270 pediastre;e : — general characters. 



organisms may be separated from the water ; the pieces of muslin 

 may be brought home folded-up in wide-mouthed bottles, either 

 separately, or several in one, according as the organisms are 

 obtained from one or from several waters ; and they are then to be 

 opened-out in jars of filtered river- water, and exposed to the light, 

 when the Desmidiacea? will detach themselves. 



209. Pediastrece. — The members of this family were formerly 

 included in the preceding group ; but, though doubtless related 

 to the true Desmidiacece in certain particulars, they present too 

 many points of difference to be properly associated with them. 

 Their chief point of resemblance consists in the firmness of the 

 outer casing, and in the frequent interruption of its margin either 

 by the protrusion of 'horns' (Fig. 116, a), or by a notching more 

 or less deep (Fig. 117, b) ; but they differ in these two important 

 particulars, that the cells are not made up of two symmetrical 

 halves, and that they are always found in aggregation, which is 

 not — except in such genera as Scenodesmus (Arthrodesmus, Ehr.) 

 which connect this group with the preceding — in linear series, 

 but in the form of discoidal fronds. In this tribe we meet with a 

 form of multiplication by Zoospores aggregated into Macro-gonidia,* 

 which reminds us of the formation of the motile spheres of Vohox 

 (§ 196), and which takes place in such a manner that the resultant 

 product may vary greatly in number of its cells, and consequently 

 both in size and in form. Thus in Pediastrum granulatum (Fig. 

 116), the zoospores formed by the subdivision of the endochrome 

 of one cell into gonidia, which may be 4, 8, 16, 32, or 64 in 

 number, escape from the parent-frond still enclosed in the inner 

 tunic of the cell : and it is within this that they develope them- 

 selves into a cluster resembling that in which they originated, so 

 that whilst the frond normally consists of 16 cells, it may be 

 composed of either of the just-mentioned multiples or sub- 

 multiples of that number. At a is seen an old disk, of irregular 

 shape, nearly emptied by the emission of its macro-gonidia, which 

 had been seen to take-place witbin a few hours previously from 

 the cells a, 6, c, d, e ; most of the empty cells exhibit the cross 

 slit through which their contents had been discharged ; and where 

 this does not present itself on the side next the observer, it occurs 

 on the other. Three of the cells still possess their coloured contents, 

 but in different conditions. One of them exhibits an early stage 

 of the subdivision of the endochrome, namely into two halves, one 

 of which already appears halved again. Two others are filled by 

 sixteen very closely-crowded gonidia, only half of which are visible, 

 as they form a double layer. Besides these, one cell is in the very 

 act of discharging its gonidia ; nine of which have passed forth 

 from its cavity, though still enveloped in a vesicle formed by the 



* Solitary zoospores or micfo-gonidia have been observed by Braun 

 to make their way out and swim away ; but their subsequent history is 

 unknown. 



