Manchester Memoirs, Vol. Ivii. (1913J, No. t^. 



IX. On some abnormal specimens of Didyota dichotoma. 

 By H. S. HoLDEN, M.Sc, F.L.S. 



{Lecturer in Botany, University College, Nottingham). 



(Commiinii-ated by Professor F. E. Weiss, D.Sc, F.L.S.) 



{Read May 6th, igrj. Received Jar publication, June ^rd, igrj.) 



It has long been a matter of common knowledge that 

 many marine algai are characterised by the fact that their 

 sexual and asexual reproductive organs are normally 

 borne on separate plants, this being a specially noticeable 

 feature in the higher members of the RhodophycetTe 

 {e.g. the Cohorts Gigartinales, Rhodyvieniales and Crypto- 

 nemiales') and in the Ectocai-pacece, Cutleriacem and Dictyo- 

 tacece among the Brown Algae. The Cutleriacecc alone, 

 of the forms referred to above, show z.x\y differences in 

 the vegetative structure of the sexual and asexual plants : 

 in the remainder the two are, to all appearances, identical. 



With the advent of modern cultural methods of study 

 algologists have been able to demonstrate that in many 

 of these algae there is a fixed alternation of sexual and 

 asexual plants, the fertilised egg on germination producing 

 an asexual plant, the spores of which, in turn, give rise 

 again to a new series of sexual plants, thus affording a 

 close parallel to the conditions obtaining in the Bryophyta 

 and Pteridophyta. Furthermore, this alternation of genera- 

 tions is accompanied by characteristic nuclear phenomena, 

 the nuclei of the cells of the sexual plant, including both 

 ova and sperms, containing the Jiaploid {11) number of 

 chromosomes, whilst those of the asexual plant possess 

 the diploid {271) number. The nuclei of the spore-mother 



July 4th, IQ13. 



