358 INTRODUCTION TO CYTOLOGY 



presence of the larger and more numerous chloroplasts. In Zea the mono- 

 ploid, diploid, tetraploid, and octoploid plants can be distinguished by 

 examining the size and distribution of the stomates. In Datura each of the 

 12 primary 2n + 1 mutants differs characteristically from all of the others 

 in its histological structure. ^*^ Aneuploid plants usually differ in appear- 

 ance from the normal diploid much more than do the tetraploids, triploids, 

 or other euploid forms. There is a "balance" within the complement which 

 is maintained even when three, four, five, or more sets are present, but 

 which is disturbed when any set lacks one or more of its members. 



The effect of heteroploidy upon sexual fertility has already been 

 mentioned. Autotetraploids often show comparatively little sterility 

 because the chromosomes, although present in double the usual number, 

 are all able to form synaptic groups of pven number (bivalents or quadri- 

 valents), enabling the meiotic mitoses to be carried out without great 

 irregularity. In triploids, on the contrary, the odd number of elements in 

 the synaptic groups (trivalents) and the frequently unassociated univa- 

 lents interfere with proper meiotic distribution, so that a large proportion 

 of the spores or gametes are non-functional. In the higher euploid types 

 such as pentaploids, hexaploids, etc., the meiotic irregularity and sterility 

 tend to be rather great, especially in the ones with odd numbers of sets. 

 Thus in Rubus the pollen sterility is very low in in, 6n, and 8n forms, but 

 very high in 3n and 5n forms (Longley, 1927a). For similar reasons 

 aneuploid types tend to show considerable sterility. Frequently a change 

 to the heteroploid condition causes greater sterility in one sex than in the 

 other. Thus in a triploid (Enothera the eggs are apparently able to 

 function with any chromosome number from 7 to 14, whereas the only 

 male gametes functioning are those with either 7 or 14, because the pollen 

 grains with intermediate numbers abort (van Overeem, 1921, 1922). 

 In some heteroploids the sterility is so complete in one or both sexes that 

 the type cannot persist except through asexual propagation. This is 

 notably the case in heteroploids of hybrid constitution, as will be shown in 

 the next chapter. Asexual reproduction, however, is not necessarily an 

 indication of hybridity." The fact that organisms reproducing asexually 

 without effective meiosis show no Mendelian segregation affords strong 

 support to the chromosome theory of heredity. The effect of hetero- 

 ploidy upon the sexual development of the organism will be discussed 

 further in Chapter XXIII. 



i" Winkler (1916) on Solarium, Randolph (1932) on Zea, Sinnott and Blakeslee 

 (1922) on Datura. See also Tupper and Bartlett (1916) on Oenothera. 



'' Tischler (1928c) reviews many cases of spore sterility in angiosperms. 



