318 A CYTOLOGICAL INVESTIGATION OF SOME SPECIES 
_cinarwm-varieties among which in the prophase of the division of the 
PMC part of the chromosomes fail to enter into normal geminus-for- 
mation. The greater fertility of the tetraploid hybrids and the one of 
the Saccharum-hybrids mentioned above, would perhaps find its expla- 
nation in the fact, that among these hybrids complete or nearly com- 
plete chromosome-pairing takes places which causes a regular chromo- 
some-distribution over the daughtercells. | 
Stress must be laid on the fact however, that isodiploid hybrids 
exist with complete chromosome-pairing in the gonotokonts, which 
are sterile all the same. Consequently many other causes of hybrid- 
sterility doubtless exist. 
Of the hybrids between Saccharum officinarum and S. spontaneum 
it was supposed by me, that in the zygote, from which they arose, the 
chromosomes derived from Saccharum officinarum could have been 
doubled in number by splitting, resulting in a somatic chromosome- 
number of 136 in stead of the expected number 96. 
No cases are known in the litterature of hybrids, thus augmenting 
their chromosomenumber, but there are indications in the littera- 
ture which consider the possibility of such an increase of the number 
of chromosomes. 
It has been mentioned already, that it is now generally accepted, 
that fornis with a higher chromosomenumber can arise from forms with 
a lower one. 
One has even observed the origin of forms with a higher chromosome- 
number from forms with a lower one. By crossing, tetraploid Primu- 
la’s have arisen from diploid ones; by mutation, tetraploid and triploid 
Oenothera-forms from diploid Oenothera’s. 
EL. and Em. MARCHAL have obtained experimentally tetraploid 
mosses from diploid ones and WINKLER !), experimentally also, tetra- 
ploid forms from diploid Solanum-species. 
One has moreover found, that the chromosomenumber of certain 
species of a genus frequently is a multiple of the chromosomenumber 
of other species of that genus. One has also observed that apogamous 
species, frequently, but not always, possess a higher chromosomenum- 
ber than the normally sexual species of that genus. 
1) Hans WINKLER. Ueber die experimentelle Erzeugung von Pflanzen mit 
abweichenden Chromosomenzahlen. Zschr. f. Botanik VIII. 1916. 
ù * 
