Rose Forms as determined by their Cytological Behaviour . 163 
Cvtologically speaking, they deviate but slightly from the path 
followed in Rosa arvensis , but to emphasize their great approximation in 
this respect and their total disagreement from all other British roses some 
account must be given. 
As in R. arvensis , the granular chromatin of the resting nucleus of the 
pollen mother-cell masses itself into a dense synaptic knot, which persists 
as such for some fairly lengthy period whilst the mother cells are separating. 
In the end loops are thrown outward to the periphery of the nucleus ; after 
this the knot gradually unloosens to build up the apparently continuous 
spireme (PI. IX, Fig. 4). Presently this thread thickens and shortens to be 
seen as being made up of loops divided lengthwise either completely or 
sectionally, thereby evolving the twisted hairpins of PI. IX, Figs. 5 and 6. 
To the interpretation of this split we shall revert under Rosa Sabini . 
These bivalent loops, the halves of which at first are so attenuated as to be 
barely visible, rapidly condense to yield the V’s, X’s, and O’s of a typical 
diakinesis (Figs. 8 and 9). 
The 14 bivalent chromosomes then approach one another with the 
accompanying development of a multipolar 
spindle. This, however, speedily gives place , 
to the bipolar variety, and the chromo- 
somes take their places on the equatorial 
plate. This event occurs in the usual 
fashion in fertile pimpinellifolia and in the 
spinosissima segregate ; on the contrary in 
the sterile plant 1 one chromosome may be 
detected as a fifteenth one lying in the 
region of one of the poles (Text- fig. 1). In 
all instances, except for the larger chromo- 
some number, the further stages are exactly 
parallel to those of Rosa arvensis and R. rugosa . 
The somatic count may be made out to be 28 in the trio of forms, 
although some small amount of difficulty may arise in the process, owing 
to the length and curvature of the chromosomes (PI. IX, Fig. 23). That the 
haploid number is 14, Fig. 22 amply demonstrates. 
Text-fig. i. R. pimpinellifolia. 
Heterotype metaphase showing odd 
chromosome like body, x 3,000. 
III. The Cytology of the Phenhybrids of Rosa. 
(a) Rosa Sabini (~~ R. pimpinellifolia x R. tomentosa var. sylvestris). 
The plant about to be considered belongs to that heterogeneous 
assemblage known to the early rhodologists as Rosa involuta , Smith. Now, 
of course, it is universally deemed to be a hybrid between Rosa pimpinelli- 
1 These tall plants are always sterile in Durham, and this slight abnormality, their tallness and 
sterility, may all be relics of some crossing in the previous history of the form.. 
M2 
t 
