I 

 THE COMPOSITION OF PLANT POPULATIONS 319 



protection in gathering pure seed. Especially is this true in sub-tropical 

 regions where humming birds are prevalent for they find tobacco flowers a 

 splendid source of sustenance and unquestionably often effect cross- 

 fertilization between plants. Moreover these remarks concerning tobacco, 

 although they apply to the commercial varieties, do not indicate the 

 true state of affairs in all species of Nicotiana, for a few species are 

 completely self-sterile. Thus in N. (data grandiflora some individuals 

 are actually completely self-sterile and others exhibit no bar whatever to 

 self-fertilization. It is especially important, therefore, in dealing with 

 plants in this class to determine these data for the particular species and 

 varieties and the special conditions attending the experiments. 



Under Ba we have included rye in spite of general statements as 

 to its self-fertility. This classification appears to be justifiable in view 

 of reports of von Riimker and Leidner on results of inbreeding rye. 

 The difficulties in the self-fertilization of rye appear to be technical ones, 

 rather than physiological, consequently reports as to its self-sterility must 

 be in error. This is of interest in connection with the next following class 

 which includes plants which are self-sterile. We have already mentioned 

 the case of Nicotiana alata grandiflora in a given population of which 

 both self -fertile and self-sterile individuals may be found. Other 

 complications arise from contradictory reports as to self-sterility in some 

 species belonging in these two groups. Thus there are reports that 

 flowers on a given plant are sterile with their own pollen, but exhibit a 

 certain degree of fertility when pollinated from some other flowers on 

 the same plant. In effect such relations give results which are equivalent 

 to self-fertility, but in some breeding operations it is important to know 

 the exact relations, because it may be necessary to take advantage of them 

 in special cases. It is probable that in general any difference which may 

 be found in the fertilizing power of pollen derived from different flowers 

 on a given plant are non-essential, and dependent upon some such 

 factor as relative maturity of pollen with respect to the receptive 

 period of the stigma. 



Among plants which are self-sterile are included a large number of 

 the horticultural varieties which are normally propagated by means of 

 clonal multiplication, but in which suitable pollination is necessary for 

 fruit-setting or for the fullest abundance of fruit-setting. Orchard 

 planting methods provide for this by mixing varieties which are known 

 to act as efficient interpollinating agents. It is important to note that 

 something more than a mere mixing of varieties is necessary; for the 

 best results accurate knowledge should have been gained beforehand of 

 the particular varieties which are most effective when planted together. 

 Self-sterility in improved tree and bush fruits is a not unimportant con- 

 sideration in practical horticultural operations. It is, also, of interest 



