PARTHENOGENESIS 327 



families. In some species it is habitual and in others it 

 exists alongside typical sexuality. In all cases the genera 

 which have parthenogenetic species have also normal 

 species. In Alchemilla (Murbeck, 1901) the spore mother 

 cell does not undergo reduction division and becomes 

 itself the megaspore ; from it there develops an embryo 

 sac, perfectly normal except that the nuclei have the 

 diploid number of chromosomes. The ovum develops into 

 an embryo without fertilisation. In Hieracium flagellar e 

 (Rosenberg, 1906) some flowers of a head show normal 

 reproduction, while in others parthenogenesis takes place. 

 Here, however, reduction division occurs and an embryo 

 sac is formed in the ordinary way. This disintegrates and a 

 cell of the integument enlarges and gives rise to a second 

 embryo sac with diploid nuclei, the egg cell of which develops 

 parthenogenetically. This is a case of parthenogenesis com- 

 bined with a second abnormality — apospory, that is asexual 

 reproduction without true spore formation, a cell of the 

 sporangial tissue taking the place of the megaspore. 



2. Apogamy : the egg cell is dispensed with and the 

 embryo is formed (parthenogenetically) from another, 

 vegetative, cell of the gametophyte. Among the flowering 

 plants this is an uncommon occurrence. In Alchemilla 

 sericata the embryo sac is derived from a megaspore with 

 diploid nuclei, and sometimes an embryo is developed in 

 it from one of the synergids (the two cells which lie beside 

 the ovum) as well as from the egg cell. 



3. Adventitious Embryos.— A third case is that in which 

 embryos are formed directly from the tissues of the nucellus 

 without the intervention of the gametophyte generation 

 at all. The development of several such embryos leads to 

 the occurrence of polyembryony in the seed. The best 

 known example is the orange ; another common plant with 

 polyembryony is the garden Funkia ovata. The embryos 

 in these cases do not develop unless fertilisation of the 

 normal egg cell has taken place. In Ccelebogyne (Euphor- 

 biaceae) adventitious embryos are formed and seeds ripen 

 without fertilisation of the egg cell. The production of 



