94 CHLOROPHYCEAE 



branched aerial thallus is composed of flat, cordate or reniform 

 segments which are strongly calcified on the outside, the segments 

 being separated from each other by non-calcified constrictions. 

 The segments are composed of interwoven threads with lateral 

 branches that develop perpendicularly and produce a surface of 

 hexagonal facets through fusion of the swollen ends. Sporangia 

 develop at the ends of forked threads which vary greatly in their 

 mode of branching: these threads, which are cut off from the 

 parent thallus by basal plugs, arise from the surface of the segments 

 or, more frequently, are confined to the edges. The sporangia 

 produce biflagellate swarmers whose fate is not known although 

 they are probably gametes. 



Vaucheriaceae : Vaucheria (after J. P. Vaucher). Figs. 65, 66. 



This genus differs in many of its characters from the other 

 members of the Siphonales, and it should perhaps be removed into 

 the Xanthophyceae. Whereas most of the Siphonales are tropical 

 genera Vaucheria is essentially temperate, inhabiting well-aerated 

 streams, soil or saline mud flats, and although some of the species 

 (e.g. V. Debaryana) may be lime-encrusted it is never to quite the 

 same extent as in the preceding genera. There is a colourless basal 

 rhizoidal portion from which arise green, erect aerial filaments with 

 apical growth and monopodial branching. The cell walls contain 

 cellulose and pectins whilst the discoid chloroplasts, which lack 

 pyrenoids, contain more than the normal amount of xanthophyll. 

 Oil forms the principal food reserve, except that under constant illu- 

 mination starch is formed, and it is in these biochemical characters 

 that Vaucheria shows considerable similarity with members of the 

 Xanthophyceae (cf. p. 113). Septa are only formed in connexion 

 with the reproductive structures or after wounding. Vegetative 

 reproduction is secured through fragmentation, whilst asexual 

 reproduction is brought about by the well-known compound multi- 

 flagellate zoospores, which are produced singly in club-shaped 

 sporangia that are cut off from the ends of the erect aerial branches. 

 The chloroplasts and nuclei congregate in the apex of a filament 

 before the septum is laid down and the nuclei then arrange them- 

 selves peripherally. Finally, two equal flagellae develop opposite 

 each nucleus and then the zoospore is ready for liberation, a process 

 which is achieved by gelatinization of the sporangium tip. This 



