408 INTRODUCTION TO CYTOLOGY 



pilosella, which is not apomictic, normal synapsis and disjunction are 

 constant in both megasporocytes and microsporocytes. In the species 

 horeale, Icevigatum, and pseudoillyricum, all of which belong to the sub- 

 genus Archieracium, are seen three stages in the transition from normal 

 meiosis with sexual reproduction, such as occurs in H. pilosella, to ameio- 

 tic sporocyte divisions with apomixis. These cases show clearly that 

 there is a connection of some sort between apomixis and a weakness 

 or absence of the synaptic attraction between homologous chromosomes. 



Another notable fact is the common association of apomixis with 

 polyploidy. When the chromosome number of an apomictic species is 

 compared with that of a nearly related sexual species, it is commonly 

 found that the former is larger. In about one-half of the known cases 

 in angiosperms the apomictic species have twice the numbers found in 

 the related sexual species, and in some cases the multiple is still larger. 

 Furthermore, polyploid forms, like apomictic forms, are often character- 

 ized by conspicuous irregularities in chromosome behavior at the time of 

 meiosis. A common cause underlying meiotic irregularity, polyploidy, 

 and apomixis has consequently been sought. 



That a primary cause of these chromosomal aberrations, together with 

 the associated substitution of apomictic for sexual reproduction, is 

 hybridization, is a theory which has been supported by a number of 

 students of such phenomena. ^^ The phenomena in question are so 

 exactly paralleled in hybrids of known origin that little doubt can be 

 entertained regarding the essential correctness of the theory, though 

 the limits of its application cannot be stated at present. The importance 

 of deficient synapsis in bringing about polyploidy and apomixis has been 

 emphasized by Winge in a special hypothesis involving the synaptic 

 reaction (see p. 369). The sexual sterility so common in hybrids and 

 polyploid forms is largely a result of the incompatibility of chromosomes 

 of unlike origin; indeed, it is highly probable that many apomictic forms 

 have arisen through the crossing of normally sexed forms, as Ernst 

 maintained. In certain moss hybrids apomixis is believed to be due to 

 the action of genes which are freed for action by hybridization or 

 polyploidy. ^^ 



The advocates of the hybridization theory do not hold hybridization 

 to be the sole cause of apomixis. There is evidence that sexual sterility 

 with apomictic development may also be the outcome of nutritive dis- 

 turbances set up by other causes, such as abnormal cultural conditions, 

 parasitism, and wounding. The cells involved in sexual reproduction 

 seem to be especially sensitive to unfavorable environmental influences, 

 so that they can easily be made to undergo abnormal development or 



1* Ernst (1917, 1918), Rosenberg (1917 et seq.), Winge (1917), I. Holmgren (1919), 

 Tackholm (1920, 1922), Ostenfeld (1921), Turesson (1930). 

 16 Von Wettstein (1927). See Zattler (1924). 



