FLO RIDE ^. 219 



certain spots the cells divide in a direction parallel to the plane of the 

 thallus, and a small projection is thus formed which becomes detached 

 and develops into a new ' frond.' Geddes (Trans. R. Soc. Edinburgh, 

 1881, p. 555) describes also a process of gemmation in Enteromorpha. 

 The ordinary mode of reproduction in the Ulvaceae so closely resembles 

 that in the Confervacese that the two families are generally considered 

 as nearly allied ; but the difference in the structure of the thallus is so 

 great that it is difficult to believe in any near affinity between them. 

 On the other hand, the structure of the thallus in Ulva and Porphyra is 

 almost identical ; and it is at least as probable that the Ulvacese are 

 derived from the Porphyraceae by further retrogressive metamorphosis 

 displayed in the complete suppression of the antherids and carpogones, 

 and the reversion of the tetraspores into motile swarm-spores of two kinds, 

 the smaller of which are zoogametes. On the other hand, Monostroma 

 may be allied to Tetraspora among the Protococcaceae. Several species 

 of Enteromorpha form a large portion of the green vegetation in salt 

 ditches or on muddy sea-shores, or on rocks between high and low water 

 mark ; Prasiola grows on bare rocks or stones, or on salt soil ; the 

 substance known as ' green laver ' consists of several species of Ulva. 



Literature. 

 Jessen — Prasiolse Monographia, 1848. 

 Thuret — Mem. Acad. Sc, Nat. Cherbourg, 1854, p. 9. 

 Wittrock — Monogr. Monostroma, 1866. 



Janczewski et Rostafinski — Mem. Acad. Sc. Nat. Cherbourg, 1874, p. 369. 

 Areschoug — Bot. Notiser, 1876. 

 Thuret et Bornet— Etud. Phycol., 1878. 

 Reinke — Pringsheim's Jahrb. wiss. Hot, 1878, p. 531. 

 Borzi — Studi Algologici, 1883. 



Class XL— Confervoidese Heterogamae. 



The small group of green fresh-water Algae comprised in this class 

 form a connecting link between the lower Florideae and the Isogamous 

 Confervoideae,- from which latter they are distinguished by the sharp 

 differentiation of their male and female organs of reproduction. In the 

 highest order, the Coleoch^tace^, the mode of reproduction bears a 

 striking analogy to that in the Nemalieae, being effected through the 

 agency of a tubular trichogyne ; but the male elements are no longer 

 immotile poUinoids, but motile swarming antherozoids, naked biciliated 

 or multiciliated protoplasmic bodies, but much more closely resembling 

 the corresponding structures in the lower Algae than the coiled anthero- 



