On the Organization of the Nuclei in the Pollen Mother- 
cells of Certain Plants, with Especial Reference to 
the Permanence of the Chromosomes. 
BY 
JAMES BERTRAM OVERTON. 
Assistant Professor of Botany in the University of Wisconsin. 
With Plates I-III. 
ECENT cytological investigation tends to strengthen the view that the 
JL V chromosomes are everywhere permanent cell-structures. Beside the 
evidence as to constancy in numbers, size, and form, many newer facts furnish 
indisputable proof that thechromosomes retain theirpermanency as individual 
units from one cell-generation to another. In common with Rosenberg and 
several more recent authors, I have presented considerable evidence to show 
that the chromosomes persist as individual structures in the germ cells of 
such plants as Campanula grandis, Helleborus foetidus , Thalictrum pur pur - 
ascens , and Calycanthus floridus. In the last two plants I have described par- 
ticularly the presence of definite chromatic collections, or ‘ prochromosomes 
in the somatic and germ cells. Numerous countings and careful study show 
that these bodies usually correspond to the somatic chromosomes both in 
number and size. These collections furnish excellent proof of the per- 
manence of the chromosomes as individual units during the so-called resting 
condition of the nucleus. I have continued the study of these plants, and 
shall describe how the structure of the resting nucleus at the period of its 
maximum chromatic distribution differs markedly from the usual reticulum. 
I find the formation of a pre-synaptic heterogeneous spirem which 
consists of a double series of chromosomes alternating with linin segments. 
I am able to follow the disappearance of these linin segments in the 
post-synaptic spirem, a process which separates the definite diakinetic 
chromosomes. Each member of the chromosome pair splits longitudinally 
in diakinesis, thus producing tetrads, which can be followed through all 
stages in the formation of the spindle until they are separated in the division. 
The chromosomes have also been traced through the various stages of the 
[Annals of Botany, Vol. XXIII. No. LXXXIX. January, 1909.I 
