— 154 — 



be seen (at a). It is of course difficult to fix the value of such 

 figures with any degree of certainty, but it seems to me probable 

 that this represents a chromosome which is not united with others, 

 but which has become univalent. 



The irregularity just mentioned becomes more distinct in later 

 stages. These univalent chromosomes are left behind in the proto- 

 plasm during the first division as is the case with the Drosera- 

 hybrid. They then form dwarfed nuclei between the two daughter^ 

 nuclei, but are destroyed or possibly united with the greater nuclei 

 in the following division, insomuch as such dwarfed nuclei are not 

 so often observed, when the tetrad division is 

 completed (Fig. IV, B). 



Soon after the second division and when 

 the pollen-cells have received their mem- 

 brane, signs of desorganisation appear. The 

 pollen-cells are never separated from each 

 other, their protoplasm becomes vacuolated 

 and they are gradually pressed together by the 

 encroaching tapetum, and destroyed. It often 

 happens, that the membranes between the 

 pollen-cells are not developed, as in Fig. V 

 where the four pollen nuclei lie quite free in 

 the PMC. When the anthers are full deve- 

 loped, any traces of pollen-cells are lacking. 



As above mentioned, sometimes, but very 

 seldom, the reduction process can take place 

 morphologically quite normal, but still a pollen- 

 formation never occurs. 



Hieracium flagellare. 



In H. flagellare a typical pollen formation seems to take place 

 as far as one can judge. Just as in the preceding species the 

 chromosomes in the heterotypic spindle figure are very different in 

 shape and form — they are however, all bivalent. Their number 

 is 21, as a great many calculations both during the first and 

 second division has taught. In the somatic cells their number was 

 about 42 (Fig. VI, B, C). 



It is noteworthy that the number of chromosomes in the 

 genus Hieracium is 7, 9, J 7 and 21, and according to Juel (7) 9 



Fig. V. H. excellens, 

 pollen mother cell, with 

 four nuclei. 



