6 4 o SCIENCE PROGRESS 



Some of the genera here included (e.g. Hormospora) are no 

 doubt peculiar stages of the common filamentous Alga Ulothrix, 

 while others (such as Hormotila and Hauckia) diverge widely 

 from the typical Tetrasporaceae in the presence of several 

 chloroplasts in the cells and in the liberation of a large number 

 of zoospores from each cell ; it is not impossible that these 

 forms are again merely stages in the life-history of some of 

 the higher Algae, but in any case our knowledge of them is so 

 imperfect that it is not possible to classify them at present. The 

 higher filamentous Algae, as we shall see, probably arose from 

 another source (cf. p. 645, and the second part of this article). 8 



Passing over a large number of more or less imperfectly 

 known forms (Glceocystis, Palmella, Bottyococcus, Palmodactylon, 

 etc.), which all represent mere modifications of the palmelloid 

 tendency, we may turn our attention to a new line of evolution, 

 the Protococcales of Oltmanns, which is one of the most 

 important and instructive of the series derivable from the 

 Chlamydomonas-stock. We owe our present clear insight into 

 this series to Senn (62) and Oltmanns, who have done more 

 to cast light on this difficult assemblage of forms than any 

 one in the last twenty years. There are two distinct tendencies 

 represented in this series of the Protococcales, and the forms 

 involved are placed by Oltmanns (53, p. 169) in the Scene- 

 desmaceae and the Protococcaceae. The simplest form of the 

 Scenedesmaceae is the genus Chlorella (including a considerable 

 number of forms referred by Artari (2) to Pleurococcus). This 

 Alga (fig. 2, h) consists of rounded cells with an almost spherical 

 chloroplast, which is interrupted only by an aperture on one 

 side of the cell, a pyrenoid being embedded in the chloroplast 

 at the point opposite to the aperture. This chloroplast is of 

 course to be regarded as an enlargement of the Chlamydomonas 

 chloroplast. The reproductive process is very simple, the 

 contents subdividing into two or four parts, each of which 

 acquires a cell-wall (fig. 2, h), and is then liberated by the 

 rupture of the membrane of the mother-cell. The daughter- 

 cells may either separate or cohere to form small groups. Such 

 a form is very readily derivable from the C/ilamydomonas-type ; 

 it is only necessary to imagine that an enlargement of the 

 chloroplast took place and that the products of division lost 

 their power of free movement. Incidentally we may also notice 

 that if the products of division cohere it is only necessary for 



