Kraus: The Self- Sterility Problem 



551 



fruitful and self-fertile. Barren and 

 self-barren mean the absolute failure of 

 a tree to produce fruit; the latter term, 

 naturally would imply the lack of 

 application of foreign pollen. The term 

 fertile has been used to express the idea, 

 both of production and capability of 

 production. There is a definite distinc- 

 tion between the two ideas which should 

 be observed, though it would be pre- 

 sumptuous to attempt to settle that 

 question here. Since it seems more in 

 line with usage in other biological 

 sciences to regard fertility as synonym- 

 ous with the actual production of 

 viable offspring, germinable seeds are 

 considered the true measure of fertility. 

 Let us consider briefly the term self- 

 fertility. By it is meant the seminal 

 production of independent offspring 

 by an individual when not pollinated 

 or fertilized by another individual. 

 Generally this term has been applied to 

 any plant which produces fruit with its 

 own pollen. As indicated above, how- 

 ever, such an application is far too 

 generalized, since a plant may be self- 

 fertile and yet not be self-fertilized. 

 Examples of such a condition at once 

 come to mind in all cases of apogamy 

 (including parthenogenesis), or vegeta- 

 tive apogamy. Again a plant may be 

 self -fertilized and still not be self -fertile, 

 if fertility is measured by viable seeds. 

 Oftentimes even when actual fertiliza- 

 tion has taken place, there is a failure of 

 the embryo either to complete its 

 development, or of the seeds to germi- 

 nate. For the present discussion, the 

 line which marks the division between 

 the fertility and sterility of an individual 

 is determined, arbitrarily, by whether 

 its seeds do or do not germinate under 

 normally favorable circtimstances. This 

 line of demarcation is arbitrary for 

 several reasons; the chief of which is 

 that whether a proembryo or an embryo 

 fails soon after fertilization has taken 

 place, or whether a fully formed embryo 

 actually germinates and the seedling is 

 markedly weak in constitution, the 

 difference in development seems not 

 so much one of kind as of degree. It 

 does seem, however, that one of the 

 definite division lines in the life cycle 

 of a plant is marked by the phenomena 



of germination of the seed. While a 

 fully matured seed may be called the 

 new individual, throughout the following 

 discussion only the germinated seed is 

 so regarded. Consider in this connec- 

 tion, if you will, the frequent failure 

 to hatch of the eggs of closely inbred 

 domestic fowls, even though the best 

 of incubation conditions have been 

 provided and the eggs contain appar- 

 ently perfect embryos; and also the 

 premature birth which frequently takes 

 place among animals when species are 

 crossed, or, in common parlance, when 

 the matings are not congenial. 



For convenience, self-fertility may be 

 considered to be of two types, first, that 

 directly due to morphological factors, 

 and second, that directly due to physio- 

 logical factors. Like all classifications, 

 no hard and fast line can be drawn 

 between the two types — they are inter- 

 dependent to no inconsiderable degree, 

 as will be pointed out later. 



MORPHOLOGICAL FACTORS 



Morphological factors as causes of 

 self -fertility are, as a rule, the most 

 readily understood because of the re- 

 lative ease with which they may be 

 studied. Instances are of common oc- 

 currence. Beach and Booth have shown 

 that under normal conditions certain 

 self-sterile varieties of grapes are in- 

 capable of fertilizing other self-sterile 

 varieties, which "is in many cases, if 

 not all, due to a lack of potency in the 

 pollen." A large proportion, or all, 

 the grains are morphologically imper- 

 fect. Dorsey has recently shown that 

 the cause of impotent pollen in the 

 grape is due to "degeneration in the 

 vegetative nucleus," and that in the 

 impotent pollen of certain varieties no 

 germ pore is present. 



There is a wide variation in the 

 germinability of apple pollen; it ranges 

 from less than 1% to 100% according 

 to variety. It also should be added 

 that the percentage germination of any 

 variety, even though apparently per- 

 fect morphologically, will vary from 

 year to year and is dependent upon 

 many factors such as age and general 

 vitality of the tree, huniidity before and 

 during blooming time, soil conditions; 



