EUPHYCOPHYTA 87 



to form akinetes in which the walls are thickened and food is 

 stored. 



In the section Aegagropila most of the species reproduce vegeta- 

 tively, but biflagellate s warmers have been reported for one species, 

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

 whilst still within the sporangiimi (Fig. 46). Asexual reproduction 

 in the other species, excluding the section Aegagropila, is by means 

 of quadriflagellate zoospores (biflagellate in two species) which 

 escape through a small pore in the cell wall. Biflagellate isogametes 

 are the means of sexual reproduction, all the species so far investi- 

 gated being dioecious. The zygote develops at once without a 

 resting period. In a number of species alternation of two morpho- 

 logically identical haploid and diploid generations has now been 

 estabUshed with meiosis taking place at zoospore formation. In 

 one or two cases, e.g. Cladophora flavescens, the zoospores some- 

 times fuse, and this irregular behaviour is very comparable to 

 similar phenomena found in the more primitive brown algae 

 (cf p. 130). 



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 Ufe cycle is known, and this difference may perhaps be 

 compared with the various cycles found for Ectocarpus siliculosus 

 under different conditions (cf. p. 131). 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 

 probably indicates polyploidy. The following diploid chromosome 

 numbers have been recorded: C. repens 8 + 1, C. suhriana 12 + 1, 

 C flavescens 24, C. flaccida 24, C. pellucida 32, C. glomerata 92 + 4 

 (octoploid). 



The subgenus Spongomorpha, in which the threads are held 

 loosely together by rhizoids, but not so compactly as in Aegagropila, 

 will have to be regarded as a separate genus, because recent work 



