— 155 — 



in H. umbeüatum. Such a variation in one and the same genus 

 is very rare. In the group Pilosella the numbers 7 and 9 seem 

 to be the basis numbers for the greater amounts. 



The first and second divisions proceed quite normally without 

 the formation of such dwarfed nuclei as in H. excellens. The 

 nucleus of the pollen cell divides in the ordinary manner into one 

 vegetative and one generative nucleus. 



The Formation of the Embryo Sac. 



In a preliminary note which has just lately been published 

 (Rosenberg 25) I have shown, that in H. excellens and flagellare 

 perfectly normal ES are deve- 

 loped after a preceding te- 

 trad-formation with a redu- 

 ced number of chromosomes. 

 In fig. 16 a synaptic stage 

 is depicted. In certain cases 

 the heterotypic spindle figure 

 often shows the peculiarities 

 of bivalent and univalent 

 chromosomes, which are 

 characteristic for the PMC. 

 This is the case in H. ex- 

 cellens. The univalent chro- 

 mosomes in the anaphasis 

 are by chance brought to 

 each of the poles or are 

 left behind in the proto- 

 plasm. In the second divi- 

 sion such irregularities also appear, where several chromosomes do 

 not enter the daughter nuclei, but form dwarfed nuclei here and 

 there in the cells of the tetrad. This ist most common in H. ex- 

 cellens and therefore in such cases the nucleus of the ES has not 

 always the ordinary number of chromosomes, but a different one. 

 I cannot with certainty contend as to whether this is the reason, 

 why so many of the embryo sacs never become fully developed. 



In certain cases the reduction process seems to proceed quite 

 normally, and then a typical ES is developed. The reason why 

 Ost en f eld got hybrids with H. excellens probably depends upon 

 the fact, that the pollen tube of an other species (the father) has 

 reached such normal ES. 



Fig. VI. H. flagellare, A, part of an ovule, with 

 aposporic embryo sac in the integument; B, 

 somatic spindle figure, polar view, 42 chromo- 

 somes; C, polar view of spindle figure in the 

 pollen-cell, 21 chromosomes. 



