Chaetomorpheae 265 



change of external form, simply by an increase in the number of chloroplasts 

 and in the food-reserves, more particularly starch. 



That the genus Pithophora is closely allied to Cladophora there is no doubt ; in fact, 

 the relationship is so close that it is most undesirable to recognize the genus as the type 

 of a distinct family the Pithophoracese as proposed by Wittrock. The curious resting- 

 spores of Pithophora, somewhat erroneously termed ' akinetes ' by Wille and others, are 

 probably the outcome of a suppression of the formation of zoogonidia. The diameter of 

 the filaments varies from 40 p. in the thinnest branches of P. kewensis to 190 p, in the 

 thickest branches of P. Roettleri. The genus is a freshwater one and almost exclusively 

 confined to the tropics. The plants are free-floating and at no period of their existence 

 attached, except in the terrestrial forms of P. Cleveana. 



The genera of the Cladophorese are : Cladophora Kiitzing, 1843 [inclus. JEgagropila 

 Kiitzing, 1849] ; Acrosiphonia (J. Agardh) Wille, 1909 ; Pithophora Wittrock, 1877. The 

 freshwater species are very variable and present many growth-stages of a transitory 

 character in which the specific characters are unrecognizable. In some species of the 

 genus Cladophora the segments attain a diameter of 250 /z. 



Sub-family CH^ETOMORPHE^E. In this small group the filamentous thallus 

 is without branches, except for the short unseptate protuberances which 

 sometimes grow out from the segments of Urospora. The filaments are of a 

 more or less uniform thickness except at the base and apex. The basal 

 segment may possess annular constrictions and has a very thick wall ; it is 

 usually elongated and develops a strong hold-fast which firmly fixes the 

 filament to the substratum (fig. 170 1 and 2). The hold-fast may consist of 

 very irregular and much branched rhizoids, which sometimes contain a great 

 deal of stored starch. In Urospora there may be a more or less large disc 

 formed by numerous intertwined rhizoids which spring from quite a number 

 of the segments in the basal part of the filament and grow downwards along 

 the sides of the shoot ; and in U. mirabilis Aresch. there are intracuticular as 

 well as extracellular rhizoids. 



In most forms there is a gradual attenuation near the apex of the filament, 

 so that the apical segments are considerably narrower than those in the 

 median part of the filament ; they are also more elongated. The median 

 segments are thick-walled and somewhat tumid, attaining a diameter of 500 /A 

 in Chwtomorpha crassa (Ag.) Klitz. The chloroplasts of the adult coenocytes 

 are reticulated parietal plates with many pyrenoids, and closely resemble 

 those possessed by various species of Cladophora. 



Reproduction takes place by comparatively small zoogonidia, which are 

 biciliated in Chwtomorpha and quadriciliated in Urospora. In the latter 

 genus they are also more elongated and seen from the front are four-angled ; 

 they are also capable of slight changes in external form. Large numbers of 

 zoogonidia are formed in each ccenocyte and are usually set free through a 

 lateral aperture formed by the dissolution of the wall at that point (fig. 170 4)- 



