374 
Spratt. — Some Observations on the 
heterocysts are detached (PL XXXII, Fig. 13). This development is in har- 
mony with that described by Fritsch for some other Anabaenct sp., but differs 
somewhat from Brand’s account of Nostoc commune and Tolypothrix sp., 
since he only mentions granules in connexion with fully formed heterocysts, 
and does not describe the transition from the lip-like thickenings to a plate. 
Intercalary heterocysts usually occur singly, but in old filaments the 
vegetative cell on either side of a heterocyst may be thus differentiated, in 
which case the filament breaks and liberates the old one (Fig. 14). A dark 
green intercellular substance is sometimes excreted between the two adjacent 
cells (Fig. 1 5,#), which assumes a flat, bi-concave shape, and loses its colour, 
while the two cells are transformed into heterocysts (Fig. 15, b) ; then the 
filament breaks at that point (Fig. 15, c). This phenomenon was observed 
in Andbaena obtained from old nodules, and also in hanging-drop cultures 
about a fortnight old. Brand describes a similar formation in connexion 
with the production of pseudo-branches and heterocysts in Tolypothrix and 
in Nostoc commune during the disjointing of heterocysts in old cultures. 
The heterocysts are stained bright green by methyl green, while only the 
central granules of the vegetative cells are thus affected. Their contents are 
also stained by aniline gentian violet, haematoxylin, fuchsin, and orange G. 
Many suggestions have been put forward with regard to the functions 
of the heterocysts. Some of the earlier observers, amongst whom are 
Borzi, Hansgirg, and Kirchner, regarded them as subserving vegetative 
reproduction, and in some cases, as described above, their formation is con- 
nected with the breaking up of a filament, and the consequent formation of 
hormogonia. Hieronymus and Hegler interpreted them as receptacles for 
the storage of reserve substances. Brand described the liberation of their 
contents as gonidia, capable of producing new filaments in Nostoc com- 
mune and microscopicum ; he also ascribed to them a storage function, and 
considered their contents as replenishing the exhausted adjoining cells. 
Fritsch points out that their differentiation into terminal and intercalary, as 
well as their occurrence in numbers side by side, compel us to attribute to 
them some function other than that of limiting the filaments, although this 
may be part of their work. He also regards them as recipients of reserve 
food material, serving for its storage under certain conditions. The fact that in 
Anabaena they only occur after some vegetative cells, capable of assimilat- 
ing food material, and producing structures suitable for storing the surplus 
organic material manufactured, have been formed, together with the varia- 
tion in their cell contents at different periods, suggests that this is a probable 
function. If, however, these are the only functions, what is the meaning of 
detached heterocysts, which certainly occur under both natural and artificial 
conditions? Brand’s observations on the germination of their contents 
hitherto stand isolated, but this would account for their occurrence de- 
tached, and would also be an important stage in the life-history of 
