278 Ulvales 



budded off growing into new plants. In Ulva new thalli can apparently be 

 formed from ' multinucleate cells ' which may be cut off from the swollen 

 ends of the disc-filaments. 



Asexual reproduction takes place by zoogonidia in Ulva and Monostroma. 

 In the latter genus they may have either two or four cilia, but in Ulva they 

 are always quadriciliated. On coming to rest they germinate at once. 



Gamogenesis of isoplanogametes is known to occur in Ulva, Monostroma 

 and Enteromorpha. Ordinary vegetative thallus-cells become gametangia 

 without change of form and give rise to eight (sometimes four or sixteen) 

 ovoid planogametes, which are biciliated and of smaller size than the 

 zoogonidia. On conjugation the gametes slowly coalesce and a ' zygozoospore ' 

 with two pigment-spots and four cilia is first formed ; this soon comes to rest, 

 loses its cilia, and becomes a zygospore (fig. 177 E). The zygospore usually 

 germinates at once, the young plants differing much in the various genera. 

 In Ulva a short filament of four cells is first formed, which soon produces a 

 flat expansion. In Monostroma bullosa the zygote divides to form a more or 

 less globular mass of cells, which after enlargement to form a hollow sphere 

 eventually splits, thus giving rise to an irregular foliaceous expansion. In 

 other species of Monostroma the embryonic plant is more irregular. Reinke 

 has found that the zygote may become a resting hypnocyst, which on 

 germination divides into four and then eight cells arranged around a central 

 cavity. By the increase of these peripheral cells a vesicular thallus is 

 produced, which ultimately becomes a flattened expansion attached by a few 

 rhizoids at its base. 



The genera are : Enteromorpha Harvey, 1849 ; Letterstedtia Areschoug, 1850 ; Ulva 

 (L.) Wittrock, 1866 ; Monostroma (Thuret) Wittrock, 1866 ; Ilea J. G. Agardh, 1883. 



Enteromorpha and Ulva are amongst the first seaweeds to attract attention on the 

 sea-shore. They are essentially littoral and sublittoral, occurring mostly from about half- 

 tide level downwards, or sometimes in the vicinity of high-water mark. Various species 

 of Enteromorpha are the principal constituents of the ' grass ' which is so largely the cause 

 of the fouling of the bottoms of ships. The flat ' fronds ' of Ulva are annual, but it seems 

 probable that the adhesive discs are perennial. 



The Ulvacese are more especially marine, but there are several freshwater species of 

 Monostroma, such as M. bullosa (Roth.) Wittr., M. membranacea W. & G. S. West 

 ('03 ; fig. 177 CE), and M. expansa G. S. West ('06), the latter being an Australian 

 species with a thallus attaining 30 cms. in length. Enteromorpha intestinalis also extends 

 from the sea-coast into brackish and fresh waters, being a frequent Alga in canals and 

 ponds. E. gracillima G. S. West ('12 A ; fig. 177 A and B) has been found in sulphurous 

 springs in S.W. Africa. The widely distributed Ulva often extends into brackish waters, 

 but Letterstedtia is confined to the coasts of Natal, Australia and Japan. 



Ulva and Enteromorpha sometimes occur in great quantities in muddy estuaries and 

 other coastal areas where there is some amount of sewage pollution (consult page 146). It 

 has been suggested that both genera become partially saprophytic in the presence of the 

 nitrogenous products of decay. 



