Rudolph Beer 219 



These examples might be multiplied but sufficient has been said to 

 show that, whilst hybridisation maij lead to pollen sterility, this is neither 

 the sole nor ultimate cause of this phenomenon. Any factor which upsets 

 the harmonious interaction normally taking place between nucleus and 

 cytoplasm may lead to the sterility of the pollen grains. 



I may mention here several cases which have come to my notice. 



During the summer months of 1906 and 1907 I made numerous col- 

 lections of the flower buds of Trago])ogon pratensis in order to study the 

 development of the pollen. In all these collections, without exception, 

 the development of the pollen grains took place in a perfectly normal 

 manner and four pollen grains were invariably formed from each 

 mother-cell. 



Several plants were, however, grown late in the season of 1907 and 

 anthers were examined in '6 °/^ NaCl solution upon 20th December. 

 There had been a cool, damp autumn and the thermometer had on 

 several occasions sunk to 25" F. It was found that under these conditions 

 the development of pollen had become irregular and that numerous 

 supernumerary pollen grains had been formed. In one case nine grains 

 had been produced from one mother-cell (Fig. 28) and in otheivs <mly two 

 pollen grains were formed from the mother-cell. In some instances only 

 a single pollen grain was produced from the mother-cell. The variation 

 in the size of the pollen grains Avas very great : thus in one instance two 

 neighbouring pollen grains within one mother-cell measured respectivelj'' 

 28fjL and 12 /a. 



Some similar observations which I made upon Oenothera biennis have 

 already been mentioned by Prof Farmer and Miss Digb3'(5, p. 200) in 

 a footnote to their paper "On the cytological features exhibited by certain 

 varietal and hybrid ferns." Here the late season, with cold nights, was 

 also effective in producing abnormalities in the pollen development of a 

 plant w-hich usually exhibits perfecth' normal pollen formation. 



Another interesting instance of irregular pollen development has 

 come under ni}' notice. It is that of Geranium ihericum Cav. A plant 

 of this species has been growing in my garden for many years and during 

 this time it has constantly failed to develop fertile pollen. I have 

 examined a number of other plants growing at Catford and at Kew and 

 in all these cases the stamens never developed any fertile pollen grains. 



A study of the pollen development showed that the distribution 

 of chromosomes was very irregular during meiosis, and that a varjnng 

 number of pollen grains were initiated in each mother-cell. 



Fig. 25 shows a case in which eight cells had been formed from one 



