01} Reproduction in lower and higher plants~ part 06-Outbreeding devices

01 Reproduction in Lower and Higher Plants - part 06 - Outbreeding devices


Outbreeding devices (contrivances): 
  • Many plants have mechanisms that discourage or prevent self pollination. 
  • To promote cross pollination and increase genetic diversity, plants have evolved a wide variety of sexual strategies. 
  • Genetic diversity is an essential factor for evolution by natural selection.
  • Continued self pollination results in the inbreeding depression. 
  • Thus plants have developed many devices to encourage cross pollination. 
  • The examples of outbreeding devices are as follows: 
Unisexuality : 
  • In this case, the plant bears either male or female flowers. 
  • It is also called as dioecism. 
  • As flowers are unisexual, self pollination is not possible. 
  • Plants may be monoecious, e.g. Maize or dioecious, Mulberry, Papaya.
Dichogamy :
  •  In this device, anthers and stigmas mature at different times in a bisexual flower so as to prevent self pollination. 
  • It can be further divided into two types: 
  1. Protandry 
  2. Protogyny 
1. Protandry : 
  • In this type, androecium matures earlier than the gynoecium, e.g. in the disc florets of sunflower. 
2. Protogyny : 
  • In this type, gynoecium matures earlier than the androecium, e.g. Gloriosa.
Prepotency :
  • Pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower, e.g. Apple.
Heterostyly (heteromorphy): 
  • In some plants like Primula (Primrose, there are two or three forms/ types of flowers in which stigmas and anthers are placed at different levels (heterostyly and heteroanthy). 
  • This prevents the pollens from reaching the stigma and pollinating it.
  • In heteromorphic flowers, pollen grains produced from anther pollinate stigmas produced at the same level.
Herkogamy :
  • It is a mechanical device to prevent self pollination in a bisexual flower. 
  • In plants, natural physical barrier is present between two sex organs and avoid contact of pollen with stigma of same flower, 
  • e.g. Calotropis pentangular stigma is positioned above the level of anthers (pollinia).
 Self incompatibility (self sterility): 
  • This is a genetic mechanism due to which the germination of pollen on stigma of the same flower is inhibited. 
  • e.g. Tobacco, Thea
 Do you know ? 
  • In all breeding programmes, the plants are hand pollinated to ensure cross pollination between selected varieties. 
  • e.g. wheat, rice.



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