96 Chlorophi/cece 



arranged with their long axes at right angles to the plane of the 

 thallus (fig. 35 D). Each cell contains a single nucleus and one 

 large parietal chloroplast, often with deeply incised or lobed margins 

 and containing a single pyrenoid. In Monostroma bullosa (Roth) 

 Wittr. the cells are generally arranged in T-shaped groups of four. 



Asexual reproduction takes place in Ulva by zoogonidia (some- 

 times termed megazoospores) with four cilia. These come to rest 

 and germinate directly. Monostroma has been observed to re- 

 produce itself asexually by budding off small flat portions from 

 the surface of the thallus, each portion producing a new plant. 

 Geddes 1 has observed a process of gemmation in Enteromorpha. 



Sexual reproduction is brought about by the conjugation of 

 isogamous gametes. Ordinary cells of the thallus become game- 

 tangia and give rise to eight (sometimes four or sixteen) piano- 

 gametes, which are pear-shaped bodies, smaller than the zoogonidia, 

 with a pigment spot and two long cilia. On conjugation the two 

 gametes coalesce slowly and a " zygozoospore," or a rounded cell 

 with two pigment spots and four cilia, is first formed ; this loses 

 its cilia and becomes a zygospore (fig. 35 J). The zygospore 

 usually germinates directly, first forming a short filament of four 

 cells, which soon produce a flat expansion by dividing in two 

 directions in the same plane. According to Reinke the zygospore 

 sometimes becomes a resting-spore or hypnocyst, which on germi- 

 nation divides into four and then eight cells arranged peripherally 

 round a central cavity. By the increase of these peripheral cells 

 a vesicular thallus is produced, which in most instances ultimately 

 becomes a flattened expansion attached by a few rhizoids at 

 its base. 



Genus Monostroma Thur., 1854. The thallus in the adult 

 plant is always a thin membranaceous plate. In its younger 

 stages it is frequently vesiculose, opening out as it grows into a 

 foliaceous plate, finally becoming free-floating. It consists of a 

 single layer of rounded or more or less angular cells which are 

 often disposed in groups of four. The zoogonidia possess either 

 two or four cilia, and the gametes are biciliated and rather smaller 

 in size. 



Few species of the genus inhabit fresh water, the only British representa- 

 tives being M. bullosa (Koth) Wittr. and M. membranacea West & G. S. West 



1 Geddes in Trans. Roy. Soc. Edinburgh, 1881, p. 555. 



