106 
B. Muriel Bristol. 
single or more rarely in pairs on any side of the filament, though 
they are much more frequent on one side than on the other. The 
sheath is very definite and, though colourless at first, gradually 
assumes a deep reddish-brown colour; the breadth of the main 
filaments varies from 11-13/x and that of the branches from 7-13/x 
Towards the ends of the branches the sheaths are seen to be 
constricted at more or less regular intervals, and exhibit also a 
few indistinct longitudinal striations ; they are much thicker than 
those of the main filaments and rather less definite in outline. 
The trichomes do not always completely fill the cavity of the 
sheath, especially in some of the older filaments; the cells are 
usually elliptical-depressed in the main filaments, and are 9-11/x 
broad by 3’5-6 /a long. The cells of the branches are usually 
somewhat less depressed, and may occasionally become much 
distended and bead-like, or more rarely very narrow and elong¬ 
ated ; they are usually 5-6‘5 /a broad and 3-7/x long. Heterocysts 
have not been found; but in certain of the filaments the cells were 
seen to have become somewhat enlarged and to have acquired 
thick cell-walls in addition to the sheath, and it is probable that 
this represents some stage in the formation of spores. 
In Borzi’s species the filaments are 6-8/u, broad, -and no 
mention is made of the colour of the sheaths, while the species 
has been described up to the present only from standing or from 
running water. The alga found in the cultures thus differs in three 
quite noticeable particulars from Borzi’s species, but with the 
information obtainable it is not possible to do more than suggest 
that it may be a terrestrial form of the same species. 
Phormidium tenue (Menegh.) Gomont. 
This species has been found only in the 1868 sample of the 
Broadbalk soil, though it frequently occurs iu cultures of recently 
dried soils. It is probable therefore, since the 1868 sample possessed 
the highest water-content of all the stored samples, that this 
species is more affected by the degree of dryness of the soil than 
are the other species. The form found in the cultures exactly 
resembles the typical form. 
Summary. 
Prom cultural experiments with old stored soils it is found that 
certain algae belonging to the groups Myxophycecc, Bacillariece and 
Qhlorophyceit possess extraordinary powers of retaining their vitality 
