628 Journal of Agricultural Research voi. xii. no. io 



20 fruits, of which 15 were borne on primary flowers, 4 on secondaries, 

 and I on a tertiary. Although this variety shows an extreme case of 

 sterility, the condition found as regards variability of sterility may be 

 an indication of what will be found when a thorough study is made of 

 this point in our cultivated varieties. 



Thus far the study of sterility has dealt mainly with those types of 

 sterility induced by a decided tendency toward dieciousness in species 

 of Fragaria. Another type of sterility very prevalent in cultivated 

 varieties and undoubtedly a factor in pollination is expressed in the 

 appearance in ripe pollen of varying amounts of defective grains. It is 

 with this type of sterility that the remainder of this paper deals. 



POLLEN DEVELOPMENT AND STERILITY 



A careful cytological examination of the pollen condition in the straw- 

 berries, both wild and cultivated, was made with the objects of deter- 

 mining (i) the amount of viable pollen in cultivated varieties and its 

 relation to the setting of fruit and (2) the cause of pollen abortion in 

 plants of hybrid origin. 



The material used as a basis in determining the general pollen condi- 

 tion in Fragaria spp., consisted of (a) F. virginiana from various parts of 

 Minnesota, (b) F. americana, (c) a considerable number of cultivated varie- 

 ties, and (d) seedlings under test in the course of the fruit-breeding work. 

 The cytological study was carried on principally on the self -fertile variety 

 Minnesota No. 3, a cross of Senator Dunlap X Pocomoke, recently intro- 

 duced by the Minnesota Agricultural Experiment Station. It produces, 

 on an average, about 50 per cent of aborted grains and so furnishes 

 desirable material for the study of normal and abnormal pollen develop- 

 ment . The stages in normal development we re also studied in F. virginiana. 



POLLEN CONDITION IN WILD FORMS 



The recent work of Jeffry and his students on the pollen condition in 

 wild forms puts under suspicion the genetic purity of the Rosaceae in 

 general. The forms which have been studied most intensively, 

 Onagraceae (25), Crataegus spp. {37), Rubus spp. (23), and Rosa spp. (8), 

 show, in some species, a relatively large proportion of aborted pollen 

 and the appearance of many subspecies, some of which appear to be 

 hybrids. Because of this fact and with a view to comparing the pollen 

 condition of the wild with the cultivated forms, pollen of F. virginiana 

 and F. americana was examined. 



The methods used in determining the amount of abortive pollen were 

 as follows: Fresh flowers were collected and either were allowed to dry 

 or were kept with their pedicels immersed in water until the anthers 

 had dehisced. The pollen was then transferred to slides by holding the 

 flower over a slide and giving it a few sharp taps. In this way the 

 anthers dehisced completely onto the slide. A drop of lactic acid was 

 then added, and a small cover slide placed over the drop, forming a 



