CYTOLOGICAL DISTINCTION IN FUNGI 69 



fusion with consequent doubling of the chromosome-number taking place 

 on fertilisation, but actual chromosome-counting is difficult. There is, on 

 the other hand, notwithstanding earlier statements which tended to locate 

 reduction at the germination of the oospore, a growing opinion, based in 

 part on exact counting, that the reduction in these plants is pre-sexual, 

 and takes place at the maturing of the oogonium and antheridium. 

 This receives considerable support from Trow's results on Achlya : l he 

 finds that doubling occurs as usual on fertilisation, but the necessary 

 reduction takes place in gametogenesis in this plant, as in most animals, 

 and not in sporogenesis, as in most plants. Such a conclusion from the 

 Saprolegniae would thus correspond to what seems probable for the 

 Peronosporeae : it has also been seen to be probable according to some 

 writers for Vaiecheria, and has been conspicuously proved for Fucus. 

 In such plants the chromosome-number in the somatic divisions will be 

 " 2n," as in animals, and there will be an absence of cytologically distinct 

 generations with obligatory alternation. 



There are various cases among the higher Fungi in which, on grounds 

 of comparison of form combined with nuclear fusion, a sexual process 

 is now recognised, for instance in the simpler Ascomycetes. Here the 

 carpogonium has long been regarded as a female organ, and the polli- 

 nodium male ; a position which is now justified by the nuclear fusions 

 observed. It naturally follows to regard the ascogenous hyphae as a 

 post-sexual stage analogous to that in the Florideae : they hold the same 

 place in the life-cycle as the carpogonial filaments of the latter. The 

 condition of this stage as regards chromosome-number is still a. matter 

 of doubt ; but there is some reason for believing that reduction may take 

 place on formation of the ascospores, while their number in each ascus 

 is in itself suggestive. Further observations will be required to show 

 how far such comparisons have a cytological justification. 



But in the Uredineae the case for an alternation based upon cyto- 

 logical detail has been fully made out for Gymnosporangium and Phragmidium, 

 the facts being as follows : 2 The mycelium which bears aecidia and 

 spermogonia has single nuclei : each is usually in a separate cell, and shows 

 two chromatin-masses on division. This stage is compared with a gameto- 

 phyte, capable of bearing sexual organs. The spermatia are held to be 

 functionless male cells, and fertilisation is effected by other means. The 

 young aecidium is held to be a sorus of female reproductive organs, 

 each of which may be fertilised by the migration into it of the nucleus 

 of one of the adjoining undifferentiated mycelial cells. The male and 

 female nuclei do not fuse, however, but continue to divide simultaneously, 

 and the product of fertilisation is accordingly a growth with paired nuclei : 

 this condition is persistent throughout the rest of the life-cycle, includ- 

 ing the aecidiospores, the mycelium which germinates from them, the 



^Annals of Botany, xviii., p. 541. 



-V. II. Blackman, Annals of Botany, xviii., p. 323, etc. 



