Origin of Sexuality 253 



but suggested in the sluggishly pseudo-ciliate Tetrasporacese) 

 palmella spores became active ciliate swarmspores. 



Unfortunately we do not as yet know the exact and minute 

 nuclear or chromatin changes that go on in the above cells, 

 and specially in swarmspores, when they fix down, or change 

 into another and later stage. But again we would consider 

 that these have all resulted from long-continued environal 

 stimulatory action, that at one period of the year or season 

 has differed much from the condition at another. Distinct 

 evidence of this is got when the life history of many algae is 

 traced. 



Thus in Ctenocladus of the Chsetophoracete, that is largely 

 intermediate in structure and aspect between the Chamse- 

 siphonese and the higher algse, autumnal akinetes with thick 

 walls arise on the ordinary branches and these act as peren- 

 nating winter spores. In late spring on lower branches of the 

 plant, cell groups are formed that become summer akinetes. 



Again in five genera of the same group (Chcetophoray Cteno- 

 cladus, Endoclonium, Stigeoclonium, and Trichopilus), that seem 

 all to be transition types between the Cyanophycese and higher 

 algae, two sizes of swarmspores — large and small — arise at 

 different times. Even in Trichopiliis, these — as in the palmel- 

 loid spores of Choanophoracese — are destitute of cilia, though 

 in the others two to four are formed. In Draparnaldia (99, 1: 

 232) two readily distinguished types of swarmspore arise, 

 the so-called macro- and microzoospores. Both have four 

 cilia, but the latter have a denser aspect, a more deeply placed 

 eyespot, and can pass directly into the resting-cell stage. While 

 the former can fix down and reproduce new plants directl3% 

 the microzoospores may become active and conjugate as sex- 

 cells according to Klebs. 



Many other cases might be cited which demonstrate the 

 frequent alternation or interrelation of akinetes, resting-spores 

 and swarmspores, in the life history of single genera. 



From evidence advanced by them, Chodat, Klebs, and other 

 workers have considered that such varying structural states 

 are due to varying environal factors. 



