84 CHLOROPHYCEAE 



the next year or years a new cylinder arises that bears one or more 

 sterile whorls of branches, until in the third or even a later year, a 

 shoot develops which produces one deciduous sterile whorl and a 

 single fertile whorl or umbrella. Each sac-like sporangium, or 

 umbrella lobe, gives rise to a number of multinucleate cysts which 

 are eventually set free through disintegration of the anterior end of 

 the sporangium. In the spring biflagellate isogametes are liberated 

 from these cysts and fuse in pairs, or else develop parthenogenetic- 

 ally. In A. Wettsteinii meiosis occurs at gametogenesis and the 

 adult plants are therefore diploid. According to Hammerling 

 the immature plant contains only one nucleus, which is to be 

 found in one of the rhizoids, and at cyst formation this divides, the 

 daughter nuclei being carried into the sporangia. The resulting 

 cysts in the umbrella lobes are uninucleate, but as the single 

 nuclear condition is in direct contrast to the reports of other 

 workers it would seem that further cytological study is desirable. 



REFERENCES 



Cladophora. Acton, E. (1916). New Phytol. 15, i. 



Neomeris. Church, A. H. (1895). Ann. Bot., Lond., 9, 581. 



General. Feldmann, J. (1938). Rev. Gen. Bot. 50, 571. 



Acetabularia. Hammerling, J. (193 1, 1932). Biol. Zbl. 51, 663; 52, 42. 



Cladophora. List, H. (1930). Arch. Protistenk. 72, 453. 



Neomeris. Svedelius, N. (1923). Svensk bot. Tidskr. 17, 449. 



SIPHONALES 



This group is characterized primarily by possession of a coeno- 

 cytic structure in which true septa are rare or absent, the coenocyte 

 normally having a cytoplasmic lining surrounding a central vacuole 

 and containing numerous disk-shaped chloroplasts. Cellulose is 

 largely replaced by callose as the principal component of the walls. 

 The group may be polyphyletic in origin, and the fact that it 

 reaches its maximum development in warm waters may be signifi- 

 cant, not only in respect of the phylogeny of the group itself, but 

 also in considering the evolution of the Chlorophyceae as a whole. 

 Most of the genera possess the power of regeneration to a marked 

 degree, but this can perhaps be regarded as a primitive character 

 that has persisted throughout the course of time. 



