CO-OPEKATION AND COMPETITION 



only the A pollen grains, and only the B embryo-sacs, germinate or 

 grow. Thus the two kinds of cytoplasm, in pollen and embryo-sacs, 

 favour the development of the available alternatives. 



This reaction has another aspect. Not merely does the cytoplasm 

 discriminate but, on the female side, the nuclei compete. Of the 

 four potential megaspores, two B's at one end and two A's at the 



Mofi> 



Haploid Competition 



-p fron\ FIura//fy <^/ ■'- 



9 



is: 



Lmoiyo-sQcs 



(n-2n) 



Ai.1. TrfSi 



AfoA/ot/'OA/c £.S 



25% ro/yji>e/b/ce 

 g'4 Symjiefa/cp 



Ca.3 ^ C YMNOSPEKMS 



mosf ortieri j 

 occasior>Qll>l 



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 Onacrace/^ 

 Rosa 



pro-cholaZQl: 

 'Jug Jans 

 Galiu/n 

 /\speruIcL 

 TaKus 



TCTKASPORIC £.S. 



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Act. Types 



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 Art€n\isici 

 OcAr\.a. 

 Citrus 

 So r bus 



Fig. 51. — Competition of cells depending on the interaction of the genetically 

 different products of segregation with physiologically determined gradients at 

 different stages in development of the embryo sac in the flowering plants. Rubiaceae, 

 Fagerlind, 1937. Juglans, Nast, 1935. Oenothera, Rudloff and Schmidt, 1932 {cf. 

 Schnarf, 1936; Darlington and La Cour, 1941). 



other, the one at the micropylar end is favoured by its position 

 in the gradient, that is by its cytoplasmic content. If a B is at the 

 more favourable end it always grows. If it is at the less favourable 

 end there is conflict between the relative advantages of the cyto- 

 plasmic gradient and the nuclear genotype. After a short contest the 

 nucleus wins and A is strangled by B. Or vice versa. The proportion 

 of victories one way or the other is characteristic of each species 



(Fig- 51). 

 This Renner Effect shows that nuclei and cells of different gcno- 



20 T 



