VARIATION IN THE DESMIDIEX. 381 
than usual, but the apices were proportionately broader and more 
inflated than is generally the case in this species. The Yorkshire 
specimens had no trace of an inflation at the apices, and the 
cell-wall was almost colourless, this being a common feature of 
mountain specimens of Closterium striolatum. It might be 
inferred from the preceding table that the shortest and broadest 
forms possess the fewest striations (only 13 in 20 u), but that 
this is not so is shown by the fact that equally short specimens 
(breadth : length —1 : 1:75) from Cam Fell, W. Yorks., possessed 
17 striations in 20 4. In a large series of specimens a con- 
siderable range of curvature is found, from the ordinary regu- 
larly eurved examples to individuals which are almost straight 
(and generally short) Some of the longer forms have the 
central portion of the cell straight and apices considerably 
eurved ; these forms have been named var. orthonotum, Roy (in 
Journ. Bot. xxviii. 1890, p. 336). 
Towards the attenuated ends of the cell the striolations become 
fewer in number, and this reduction may be caused in two ways: 
either by the gradual fading out of a few of the striolations, or by 
the fusion of some of them before they reach the apex (fig. 3). 
This fusion takes place very gradually (fig. 3, a), or, more rarely, 
suddenly (fig. 3,5). The cell-wall between the striolations is 
very minutely punctulate, the punctulations being marked in 
some instances, but almost inappreciable in others. 
Fig. 3. 
ihe 
Part of the cell-wall of three specimens of Closterium striolatum, Ehrenb. 
From Frensham, Surrey. x 1280. 
The average British specimens of this species have a gradual 
2n2 
