27 
Pollen Mother-cells of Certain Plants. 
nuclear membrane disappears and the chromosomes become arranged 
in the equatorial plate, each portion of the bivalent chromosomes shortens 
and thickens to form double rods, which eventually lie parallel to each 
other (Figs. I'ld and 22 <?, PL I). 
In diakinesis I have observed beside the chromosomes in most 
nuclei a linin-like substance present, very similar to that which Miyake (’05) 
and I have already described. This linin-like substance binds chromo- 
somes together or to the nuclear membrane. In many cases portions of 
the linin intervals of the spirem remain which have not entered into the 
chromosomes during their concentration. Fine cross-threads are also 
present, resulting in a sort of linin reticulum. This arrangement of linin 
is plainly visible in Figs. 16, 19, and 20, PI. I. 
The formation of the spindle has not been followed in this plant with 
any particular detail. The formation of a felted zone, the dissolution of 
the nuclear membrane, and the formation of a multipolar spindle, which 
later becomes bipolar, seems to agree, in so far as I have been able to observe 
the stages, with the more recent results of several authors on spindle 
formation in the heterotypic division of higher plants. Allen (’03) has 
thoroughly described spindle formation in the pollen mother-cells of Larix , 
and Berghs (’05) has studied the process in Paris quadrifolia. 
In Thalictrum purpurascens the nuclear membrane disappears and the 
chromosomes are arranged by means of the spindle fibres into the equatorial 
plate in characteristic fashion. The spindle fibres are attached to the 
chromosomes in bundles at definite points. Figs. 23 and 24, PI. I, illustrate 
the position of the chromosomes during the formation of the multipolar and 
bipolar spindle. Each chromosome may be seen in these figures to be 
bivalent, with each of its parts forming a rather straight rodlet. I have 
never observed that the bivalent chromosomes of this plant at this stage 
ever form any other figures. They are always double rods. If we 
compare the size of these chromosomes in the equatorial plate with that 
of the bivalent structures of diakinesis or with the size of the chromosomes 
found in the thick spirem or even earlier stages, it is seen that they all 
are of essentially the same size. (Compare Figs. 3, 5, 19, and 23, PI. I.) 
The chromosomes may be easily counted at this stage. Figs. 23 and 24, 
PI. I, are sections, and the whole number of chromosomes does not appear 
in them. It is evident that the chromosomes are the same double rods or 
bivalent structures which were present in the spirem and also in earlier 
stages as paired chromosomes. 
The orientation of the chromosomes in the nuclear plate and their 
behaviour during metaphase and anaphase is the same as I found it in 
Campanula rotundifolia , Helleborus foetidus , and Podophyllum peltatum , and 
as it has been described by Strasburger (’00), Mottier (’03), Allen (’05), and 
Miyake (’05). Since my studies on Thalictrum purpurascens confirm my 
