176 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



bluegreen algae are largely of a negative kind, as Pringsheim has emphasized, 

 there is little reason for believing that certain negative characters are less im- 

 portant than positive characters as indicators of phylogenetic relationship. Fur- 

 thermore, if present-day bacteria and bluegreen algae had evolved from com- 

 mon ancestors affinities should be sought primarily among the simpler members 

 of both groups (the Eubacteriales and the Chroococcales, respectively), since 

 they are the forms which may be expected to have retained and consequently 

 show the largest number of ancestral characters. The morphological similarity 

 between certain members of the Eubacteriales and members of the Chroococ- 

 cales suggests that a relationship does exist between these two groups. Further 

 evolution in the bluegreen algae has resulted in the development of thalli which 

 are much more elaborate than those of the higher bacteria. But morphological 

 differences of comparable magnitude are not uncommon among groups of organ- 

 isms which are known to be phylogenetically related and their occurrence in 

 the Schizophyta do not necessarily speak against a common origin of bac- 

 teria and bluegreen algae. 



On the basis of method of multiplication, Thuret as long ago as 1875 subdi- 

 vided the bluegreen algae, or Nostochinees as he called them, into two tribes: 

 (1) the Chroococcaceae or Coccogoneae, which show vegetative propagation by 

 single cells, and (2) the Nostochineae or Hormogoneae, which reproduce vege- 

 tatively by short rows of cells (hormogonia). Thuret segregated the Hormo- 

 goneae into two subtribes: (1) Psilonemeae, in which the filaments lack hairlike 

 tips, and (2) Trichophoreae, in which the filaments possess hairlike apices. In 

 Thuret 's time the members of the subsequently established family Chamaesi- 

 phonaceae were only poorly known. 



To Nageli (1849) and Hansgirg (1888b, 1892) we are indebted for much of 

 the fundamental information on which present-day classification of the Chroo- 

 coccaceae rests. Current classification of Thuret 's Hormogoneae is largely based 

 on the systems of Borzi (1878, 1879, 1882), Bornet and Flahault (1886-1888), 

 and Gomont (1892, 1893). 



The first comprehensive treatment of the Schizophyceae as a whole was given 

 by Kirchner (1898), who followed, as far as the broad outlines are concerned, 

 the classification of Rabenhorst (1865), Thuret (1875), and Hansgirg (1888b, 

 1892). In accordance with the system of Thuret, Kirchner divided the blue- 

 green algae into Coccogoneae and Hormogoneae. In the Coccogoneae he placed 

 the Chroococcaceae and the Chamaesiphonaceae, a family established by Borzi 

 in 1882. The Hormogoneae were segregated, in agreement with Thuret, into 

 the Psilonemateae (which received the families, Oscillatoriaceae, Nostocaceae, 

 Scytonemataceae, and Stigonemataceae) and the Trichophoreae (in which were 

 placed the Rivulariaceae and the Camptotrichaceae). As has been pointed 

 out by Fritsch (1944, p. 262), this division of the Hormogoneae (^ Hormogo- 

 nales) by Thuret and Kirchner into two groups on the absence or presence of 

 hairs at the tips of the filaments overemphasized the systematic value of a minor 

 character, and is no longer adhered to. 



Adopting and amending a classification introduced by Stizenberger (1860), 

 Bornet and Flahault (1886, p. 325) and Gomont (1892) divided the Hormo- 

 goneae into the two groups Heterocysteae and Homoej^steae according as the 

 trichomes contain or lack heterocysts. Kirchner (1898, p. 49) and Fritsch (1944) 



