THE GREEN ALG^ 643 



have become adapted as Plankton-organisms (fig. 2, p), but the 

 differences between them and certain members of the Scenedes- 

 maceae are so insignificant that a separate family is scarcely 

 warranted. A peculiarity of all these forms (Scenedesmaceae 

 and Phyteliaceae) is that the daughter-individuals are only 

 liberated from the mother-cell, when they have attained their 

 complete differentiation. This has been styled autospore- 

 development, and finds an interesting parallel among the Chlamy- 

 domonadaceae in the genus Brachiomonas referred to above. 



If we derive Chlorella and the other genera above discussed 

 from a Chlamydomonad ancestry, it is only natural to expect 

 that there will be forms, which have not gone to the extreme of 

 giving up all power of adopting the motile condition but, whilst 

 sedentary during the greater part of their life-cycle, have 

 retained the power of reproducing by motile cells (zoospores). 

 This is the case with Oltmann's Protococcaceae, which occupy 

 an intermediate position (but not in the phylogenetic sense) 

 between the Chlamydomonadaceae and Scenedesmaceae. The 

 genus Chlorococcum (Artari 2, p. 11) is regarded by Oltmanns as 

 the simplest form of the Protococcaceae. It consists of rounded 

 and mostly isolated cells of rather large size, although otherwise 

 having the same structure as those of a Chlorella. These cells, 

 however, reproduce by successive subdivision of the contents 

 to form a large number of naked zoospores (fig. 3, a), which, on 

 coming to rest, become enveloped by a cell-membrane and form 

 new Chlorococcum-'mdividuals. When grown in concentrated 

 nutritive solutions, the products of division of the cell-contents 

 are not liberated as zoospores, but become enveloped by cell- 

 walls, while still within the mother-cell ; on the rupture of the 

 membrane of the latter the contained cells (so-called aplanospores) 

 give rise each to a new individual, this modification of the 

 reproductive process obviously bringing Chlorella completely 

 into line with Chlorococcum. In fact, we might call the former a 

 Chlorococcum, which has permanently acquired the property of 

 aplanospore-formation. 



The young cells of a Chlorococcum are uninucleate, but as the 

 cell becomes older the nucleus divides and so a multinucleate 

 condition is attained. This nuclear division is here merely a 

 preliminary to zoospore-formation, and in all probability the 

 subsequent fission of the cell-contents leads to the formation 

 of as many zoospores as there are nuclei. The fact remains, 



