Centrifugal Force upon the Cell 359 
the nuclei, and that the role of the cytoplasm is largely 
secondary. 
The results obtained in root-tips of Zea, Allium , Vicia , 
and Phaseolus have thrown more light upon the specific 
gravity of various cell-constituents, and especially those of 
the nucleus. There is no doubt that the nucleolus is relatively 
a very heavy body, and that its specific gravity is greater 
in the nuclei of cells destined to a greater constructive activity. 
Cells of the plerome-cylinder, which eventually develop into 
the various thick-walled elements, demand a much greater 
constructive activity of their protoplasts than the smaller 
thin-walled cells of the cortex. The nucleus, which is beyond 
any doubt connected with the secretion of cell-wall substances, 
must do more work where thick-walled elements are produced 
and consequently it must have more food. From this fact 
and from what is known of the behaviour of the nucleolus 
during karyokinesis (Mottier, ’97), the statement seems 
justified that the nucleolus represents so much food-material 
that may be drawn upon by the nucleus whenever necessity 
demands. 
During the comparatively short time at my disposal, it 
was possible to do but little more than to give a general 
survey of a part of the field, covering a tolerably wide range 
among plants, and to indicate lines along which fruitful re- 
sults might be expected. It was not possible under the 
circumstances even to make an exhaustive and quantitative 
study of a single phase of the various problems touched upon. 
In an investigation of this kind nothing is more strikingly 
evident than the necessity of combining the best indirect 
methods of the histologist and cytologist with those of the 
physiologist. 
It is hoped that some fact or suggestion contained in the 
foregoing may arouse a sympathetic interest in others having 
at their disposal the necessary mechanical apparatus for the 
prosecution of such and similar studies ; for there is no doubt 
but that careful and painstaking investigation here will yield 
fruitful results. 
B b % 
