The bearing of cytological studies on the phylogeny of the Marchantiales

Abstract

The paper gives a brief account of the cytological studies on the Hepaticeae and indicates the bearing of these on the problem of the phylogeny of the Marchantiales. The data available have been analysed and grouped under the following four headings: the sex chromosomes, hybridization, polyploidy and gene mutations, and the morphology and numbers of chromosomes. The information regarding the chromosomal numbers has been tabulated and the probable basic numbers in different families have been indicated. From the table given on page 145 it will be seen that the basic number of chromosomes (gametophytic number,i.e., ra-number) for the majority of the genera of the Marchantiaces and the Jungermanniaceae seems to be 9, and for the genera of the Ricciacete it seems to be 8. In the light of this cytological observation the existing schemes of classification of the Marchantiales have been examined with a view to knowing how far the morphological data and the cytological data are correlated. If it be supposed that the basic complex of the ancestors of the Marchantiales called Pro-Marchantiales in this paper, had 8 chromosomes in their genom and changed to a 9 chromosomal condition later by fragmentation, then there is some justification for the progression hypothesis. On the contrary, if it be imagined that the Pro-Marchantialean complex consisted of 9 chromosomes originally and got reduced to 8, the reduction hypothesis may seem plausible.But when either of these ideas are pressed further and the number of chromosomes in different genera in the various schemes of classification given by Cavers, Kashyap, Verdoorn and Evans are considered, the verdict of cytology on either of these schemes is not unequivocal, as the main argument brought in the discussion for or against a theory is based on the difference of a single chromosome only; and it may well seem doubtful whether that alone can constitute sufficient proof for or against a scheme.