. 
The ordinary cells vary much in length in different fila- 
ments, and even in the same filament, and not unfrequently 
they present evident transverse striz, which doubtless indicate 
the commencement of division ; the endochrome is in the form 
of minute oval or irregular masses. Under the action of iodine 
the contents of the cells assume a dark-brown colour, and se- 
parating from the walls contract towards the centre of the cell, 
where they appear bounded by a very definite outline (primor- 
dial utricle). The entire filament appears to be surrounded by 
an indistinct gelatinous (?) sheath. 
When the aphanizomenon first showed itself in the pond, 
the heterocysts were abundant ; but no sporangia could be de- 
tected. The heterocysts are in the form of short cylinders 
with rounded extremities, and with bluish-green contents, which 
scarcely ever present any trace of granular structure. Under 
the action of iodine the following structures may be seen in the 
heterocyst :—1. The endochrome contracted towards the cen- 
tre of the cell, and presenting a well-defined boundary. 2. 
External to this, a delicate cell-wall separated from the con- 
