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W. B. Crow 
marks on mucilage-secretion, we are dealing with phenomena of 
great variability. Colony form, which plays such a conspicuous part 
in the ordinary systems of classification, depends on several factors, 
viz. (i) the arrangement of the cells$lue to their special mode of cell- 
division, (2) the consistency of the mucilage, and (3) direct effect of 
the environment. The factors never act independently but sometimes 
one may predominate. Thus direct action of the environment may 
determine the spherical form of the colony in some species of Micro¬ 
cystis and Ccelosphcerium (cf. cell-shape), for it is significant that 
practically no large terrestrial colonies (e.g. those of Aphanocapsa) 
are spherical. It is instructive to compare the Coccochloris section 
of Aphanothece with the Aphanothece section of the same genus. The 
former is distinguished by a globose stratum, the latter has an amor- ; 
phous one. The members of Coccochloris are mostly aquatic, whilst 
the majority belonging to the Aphanothece section are terrestrial. 
Environment also acts indirectly by affecting the consistency of 
the mucilage, and indeed the actual process of secretion. The influence 
of stimuli on mucilage formation is well illustrated by comparing 
cultures of the same species in two different media. Thus Acton (l) 
records the development of asymmetrical envelopes in Chroococcus 
turgidus (Kuetz.) Naeg. when grown on damp soil: the unilateral 
growth was, in some cases, found to form a thick mucilage stalk 
longer than the cell itself. When grown in water, however, the muci¬ 
lage investment is concentric in form. Similar cases have been 
observed in Chroothece. The extensive studies of Brunnthaler on 
Glceothece rupestris (Lyngb.) Born. ( 4 ) have shown that the amount 
of mucilage produced varies greatly in different media. 
Consistency of mucilage may also vary with the nature of the 
species. This is generally assumed to be the case, and size of colony 
and distance apart of the cells may be given as diagnostic characters, 
at least in the case of those genera like Coelosphcerium, Microcystis 
and Merismopedia where the colony has a somewhat definite shape. 
In nature, however, species of the two former may show single 
colonies in which the distance apart of the individual cells varies 
from several times the cell diameter to nearly nothing. Merismopedia 
elegans A. Br. may show a graded series in distance apart of the cells 
from one side of the colony to the other, although here the difference 
is not so pronounced. In Microcystis ceruginosa Kuetz. and M. 
ichthyoblabe Kuetz. all transitions are observed from compact colonies 
to conditions where the mucilage is so diffuse that the cells are no 
longer held together. 
