DEGENERATION OF MEIOSIS 455 



mother-cell, except that the purely mitotic Antennaria and 

 Euhieracium type of development is missing. Two groups 

 must be distinguished : (i) Species with apospory or nucellar 

 embryony as well as parthenogenesis, (ii) Species that are solely 

 parthenogenetic. 



The first are naturally less dependent on the regular formation of 

 diploid gametes (for the embryo-sac) and it is found that while 

 meiosis is often suppressed owing to the reconstitution of a single 

 nucleus at the first telophase (following the lagging of a variable 

 number of unpaired chromosomes) yet often the results are entirely 

 irregular or a tetrad is formed of nuclei with different numbers of 

 chromosomes. This is the case in Erigeron annims, Artemisia 

 nitida and Ochna serrulata. In the last species pairing follows 

 precisely the frequencies observed in the similarly autopolyploid 

 Tidipa Clusiana, which is also a pentaploid and sexually fertile 

 (Figs. 74, 75). 



In purely parthenogenetic species (e.g., Eu-hieracium) meiosis is 

 rather more regularly suppressed. But several gradations have 

 been described by Rosenberg in this apparent transition towards 

 the regular formation of diploid gametes (if we may infer a 

 progressive change) . In all these the early prophase appears normal, 

 and doubtless some association of chromosomes occurs at pachytene. 

 The first stage of "degeneration" corresponds with that found in 

 such hybrids as Raphamts-Brassica. Few (in Hier actum horeale and 

 Chondrilla juncea, with embryo-sac of type i) or no chromosomes 

 (in Hieracium IcEvigatum, with embryo-sac of type 2) are paired and 

 very often, probably as a result of the uneven distribution and 

 lagging of the unpaired chromosomes on the spindle, a single resting 

 interphase nucleus is reconstituted before the two groups are 

 clearly separated. This " restitution nucleus " divides to give 

 daughter-nuclei, each of which has the unreduced number of 

 chromosomes. Various abnormalities occur, such as the formation 

 of one or more, usually small, supernumerary nuclei. 



The second stage of degeneration is found in H. pseiidoillyricum 

 and a few other species (with embryo-sac of type 3). The nucleus 

 contracts after reaching a stage immediately before diakinesis, and 

 on resuming its prophase condition develops directly into a somatic 



