22 THE CYTOLOGY OF THE SUGARCANE 
to divide irregularly by the application of chloral. KOERNICKE submit- 
ted flowerbuds of Lilium martagon to radium-radiation and so obtai- 
ned an irregular division of the pollen mothercells. 
All these agents impeach the metabolism of the cells and have a 
laming effect on the activity of the cytoplasm, which hinders a nor- 
mal celldivision. 
In the case of many hybrids, an abnormal division of the pollen- 
mothercells has been found, in the first place among such hybrids as 
arose from crosses of species with different chromosome-numbers, but 
also in hybrids the parents of which had an equal number of chromo- 
somes. Such hybrids are frequently sterile. 
We also know however sterile hybrids in which the reduction-divi- 
sion takes a perfectly normal course. The influences which cause pol- 
lensterility, in such cases, probably begin to act only after the comple- 
tion of the tetrad-division. 
It is a curious fact that many hybrids show approximately normal 
divisions besides very irregular ones. JUEL found in the case of Syringa 
chinensis, probably a hybrid of S. vulgaris x S. persica, very irregular 
divisions. TISCHLER found these also, but in his material normal divi- 
sions occurred in a pretty considerable percentage. Yet, both in the 
material of JUEL and in that of TISCHLER, the pollengrains were sterile. 
Their sterility can consequently not be explained by the irregular di- 
vision. 
E. BORGENSTAM !) found in the case of this lilac normal division of 
the pollenmothercells, but showed that lowering of the temperature 
caused similar abnormalities as those observed by JUEL and TISCHLER. 
Not an, artificially caused, unfavorable factor, but a purely natural 
one, a lowering of the temperature, has consequently here caused an ir- 
regular division. Similar other cases are known: BEER found irregular re- 
duction-division in the case of Oenothera biennis only, when the mate- 
rial had been gathered late in the year. CANNON found the same in the 
cotton hybrid Gossypium Barbadense x G. herbaceum. 
Bulbgrowers expose the harvested hyacinthbulbs in which the in- 
florescence has already been formed, during a certain time, to tempe- 
ratures of 75—80° Fahrenheit, in the bulbhouses. 
1) E. BORGENSTAM. Zur Zytologie der Gatting Syringa nebst Erörterungen 
ueber den Einfluss aiisserer Factoren auf die Keimungsvorgange. Archiv. f. Bot. 
19228. Ret) Pot CB NF bd. 11292289 
