SIPHONOCLADIALES 77 



have been recognized in the Aegagropila forms: [a) rhizoids, 

 {h) cirrhoids, both these and the rhizoids being neutral or non- 

 reproductive branches, (c) stolons or vegetative reproductive 

 branches. Many of the species of Cladophora are perennial, and in 

 the section Spongomorpha the rhizoids form a basal expanse from 

 which new threads may arise each year. In some of the fresh- 

 water species certain cells may become swollen to form akinetes in 

 which the walls are thickened and food is stored. 



In the section Aegagropila most of the species reproduce vege- 

 tatively, but biflagellate swarmers have been reported for one 

 species, Ae. Sauteri, and these are interesting in that they may 

 germinate whilst still within the sporangium (fig. 52 L, M). 

 Asexual reproduction in the other species, excluding the section 

 Aegagropila, is by means of quadriflagellate zoospores (bi- 

 flagellate in two species) which escape through a small pore in the 

 cell wall. Biflagellate isogametes are the means of sexual repro- 

 duction, all the species so far investigated being dioecious. The 

 zygote develops at once without a resting period. In a number of 

 species alternation of two morphologically identical haploid and 

 diploid generations has now been established with meiosis taking 

 place at zoospore formation. In one or two cases, e.g. Cladophora 

 fiavescens, the zoospores sometimes fuse, and this irregular be- 

 haviour is very comparable to similar phenomena found in the 

 more primitive brown algae (cf. p. 138). 



In a few species there is an odd or heterochromosome, and in 

 a cell the number of zoospores with the odd chromosome are equal 

 to the number lacking it. Haploid plants of C. Suhriana have six 

 or seven chromosomes, whilst in C. repens the cells contain either 

 four or five. In a fresh-water species, C glomerata, a wholly 

 different type of life cycle is known, and this difference may perhaps 

 be compared with the various cycles found for Ectocarpus siliculosiis 

 under different conditions (cf. p. 135). Gametes and zoospores are 

 both formed on diploid plants and meiosis takes place at gamete 

 formation so that there is no haploid generation. Whilst zoospore 

 formation takes place all the year round gametes only appear in 

 the spring, but the reason for this seasonal restriction is not under- 

 stood. Parthenogenetic development of gametes has also been 

 recorded in a number of species. Of the species so far investigated 

 the chromosomes appear to be present in multiples of 4, and this 



