160 BRITISH DESMIDIE^. 



but as they are frequently indistinct, or from various causes it may 

 be difficult to count them with certainty, I am unwilling to introduce 

 them into the specific character except in the absence of more per- 

 manent marks of distinction. 



The diaphanous vesicles, which are conspicuous in most of the 

 species, are either scattered or arranged, with more or less regularity, 

 in a single longitudinal series. 



At each extremity of the endochrome, even in its earliest state, 

 there is a large hyaline or straw-coloured globule, which contains 

 minute granules in constant motion. This globule disappears in the 

 dried specimen. 



A distinct circulation has been noticed in several species. It is 

 said to occur only in specimens obtained from water, and not in those 

 taken from moist ground. 



The empty frond is striated in some species and smooth in others, 

 but no instance of its being granulate is known. The striated species 

 are by Ehrenberg placed together in a subgenus [Toxotium), but it 

 is sometimes very difficult to detect the striae, which I have also 

 seen distinctly in Closterium attenuatum, and other species usually 

 considered smooth. 



The colour and firmness of the covering differ in different species. 

 Some are quite colourless, flexible, collapsing when dried, and in 

 general allowing the endochrome to escape by a merely partial sepa- 

 ration of the segments. These species are never striated. In other 

 species the fronds are more or less straw-coloured or even reddish, 

 probably from the presence of a small portion of silicate of iron. The 

 deeper the colour the firmer is the frond. The segments separate 

 entirely from each other and retain their shape when empty or dried ; 

 and some of the striated species, even when submitted to the action 

 of nitric acid or fire, retain their form and markings. In the coloured 

 species the extremities are generally darker than the rest of the 

 frond. 



Even in the firmest species the frond becomes flattened in drying, 

 its breadth at the centre increases, and the ends appear more atte- 

 nuated than in their living state. This fact should be attended to 

 when describing or drawing a dried specimen. 



In Closterium, of which several species have been noticed in a 

 conjugated state, the process appears to be nearly the same as in the 

 Conjugatai. Two fronds unite by means of projections arising at the 



