CHAPTER XXVII 

 UNFRUITFULNESS ASSOCIATED WITH INTERNAL FACTORS 



Stout^^" recognizes three types of sterility that are to be attributed 

 mainly to internal factors: (1) sterility from impotence, (2) sterility from 

 incompatibility, (3) sterility from embryo abortion. Sterility from impo- 

 tence arises when one or both of the sex organs fails to develop. This may 

 be complete, in which case either no flowers or no sex organs are formed, or 

 it may be partial, in which case either stamens or pistils are abortive. 

 Sterility from incompatibility arises when, though the sex organs are 

 completely formed, they fail to function properly. In the last type of 

 sterility the gametes are formed and apparently function but abortion 

 of the developing embryo takes place before maturity is reached.- The 

 same classification may hold for the internally controlled factors with 

 which unfruitfulness and the failure to set fruit are associated. It may 

 be observed that the sterility due to impotence represents an evolutionary 

 tendency in the group or species — an evolutionary tendency that finds 

 immediate expression in a distribution of the two sexes between different 

 flowers or branches on the same plant or between different plants. The 

 distinction between sterility due to incompatibility and that due to 

 embryo abortion is drawn in recognition of the time or stage of develop- 

 ment at which the male and female gametes, both structurally and func- 

 tionally perfect, show their incompatibihty — their inability to unite or 

 develop together to form a mature embryo. 



Perhaps a classification of the causes of sterility associated with 

 internal factors and based upon more fundamental processes would recog- 

 nize: (1) those due to evolutionary tendencies, mentioned above; (2) 

 those due to genetic influences, regardless of the exact time or stage of 

 development when the two kinds of gametes show their mutual aversion 

 and (3) those due to physiological factors, in which case there is not true 

 incompatibility but a failure of the plant to provide nutritive conditions 

 suitable for continued growth. This last type of sterility cannot always 

 be differentiated clearly from that due to environmental factors. 



DUE PRINCIPALLY TO EVOLUTIONARY TENDENCIES 



In nature the advantage of cross fertilization in maintaining the vigor 

 of the species has resulted in many cases in the development of certain 

 characteristics which make self fertilization difficult, if not impossible. 

 These factors, so favorable to the maintenance of the species, may, in 



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