VARIATION IN THE DESMIDIER. 379 
probably never exactly equal) that section of the cell-wall which 
previously enclosed a median portion of the cell no longer does 
so, and in this way, in the course of many divisions, a short 
eylindrical portion of the cell-wall, which once oecupied a central 
position in the cell, may become shifted about, first towards one 
end and then towards the other. Thus the widest part of the 
cell may be at any point between the two ends. 
In some specimens the apices are considerably dilated, in others 
nof at all and all stages are met with intermediate between 
these extremes. 
The striolation of the eell-wall is the most variable character 
of the species, the striolations being coarse, fine, or broken up 
into series of dots even in different individuals from the same 
gathering, thus exhibiting the main characters of the three forms 
described by Turner as Penium spirostriolatum, P. scandinavicum, 
and P. Royanum. The number seen at one time across the cell 
varies from about 8 to 13, and this variation may be seen on one 
individual, the striolations being more crowded at some parts of 
the cell-wall than at others. In one individual two striations 
were observed to be 2:3 u apart, and the distance between one of 
these and the next one was 52 y. They are generally arranged 
round the cell in a spiral manner from apex to apex, and may 
make about 13 turns in the whole length of the cell; but in the 
majority of speeimens they are much straighter than this, and in 
some are longitudinal although not quite straight. They are not 
always eontinuous from end to end, but often run only part of 
the distance and then fade away or join with a neighbouring 
striolation. In many cases they are very irregular (Pl. 8. 
figs. 2, 4, 6, 9, 12), and a reticulation is often present joining 
together several, or all of them. This reticulation may not be 
very marked (Pl. 8. fig. 2), or it may be concentrated towards 
the middle of the cell (Pl. 8. fig. 3). In many examples there is 
a marked reticulation at the end of the cell (Pl. 8. fig. 8), and 
sometimes a reticulated zone is present just below the apex 
(Pl. 8. fig. 10). Most specimens have a distinct convex apical 
eap which is strongly punctate, the punctulations being con- 
tinuous with those between the striolations. All the specimens 
Ihave examined were punctate between the striolations, some 
strongly and others faintly ; but I have seen no specimen without 
these punctulations, however minute they may have been, I 
LINN. JOURN.—BOTANY, VOL. XXXIV. 2D 
