110 CARNEGIE INSTITUTION OF WASHINGTON. 



case on the basis of the numerous other genes, already known and 

 daily being added to in Drosophila, no effort will be made to carry on 

 the experiment further at this station, since it arose as a side issue in an 

 entirely different line of work. The results obtained are almost ready 

 for publication. 



CAUSES OF STERILITY; ABORTION OF POLLEN GRAINS. 



In the study of Datura it is highly important to recognize that many 

 mutants produce pollen of which nearly half is sterile. This sterility 

 is probably due to an abnormal number of chromosomes in the pollen. 

 It is especially apt to occur in hybrids between mutants. Mr. John 

 Belling has spent some time during the past year on this topic. He 

 finds that, in the Orchidaceae, which he was able to study through the 

 courtesy of the Missouri Botanical Garden, the hybrid pollen fre- 

 quently is normally formed. The pollen-mother cell divides regularly 

 into 4 cells, forming tetrads, just like those of a wild species. Accord- 

 ingly, microcytes were very rare. In the genus Papheopedilum, 

 on the other hand, where each tetrad divides into 4 pollen-grains, the 

 species hybrids show a large percentage of aborted pollen. The 

 following types of abortion of pollen-grains have been found : 



(1) Chance abortion of a fraction of the grains, altering slightly or greatly 

 with environmental changes; and probably due to the somatic constitution 

 of the plant, and not to constitutional differences among the pollen-grains 

 themselves. This was found in some Canna clones; and, superposed on 

 types (2) or (4), in Stizolobium and Rhododendron. 



(2) Abortion of a Mendelian fraction of the pollen-grains due to the ab- 

 sence of vital genes through a mutation or as a result of crossing. This 

 was found in Stizolobium and apparently in Rhododendron, sometimes with 

 type (1) superposed. 



(3) Abortion similar to type (1), but due to the presence of an extra chro- 

 mosome. Found in some of the simple trisomic daturas; and more accentu- 

 ated in the double trisomies and in the simple tetrasomic. 



(4) Abortion, like type (2), but due to the absence of one chromosome of a 

 set. Found in the Datura with 47 chromosomes assorting 23 to 24. Appar- 

 ently all the 23 chromosome pollen-grains abort. Since some tetraploids 

 produce much 23-chromosome pollen, the same cause may be at work in them. 

 Possibly this is also the cause of abortion of half the grains in some partially 

 triploid Canna clones with one or more missing chromosomes. 



(5) Abortion due apparently to frequent non-disjunction, and varying 

 with environment from 38 to 100 per cent. This was found in one nearly 

 sterile Canna clone. 



(6) Abortion probably due to the absence of proper pairing among the 18 

 chromosomes, so that the assortment 9:9 is absent or rare. This was 

 found in one sterile Canna clone. 



Mendelian abortion among the pollen-grains is well illustrated in a 

 certain presumably hybrid Rhododendron. These plants are known to 

 have their mature pollen-grains firmly adherent in sets of four. In 

 most clones examined, and in one named hybrid kindly given by the 

 Arnold Arboretum, there were varying numbers of empty grains, but 



