IN THE DIVIDING ROOT-TIP CELLS OF THE ONION. 25 



biological necessity of having to resort to the smaller differences, it 

 seemed advisable also to select three temperatures, all between the 

 minimum and optimum for plant growth, and also near the mean 

 temperature most often found in reactions which obey van't Hoff's 

 law. Another reason for basing the first practical measurements (in 

 accordance with the method developed) upon temperature is that the 

 latter is known to exert great influence upon growth, implying bulk 

 increase and mitosis. It is, moreover, one of the external conditions 

 most readily and precisely manipulated. 



MATERIAL FOR THE EXPERIMENTS. 



Advantage was taken of the facts presented and the experience 

 gained in the preliminary study in planning and executing the com- 

 pleter one. The temperature-range having been decided upon, it is 

 next necessary to select suitable material. The onion, having proven 

 to be so well adapted to the sort of study in hand, was chosen for the 

 completer investigations. Not only has it long been known to show 

 mitotic rhythm, but it presents a homogeneity of samples not so easily 

 obtained in other types of organisms. Their root-tips closely resemble 

 each other and their mitotic processes were shown to synchronize. 

 (See p. 13.) Moreover, one sample may be taken without disturbing 

 the activity of the others, at least during the few hours of sampling. 

 They are not difficult to prepare cytologically. Furthermore, the 

 cells constituting the growing root-tip show comparatively little differ- 

 entiation. Each possesses a large number of chromosomes, which fact 

 (when the cells are longitudinally sectioned) makes the determination 

 of arbitrarily marked mitotic stages an easy and definite matter. 

 Finally the cells are large and the rate of mitotic activity permits 

 convenient (lO-minute) sampling intervals. 



Bacteria, such as Ward^ used in his investigations, divide rapidly, 

 but their smallness and the imperfections of the views obtainable of 

 their transformations render them inferior to many other materials. 

 If one desires to learn how the details of certain other mitotic struc- 

 tures — for example, centrosomes which are not present in plant cells — 

 are influenced during their mitotic transformations by various external 

 agents, other materials would be necessary; but, taking all factors into 

 consideration, the onion presents a very satisfactory source of material 

 for the type of investigation here reported. 



Many of the quantitative studies on growth have been based upon 

 the lengthening root-tips of plants. This is suitable material, whether 

 growth proper — i. e., permanent bulk-increase — is considered alone or 

 in relation to mitosis, for the root-tip grows chiefly in one dimension, 

 namely, length. But very rarely do the cells divide other than trans- 



iSeeref. No. 1, p. 13. 



