666 Journal of Agricultural Research voi. xii, no. io 



(5) The appearance of considerable amounts of aborted pollen in 

 wild plants of F. virginiana and F. americana is rare except in anthers 

 of the intermediate type. Most cultivated varieties produce considerable 

 amounts of aborted pollen of the type common in hybrids. The percentage 

 of aborted grains is not constant in the individual flowers of a variety, 

 and neither is it constant in the individual anthers of a single flower, as 

 just as great variations appear within the anthers of a flower as are 

 shown by composite pollen samples of individual flowers, 



(6) In those varieties producing high percentages of aborted grains 

 a portion, at least, of the morphologically normal pollen grains are 

 functional, as shown by germination and bagging tests. There is no 

 evidence of a physiological self-sterility in the strawberry. 



(7) In the partially sterile variety Minnesota 3 pollen development 

 is carried on normally up to the liberation of the microspores from the 

 tetrad. At this time all of the microspores appear normal and alike. 

 Following liberation, variations in rate of growth, time of division of the 

 microspore nucleus, and ability of the individual microspores to develop 

 normally are shown. At all stages, during this growth period micro- 

 spores were found in various stages of abortion. F. virginiana exhibits 

 as great regularity during this, growth period as is shown in the stages 

 leading up to liberation of the microspores. 



(8) Liberation of the microspores from the tetrad marks the beginning 

 of an independent gametophytic generation, so far as the metabolic 

 processes of growth are concerned. The individual microspores float in 

 a homogeneous nourishing medium provided by the sporophyte, but the 

 use of this food material in cell metabolism depends entirely upon the 

 individual organization of the microspores. 



(9) Specific chromosome combinations have been shown by various 

 investigators to be a potent factor in the developm^t or lack of develop- 

 ment of individual plants or animals. In plants heterozygous for a 

 number of factors, as are the varieties of strawberries, numerous new 

 chromosome combinations occur for the first time in the microspores. 

 The varying rates of growth, time of microspore division, ability to 

 increase the cytoplasm, and inability in many cases to develop normally 

 seem to be the outward expression of the differential ability of these 

 new chromosome combinations to carry on cell metabolism. 



LITERATURE CITED 

 (i) Allen, C. E. 



1905. NUCLEAR DIVISION IN THE POLLEN MOTHER-CELLS Olf LILIUM CANADENSE. 



/n Ann. Bot., v. 19, no; 74, p. 189-258, pi. 6-9. Literature cited, 

 p. 252-256. 



(2) 



1905. DAS VERHALTEN DER KERNSUBSTANZEN WAHREND DER SYNAPSIS IN 

 DEN POLLENMUTTERZELLEN VON LILIUM CANADENSE. In Jahtb. 



Wiss. Bot. [Pringsheim], Bd. 42, Heft i, p. 72-82, pi. 2. 



