STOUT AND SUSA, CHROMOSOME IRREGULARITIES 23 



show very marked irregularities throughout the processes of sporogenesis. 

 In reviewing these Sharp (1926, p. 356) has noted that there is "a 

 series of transitional conditions between normal meiosis associated with 

 sexuality and failure of meiosis associated with apomixis." It seems 

 that a rather common rule of behavior in apogamic plants is that the 

 egg mother cells merely continue in somatic divisions with slight signs, or 

 even no signs at all, of the stages of synapsis and meiosis, while for the 

 pollen mother cells there are such irregularities that the pollen is usually 

 all aborted. Examples of this are the common dandelion (Jeffrey, 1927), 

 and the hawkweeds, which have been studied by Rosenberg (1917, 1927). 



In respect to many of the irregularities in sporogenesis, there is much 

 similarity between the conditions found in hybrids, polyploids and par- 

 thenogentic forms. In the latter, however, there is the ability to produce 

 embryos vegetatively. Parthenogenetic and apogamic forms include both 

 hybrids and polyploids, and the evidence of relationship is so strong that 

 it has been considered by some (Ernst, 1918; Jeffrey, 1927) that hy- 

 bridization and polyploidy of themselves frequently stimulate partheno- 

 genetic tendencies in the ovule apparatus. 



There is no evidence that plants of the Europa clon are in the least 

 degree parthenogenetic or apogamic. The few seedlings that have thus 

 far been obtained from them are the result of cross-pollination with 

 other species and are obviously all hybrids. 



5. In the four classes of plants mentioned above which exhibit irregu- 

 larities in meiosis and abortion of spores, the fundamental condition 

 responsible for the irregularities may be considered as inherent in the 

 plants themselves or as inherent in some condition or set of interrela- 

 tions between the mechanisms of the cells and particularly of the germ 

 cells. A somewhat similar irregular behavior of chromosomes both in 

 mitosis and in meiosis has in various cases been induced by subjecting 

 plants to chemical influence (to chloral hydrate, Nemec, 1904), to 

 mechanical treatment such as shaking, and to higher and lower tempera- 

 tures than usual, all of which temporarily modify the physiological con- 

 dition of the cells concerned in sporogenesis. 



In considering the causes of pollen abortion in Hemerocallis, Timm 

 (1928) emphasizes the possibility of the action of climatic influences. 

 But it can scarcely be considered that a purely external influence, such 

 as temperature, plays an important role in producing the high percent- 

 age of gametic sterility always and very constantly seen in plants of the 

 Europa clon, as they grow rather widely in the North Temperate Zone. 



