On the Retention of Vitality by Algce. 101 
intermediate between the two can be observed, showing beyond 
any possibility of doubt that the two are only forms of the same 
species, and not even different varieties. 
Nostoc Passerinianum Bornet et Thuret. 
The Nostoc that has been been identified as this species 
appeared only in the 1856 Broadbalk sample, so that it had ger¬ 
minated after a period of about sixty years. Unfortunately, 
probably owing to cultural conditions, no heterocysts were pro¬ 
duced so that full particulars could not be obtained. The size of 
the vegetative-cells and of the spores agrees very well with 
Lemmermann’s description, while the almost parallel arrangement 
of the filaments makes it very probable that this identification is 
the correct one. It is very conspicuous that the long parallel 
strands of filaments often undergo a slight spiral twist, and the 
long rows of spores with their yellowish-brown walls often assume 
the appearance of the strands of a rope that has been frayed out 
in places. 
Nostoc Sp. ? Juvenile form. 
In the cultures of the three Broadbalk samples, 1881, 1865 
and 1856, a blue-green alga has been observed that is certainly 
a Nostoc, but the characters have never become sufficiently distinct 
for any identification of the species to be made. The cells con¬ 
stitute filaments that are twisted together in close irregular spirals 
to form irregularly lobed or cylindrical colonies invested with a 
close gelatinous covering which gradually assumes a yellowish 
colour. As the colonies become larger the spiral arrangement 
of the filaments becomes less distinct until finally the colony 
appears to consist of an irregular mass of closely packed cells. 
In the young colonies the cells are usually barrel-shaped, being 
4-6/x in diameter and rather shorter than broad, but in the older 
colonies they become more globose and are sometimes even angular 
by compression. Heterocysts are exceedingly rare in these 
colonies, but when found they are spherical and about 5/z in 
diameter. Very occasionally the cells become converted into 
spores with yellowish-brown shiny walls ; they are only slightly 
bigger than the vegetative cells. 
It is probable that these colonies are only juvenile forms of 
some well-known species, which owing to cultural conditions have 
retained their juvenile characters for a considerably longer period 
than would be the case under quite normal conditions; and the 
