INCOMPATIBILITY 



Incompatibility 



Secondary systems favouring either self- or cross-pollination in 

 bisexual flowers depend on control of a habit which is uniform for 

 all individuals, and therefore adapted to the use o( the species very 

 much as are the shape and colour of petals. We now have to deal 

 with another system which depends, like dioecy, on genetic dif- 

 ferences, not genetic uniformity, for its maintenance, but which 

 operates in bisexual flowers. This system operates through 

 incompatihiUty of pollen and style. 



Separation of pollen from the styles of the same flowers — 

 separation either in space or time — can only hinder, not preclude, 

 self-fertilization. Incompatibility acts later and can be absolute 

 in its effect, as we can see if we consider an example. 



The Sweet Cherry, Pmnus avium, is a diploid species. It consists 

 wholly of individuals, wild or cultivated, which are incapable of 

 setting fruit or seed when self-pollinated (with which we include, 

 of course, pollination with trees of the same vegetatively propagated 

 variety). When cross-poUinated one of two things may happen. The 

 seed set is either complete or is as much a failure as with selfmg. 

 For example, Bedford Prolific is completely successful with 

 Napoleon or Waterloo, but it fails with Early Rivers. With 

 reciprocal crosses the success or the failure is always the same. 

 Moreover there are groups of varieties (one of them includes 13 

 names) which are mutually unsuccessful or incompatible, while being 

 compatible with all other groups of varieties of which some 16 are 

 known (Fig. 58). Thus to ensure a crop it is necessary to mix 

 together compatible varieties from different groups when planting 

 the orchard. 



How does this incompatibility work; It is clearly due to like 

 things failing to agree in pollen and style. When we look into the 

 matter we fmd that witl; incompatible pollination the pollen grows 

 too slowly down the style ever to complete its journey to the egg. 

 With compatible pollinations, we fmd one of two situations. Some- 

 times, as in Bedford Prolific by Napoleon or vice versa, all the pollen 

 grows quickly and well; and sometimes, as in Bedford Prohfic by 

 Waterloo and vice versa, half of it grows well, and the other half 

 behaves as though it is incompatible. The good half, however, 



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