36 
S. L. Ghose. 
The filament. The filaments measure up to one and a 
quarter m.m. in length, the matureones being curved in a more or 
less semi-circular manner. Occasionally pseudo-branches are given 
off, either singly at the base of an heterocyst (Fig. 3) as in Tolypothrix, 
or rarely in pairs—generally in old filaments—-between 
two heterocysts (Fig. 4) as in Scytonema. Trichomes 6-9 /a thick, 
bluish-green, slightly constricted at the joints. Cells 5-1 1/a long, 
with coarsely granular contents. Dissepiments are generally not 
very distinct in mature filaments, but are easily seen in hormogones 
or young filaments. 
The sheath. The sheath is at first thin and hyaline, but 
later on becomes thick, often lamellose and yellowish-brown, up 
to one micromillimeter thick. It is very inconspicuous in filaments 
or portions of filaments embedded in mud, but in exposed filaments 
or parts of filaments it is brown, firm, thick and tightly adhering. 
Heterocysts. One heterocyst is, as a rule, found in the middle 
of the filament (Fig. 2). Sometimes, however, two heterocysts are 
found side by side at this place (Fig. 5). Other heterocysts 
are situated at intervals through the whole length of the filament 
(Fig. 3). The heterocysts are at first pale yellow in colour and 
have some finely granular contents. Later on they lose these 
contents and become hyaline. The refractive granules situated 
adjacent to the pores at each pole, as mentioned by West (4) and 
Fritsch (1), are quite easily seen in most cases (Fig. 2). The 
heterocysts are 12-21 /a long, 7-9/a broad, and rectangular or 
ellipsoid in shape. 
