farr: cytokinesis of pollen-mother-cells 285 



Sections were made for the most part five microns thick. In 

 some instances, however, it was found advisable to cut them only 

 three or four microns thick so as to remove all of the superficial 

 material from median sections of the dividing mother-cell. 



To Professor R. A. Harper, under whose supervision this study 

 was pursued, are due many thanks for his valuable suggestions and 

 criticism, which are deeply appreciated. 



IV. Observations on living mother-cells 

 The study of living material is without doubt a very valuable 

 method in cytological research. So far as the more obvious facts 

 of cell division are concerned, such studies give unimpeachable 

 data, as is evidenced by the accuracy and suggestiveness of the 

 studies of such early cytologists as Naegeli, Pringsheim, and 

 Hofmeister. Lundegardh and others are certainly pursumg a 

 wise course in devoting a large amount of attention to the living 



cells. 



The chief difficulty in the study of living cells lies in the fact 

 that they can be retained in their normal condition for so short 

 a time. In this respect the students of animal cells are more 

 fortunate, since body-fluids may be used to keep the cells alive 

 almost indefinitely. The pollen-mother-cells of the larch are found 

 to break down within ten or twelve minutes after crushing out the 

 content of the anther into tap water. Even before this time has 

 expired, and often almost immediately, the nucleus appears to 

 enlarge, doubtless due to a decrease in the osmotic pressure of the 

 medium. The introduction of a trace of weak alkali was found to 

 prolong the period of normal appearance to between fifteen and 

 eighteen minutes. The pollen-mother-cells of Tropaeolum, Chry- 

 santhemum, Helianthus, and Nicotiana were studied in the living 

 condition. This was attempted with Primula; but here the 

 mother-cells were apparently bound together so firmly that the 

 entire content of the pollen-sac remains intact during the process 

 of crushing out, and to separate the cells would involve so much 

 abuse of the protoplast as to make the results of no value. In 

 the other dicotyledons noted, the mother-cells readily wash out 

 separately from the anther as soon as it is cut. In these forms the 

 protoplast retains its natural appearance longer than in the larch. 

 This may perhaps be attributed to the enormously thickened 



