106 PRINGSHEIM, ON THE 



micro-spores are united into a new family. These micro- 

 spores are distinguished by their Avant of movement from the 

 chronizoospores of Hydrodictyon in general, in Caelastrum 

 the movement of the spores is retrograde and the largest often 

 form a network, without having previously given any signs of 

 spontaneous motion. 



The other genera, which appear to bear some resemblance 

 to the preceding, have at present been too little studied even 

 to indicate the organs that might, in them perhaps, corre- 

 spond to the chronizoospores ; but it is evident that these 

 genera, tlie generations of which, so far as is hitherto known, 

 are always associated in families which possess other repro- 

 ductive bodies which have doubtless an isolated existence 

 after the fashion of the polyhedrons of Hydrodictyon and 

 under analogous forms, may we not then ask whether those 

 observers Avho have devoted themselves to the study of 

 microscopic algaj, may not have seen and described these 

 polyhedral bodies whose characteristic form could not but 

 attract their attention. 



And this really appears to be the case. The jjolyhedrons 

 of Hydrodictyon have, in fact, never, so far as I know, been 

 described by any one ; but 1 tliink I recognise the jjolyhe- 

 drons of other algce of the same family, and especially, perhaps, 

 those of the genus Pediastrum, in ths remarkable bodies 

 which jNI. Isageli has taken as a type of a new genus of al[ja> 

 and termed by him polyhedr'mm. It is in allusion to this 

 denomination that I have given the name of polyhedron to 

 tlie isolated and enlarged zoospores oi Hydrodictyon. 



iSTothing is known of any of the polyhedron of M. Nageii 

 beyond the external form, it therefore appears to me that 

 they are most probably, merely the first isolated generations 

 of difierent kinds of algae belonging to the group of Hydro- 

 d/tctyece. 



In the same manner, the solitary and unicellular forms 

 that ]M. Braun has described and figured iu some species of 

 Pediastnun, may be regarded as the polyhedrons of this same 

 species, or of others analogous to them. I might say the 

 same of the Asteriscium caudatum of Corda ('Alman. de 

 Carlsbad,' p. 238, pi. i, figs. 1, 2) . 



The spores of Hydrodictyon which remain free and became 

 dispersed, and of which I believe my observations have shown 

 the true morphological interpretation, appear from the facts 

 already stated, to belong to a certain order of a sexual gene- 

 ration the products of which, remarkable by their inter- 

 mittent vegetation, may be classed in two distinct groups. 

 The first group will comprise the chroni-zoospores, such as 



