378 MR. G. S. WEST ON 
speeimens obtained from many parts of the British Isles and the 
United States, and exhibit many points in the morphological 
structure of the plant that were previously unknown, as well as 
proving conclusively that the above-mentioned species are merely 
forms of one plant. 
This rare species of Penium generally frequents the few Des- 
mid-bearing places in the bogs of mountainous countries, and is 
remarkable among all other Desmids as possessing a considerable 
number of transverse sutures. Each transverse suture observed 
on the cell-wall of a Desmid represents a position at which cell- 
division has taken place, and as I have observed as many as 
sixteen distinct transverse sutures in one individual of this 
species (long. 242 a, lat. 254) from Oughtershaw Tarn, W. 
Yorks., distributed at intervals from end to end, it follows that 
this plant had undergone cell-division at sixteen different points 
along its length. This is noteworthy, as in the vast majority of 
Desmids eell-division ean take place only at one point (the 
isthmus) situated in a median position between the two ends *. 
There is a wide range of variation in the comparative 
length and breadth of this species as well as in outward form. 
One of the shortest specimens (long. 123 p, lat. 24 u) was only 
5:1 times longer than broad, and one of the longest forms (long. 
274 u, lat. 28.) was 11:9 times longer than broad, being over 
twice as long as the former. Two specimens of the same breadth, 
26 u, had lengths of 179 u and 224, respectively. This large 
range of variation in comparative length and breadth is owing to 
the inequality in the positions of the sutures, for if division were 
to take place at a suture near to one end of the cell, and the 
newly developed halves ultimately became of approximately the 
same proportions as the corresponding parent halves, then a long 
and a short individual would be produced. The irregularity in 
the position of the sutures also accounts for the marked varia- 
tions in outward form, because if the newly developed half be 
not exacily equal in length to the correspondent parent half (and 
if division takes place at a suture near one end these halves are 
* In some species of Closterium there are two, or even three, transverse 
sutures, and cell-division can take place at three points along the cell. I have 
seen as many as 2l transverse lines at one of these points in a specimen of 
Closterium striolatum from West Ireland; yet, in this specimen, cell-division 
could take place at only two pointe distant about one third the length of the 
cell from each apex., 
