APOMIXIS AND RELATED PHENOMENA 407 



2a) (Longley, 1926a; East, 1930). In Atamosco the male nucleus entered 

 a diploid egg and later degenerated, the embryo continuing development 

 with maternal chromosomes only (explanation, 26) (Pace, 1913). In 

 certain species and biotypes of Potentilla pollination is necessary for the 

 setting of seeds, but the offspring of crosses are purely maternal in 

 characters and chromosome number (Miintzing, 1928). 



B. Androgenesis. — Development of offspring with the paternal 

 chromosomes only. How the condition arises is not yet known. 



Examples: Tripsacum dadyloides 9 X Euchlcena mexicana cf yielded 

 a plant which was morphologically and genetically like the male parent, 

 although the seed from which it arose was outwardly like that of the 

 other parent (Collins and Kempton, 1916). Nicotiana digluta 9 X N. 

 tabacum c^ yielded, among other types, one plant which was morphologi- 

 cally and cytologically like other tabacum plants with the reduced chromo- 

 some number (Clausen and Lammerts, 1929). Similarly, N. tabacum 

 macrophylla 9 X N. Langsdorffii cf gave a typical monoploid Langsdorffii 

 individual (Kostoff, 1929). 



C. Merogony. — Similar in some respects to androgenesis is merogony, 

 in which an egg fragment without a nucleus develops after the entrance of 

 a sperm. This has been induced in the brown alga Cystosira barbata by 

 Winkler (1901) and in several animals (p. 412). 



Causes of Apomixis. — It is a noteworthy fact that apomictic plants 

 are usually characterized by various irregularities in sporogenesis. The 

 megasporocyte, for example, may form a quartet of unreduced spores, a 

 pair rather than a quartet, or no spores at all. Similar irregularities in 

 microsporogenesis lead to the production of varying numbers of function- 

 less pollen grains; in extreme cases no pollen is formed. 



It is evident that abnormal chromosome behavior is largely responsible 

 for such defective sporogenesis. In the megasporocyte of Marsilia 

 Drummo7idii the chromosomes undergo synaptic pairing but then 

 dissociate and split longitudinally as in a somatic mitosis. In Antennaria 

 the meiotic phenomena are fewer, and the mitosis passes sooner into the 

 vegetative form. In Wikstroemia there is no trace of synaptic association, 

 the mitosis being purely vegetative in character; here only two cells are 

 formed. Finally, in Elatosiema sessile there are no meiotic phenomena 

 and no cell-division; the sporocyte forms the female gametophyte directly. 

 These cases^^ illustrate a series of transitional conditions between normal 

 meiosis associated with sexuality on the one hand and the failure of 

 meiosis associated with apomixis on the other. 



The situation is better known in microsporogenesis, where a series of 

 increasingly aberrant modes of chromosome behavior may be seen within 

 a single genus, notably in Hieracium (Rosenberg, 1917). In Hieracium 



"Strasburger (1907a, 1904c, 1909a), Modilewski (1908). 



