124 A CENTURY Of PROGRESS IN THE NATURAL SCIENCES 



has evolved independently in a number of different groups of algae — simple 

 forms as well as advanced types— and it is doubtful that any great weight should 

 be placed on this character in the delimitation of major taxa. In the family 

 Aerochaetiaceae of the red algae, for instance, some species of a genus have a 

 single basal cell whereas others produce an extensive prostrate system. 



Although the majority of recent systematists have accepted the order Ul- 

 vales, proposed by Blackman and Tansley (1902), there is a great deal of 

 justification for the view of Fritsch (1935, 1944) that they are advanced Ulo- 

 trichales. The possession by the Ulvales of a parenchymatous thallus does not 

 distinguish them from all Ulotrichales since some of the latter (e.g., Fritscliiella) 

 also form parenchymatous thalli. Some Ulvales show an alternation of isomorphic 

 generations but this is now believed to be true also of certain Ulotrichales (e.g., 

 DraparnaldioiJsis and FritschieUa; ^ingh, 1945, 1947). 



The order Microsporales, which was established by Bohlin (1901) to accom- 

 modate the genus Microspora, has never received wide acceptance. Eecently, how- 

 ever, it was resurrected by Prescctt (1951), who in the same work also erected 

 an order Cylindrocapsales for the genus C ylindrocapsa. Although these two 

 genera, especially the oogamous C ylindrocapsa, occupy a somewhat isolated po- 

 sition among the Ulotrichales it seems best to regard them in agreement with 

 Fritsch (1935) and Smith (1950) as constituting well-defined families within 

 the Ulotrichales. Printz (1927) does not even recognize the family Microspora- 

 ceae; he places Microspora in the Ulotrichaceae. 



The Oedogoniales occupy an isolated position in the Chlorophycophyta. The 

 peculiar method of cell division shown by the three genera comprising the order 

 is not met with am^vhere else and the collar of subterminal flagella present in 

 the sperms and zoospores is encountered elsewhere only in Derhesia (Sipho- 

 nales). Monographic treatments of the order have been presented by Him 

 (1900), Tiffany (1930), and Gemeinhardt (1938-1940). 



The Schizogoniales, established by West (1904), comprise a small group of 

 terrestrial, freshwater, and marine algae placed by some (e.g., Fritsch, 1935) 

 in one, by others (e.g., Knebel, 1935) in two. genera. The group is character- 

 ized by the formation of parenchymatous thalli, stellate plastids, and the ap- 

 parent lack of motile reproductive cells (cf., however, Fujiyama, 1949). The 

 order has recently been the subject of a monograph by Knebel (1935). 



The more important early discoveries relating to sexuality in the green algae 

 have been considered briefly in the introduction to this chapter. In the present 

 section attention will be focused especially on some of the more recent work 

 on the life histories and the associated nuclear phenomena. 



A fusion of the gamete nuclei in the zygote was observed by Schmitz 

 (1879c) in Spirogyra. Klebahn (1888, 1891, 1892) observed it in desmids and 

 Oedogonium, and Goroschankin (1890) saw it in ChJamydomonas. 



Following the postulate of Weismann (1887) that the doubling of the chro- 

 matin mass at syngamy must be followed by a regulatory reducing process, 

 many investigations were undertaken with the view of testing this hypothesis 

 and of determining the place in the life history where the reduction may occur. 

 The first observations on meiosis in the green algae were by Allen (1905) in 

 Coleochaete. The life history of Coleochaete had previously been investigated 

 by Pringsheim (1860) who showed that the contents of the zygospore at germi- 



