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. alata 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 inerror. 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 
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