K.— BOTANY. 187 



of the filamentous Green Algse are practically unknown, and a reinvestiga- 

 tion of Coleochaete (and especially of other species than C. scutata) from 

 this point of view is advisable. Lambert in 1910 reaffirmed 4 '' the presence 

 in the life-cycle of Coleochaete of a succession of small plants that are 

 purely asexual, a feature already emphasised by Pringsheim. 26 



Klebs' classical investigations 27 on the conditions of reproduction in 

 various Algae have generally been regarded as disposing of the possibility 

 of any alternation between asexual and sexual filaments such as Pringsheim 

 postulated, but on closer consideration they afford no absolute proof of 

 its non-existence. In the usual dense tangle of filaments asexual and 

 sexual individuals may easily be intermingled, either remaining purely 

 vegetative until conditions suitable for the formation of reproductive cells 

 are realised. But there are more important considerations than these. 

 With reference to Ulothrix zonata Blebs 2 * especially remarks that it always 

 depended on chance whether he was able to get threads to form gametes 

 or not. 29 He also records how at one and the same time threads from one 

 habitat readily formed gametes, while those from another failed to do so. 

 And Ulothrix is the only member of Ulotrichales and Chaetophorales in 

 which Klebs deals at all fully with the sexual reproductive process. 

 Yet from this one case conclusions have been drawn for the whole 



group ' 



There is, so far as I can see, little positive evidence that asexual and 

 sexual reproduction takes place at all frequently in one and the same 

 filament of these forms, although there is no reason why sexual individuals 

 should not reproduce themselves asexually after the manner of PylaieUa. 

 An important matter for investigation is : are there in these filamentous 

 Green Algae threads that can only reproduce asexually and that by no 

 manner of means can be brought to sexual reproduction ? This may be 

 more readily established than possible cytological differences, which call 

 for a highly skilled investigator owing to the small size of the nuclei in 

 most of these forms. The fact that in so many Ulotrichales and Chaetophorales 

 the zoospores have four, and the gametes two cilia, is perhaps significant 

 from this point of view. 



Even if, however, further investigation should altogether support the 

 present view that there is no alternation between asexual and sexual 

 filaments in the Green Algae, the example afforded by PylaieUa serves to 

 show how easily alternation can arise in lowly filamentous types, and in 

 the case of terrestrial plants it may have originated after, rather than 

 before, the adoption of the land-habit. I have elsewhere 30 indicated 

 how the dual development of the plant-body in types like the Chaetophorales 

 might readily, in the course of further evolution, afford an upright sporo- 

 phyte and a prostrate gametophyte. I have nothing to add to this and 

 I do not propose to pursue the topic further. 



25 Tufts Coll. Stud., Scient. Ser. iii, 1910, p. 61. 



26 Gesammelte Abhandl. i, 1895, p. 305 (Jahrb. wiss. Bot. ii, 1858). 



27 Beding. d. Fortpflanzung, Jena, 1896. 

 * Op. cit., p. 314. 



29 cf. also Dodel, Jahrb. wiss. Bot. x, 1876, p. 539. 

 80 New Phytol. xv, 1916, p. 233. 



