58 THE ALGAE 



the divisions of an apical cell. In some species the plastids are said 

 to exhibit polarity, occurring normally in the apical portion of the 

 cell. Such polarity is apparently absent in Ulva and Monostroma. 

 When the tubular thallus is damaged near the apex the wounded 

 cells put out papillate outgrowths, but if the damage occurs near 

 the base rhizoids are produced. Asexual reproduction is by means 

 of zoospores, and as meiosis normally takes place at their formation 

 the Ufe cycle in these species is identical with that of Ulva, because 

 morphologically similar haploid plants are known. The first divi- 

 sion of the germinating zoospores is transverse, the lower segment 

 forming an embryonic rhizoid. 



The sexual haploid plants are dioecious, usually with isogamous 

 reproduction, the gametes commonly being Hberated around day- 

 break. Anisogamy occurs in E. intestinalis where the male gamete 

 is small with but a rudimentary pyrenoid. The gametes may remain 

 motile for twenty-four hours, whilst the zygote may retain motiUty 

 for an hour, although the first division does not take place usually 

 until after several days' dormancy. In E. procera and E. nana 

 gametes are not known and the asexual swarmers are called neutro- 

 spores because meiosis does not occur prior to their formation. 

 These two species are presumably diploid and the haploid genera- 

 tion has been lost (cf. Ulva linza). Parthenogenetic development of 

 gametes has been recorded for some species, e.g. E. clathrata, and 

 this presumably results in new sexual plants. In the reproductive 

 areas some of the cells fail to divide. These are termed rest cells 

 and later they may germinate and give rise to sporelings which 

 eventually become separated from the parent thallus. 



In the majority of species there appears to be a regular alterna- 

 tion of morphologically similar generations. In E. prolifera, how- 

 ever, there is alternation between heteromorphic generations, one 

 being simple and the other much branched. Branching may not 

 always be a good character because it appears to be dependant to 

 some extent upon temperature and degree of salinity. Within the 

 genus there are several lines of development including two probable 

 lines of reduction in size. One such reduction series is represented 

 by the sequence E. clathrata — E. ralfsii — E, salina — E. per- 

 cursa, whilst E. torta may be a reduction form from E. prolifera. 

 In the new genus Feldmannodora the individual cells are arranged 

 in rows and are capsulated, groups of cells coming from a common 

 parent cell. 



