1922] SEARS—TARAXACUM 319 
produces a diad. One of the cells, usually the apical, disintegrates, 
while the other develops into an eight-nucleate sac by regular vege- 
tative mitoses. A prophase of the first of these mitoses is shown 
in fig. 30a, 6, with twenty-six somatic chromosomes segmented. 
Sequence C is illustrated in figs. 31-35. It comprises a rather 
wide range of gradations in behavior, completely bridging the gap 
between types B and D. Following segmentation, the nucleus 
elongates, and the membrane disappears, with the twenty-six 
cuboids widely scattered and quite unpaired. As the spindle 
fibers appear, orientation and pairing are quite variable in their 
degree of perfection. In fig. 32 spindle, synapsis, and orientation 
seem rather perfect, excepting that chromosomes from the extreme 
ends of the nucleus have been caught at the poles and will doubtless 
remain there. In other cases pairing cannot be detected, and 
the majority of the cuboids may be caught at the poles, only a few 
or none reaching metaphase position. These latter constitute the 
“‘delayed”’ chromosomes familiar in descriptions of pollen abnor- 
malities, although actually the lagging ones are those at the ends. 
Obviously it is but a short step from this condition, where no 
cuboids reach metaphase, to amitosis as already described for 
sequence D. 
Sequence C is best exemplified in the pollen. In the embryo 
sac it has been traced through orientation. With amitosis it 
shares most of the responsibility for pollen abnormalities recorded 
in a previous paper. The chromosomes which never reach meta- 
phase position are likely to be reorganized into nuclei before those 
at the center reach the poles. These latter “delayed” chromo- 
somes then reorganize as supernumerary nuclei. Additional causes 
of supernumerary nuclei are (1) irregular lobing during first ami- 
tosis, (2) a second amitotic division, (3) extrusion of chromosome 
substance and formation of membranes about it. 
It should be understood that the four sequences described 
intergrade almost insensibly. It should also be noted that sequence 
D in its extremest fluctuations shows nuclear elongation and 
amitosis beginning so soon after synizesis that the chromatin has 
no opportunity to organize beyond the condition of a granular 
read. 
