ULOTRICHALES 51 



multinucleate rhizoids grow down from the lower cells and a basal 

 attachment disk is formed which may persist throughout the 

 winter, new plants arising from it in the spring. Detached frag- 

 ments are another frequent means of forming new thalli, whilst 

 normal asexual reproduction is by means of quadriflagellate zoo- 

 spores. In sexual reproduction, which occurs in plants other than 

 those producing zoospores, fusion takes place between isogametes 

 from separate plants which have been described as + and - . The 

 gametes may fuse in pairs or they may fuse into "clumps", and 

 whilst they are positively phototactic before fusion, the zygote is 

 negatively phototactic, and this change in behaviour causes it to 

 descend on to a suitable substrate. Hartmann (1929) has shown 

 that in certain cases there may be relative sexuality among gametes 

 from different plants, the sex of the older and weaker gametes 

 becoming changed. Meiosis takes place at zoospore formation and 

 there is a regular alternation of diploid and haploid generations, 

 both indistinguishable morphologically, and when this life history 

 is compared with that of Monostroma the essential differences are 

 immediately apparent (cf. fig. 36). The plants occur in saline or 

 fresh water and become particularly abundant when the waters are 

 polluted by organic matter or sewage. 



*Ulvaceae: Enteromorpha {entero, entrail; morpha, form). Figs. 36, 

 39- 



The plants of this genus also commence hfe as uniseriate filaments 

 which soon become multiseriate and tubular. Like Uha, many of 

 the species are attached by means of rhizoids, but there are also a 

 number of forms, especially on salt marshes (cf. p. 330), which are 

 free-floating for the whole or part of their life cycle. Growth of the 

 thallus is either intercalary or else through the divisions of an 

 apical cell. Asexual reproduction is by means of zoospores, and as 

 meiosis takes place at their formation the life cycle is identical with 

 that of Ulva because morphologically similar haploid plants are 

 known. The first division of the germinating zoospore is transverse, 

 the lower segment forming an embryonic rhizoid. The sexual 

 haploid plants are dioecious, usually with isogamous reproduction, 

 the gametes commonly being liberated around daybreak. Aniso- 

 gamy has been found by KyHn (1930) in E. intestinalis where the 

 male gamete is small with but a rudimentary pyrenoid. The motile 



