44 DURATION OF THE SEVERAL MITOTIC STAGES 



(e). THE REACTIONS OF DEFINITE MITOTIC STAGES. 



General survey.— The temperatures 10°, 20°, and 30° C. at which the 

 plants experimented with were grown are medium in the sense in which 

 the term is used in relation to physiological experiments generally. 

 At these temperatures, with mitosis as with other physiological pro- 

 cesses, we find Qio values of the expected magnitude. Here also, as is 

 usual with both simple chemical and complex physiological processes, 

 accompanying an arithmetical change in temperature, we find a geo- 

 metrical change in reaction velocity. 



In some stages, such as No. 2, it appears that the activity is chiefly 

 chemical, or at least diffusional mvolving most minute bodies, for a 

 high-power microscope reveals few structural changes. If the products 

 of reaction were immediately removed, if auto-catalysis and other 

 activating or retarding factors were absent, such a stage might, in its 

 behavior, be expected more nearly to approach van't Hoff's rule than 

 would a stage whose changes appear to be mostly physical, such as, for 

 mstance, stage 6, which seems chiefly a physical shift. This surmise 

 in reference to stage 2 holds good in the temperature-difference 10° to 

 20° C, but falls down utterly in the 20° to 30° C. rise. While other 

 stages— N OS. 4 to 10 — which seem to be characterized chiefly by gross 

 structural changes, in the 10° to 20° C. change generally respond with 

 a Qio value less than van't Hoff's expectation, but in the 20° to 30° C. 

 change are well within the range of such prediction. These differences 

 indicate an interplay of forces specific for each stage. Doubtless the 

 non-removal of products, which become thereby factors influencing sub- 

 sequent activities, constitutes a very great if not the principal cause 

 of difference between the response of a mitotic stage and a homogene- 

 ous chemical reaction to temperature-changes. 



A cell through a given mitotic stage is apt to be more homogeneous, 

 i. e. simpler, in its physico-chemical complex than the same cell traced 

 throughout its whole mitotic cycle; also the activities of a given 

 mitotic stage may be chiefly chemical or chiefly molar. We should, 

 therefore, expect to find individual stages presenting velocity-gradients 

 more elementary {i. e., less composite) than the same gradient char- 

 acteristic of mitosis as a whole. Examination of the data shows that 

 for the mitotic cycle as a whole {i. e., the 10 active stages), an increase 

 of 10° (from 10° to 20° C.) causes a reduction in duration from 

 unity to 0.8342 (velocity increase of +1.1990), while an increase of 

 10° C. (from 20° to 30° C), taking 20° as the standard, causes a 

 reduction in duration for the 10 active stages from unity to 0.7158 

 (velocity increase of + 1 .3926) . Thus the cumulative effect of increas- 

 ing temperature upon the velocity of mitosis is, in the present experi- 

 ments, greater in the higher than in the lower temperatures, in this 

 respect resembling the simpler chemical reactions. (See pp. 39 

 and 43.) 



