IN THE DIVIDING ROOT-TIP CELLS OF THE ONION. 43 



seen that the lower the temperature the higher the coefficients. It is 

 evident that here increasing temperatures exert a progressively declin- 

 ing accelerative effect upon growth. 



Mitosis. — Not only is there, in a relatively simple physiological 

 complex, a decrease in Qio values as the temperature increases, but if 

 growth, which is most complex physiologically, is measured in terms 

 of permanent bulk-increase, we find the same phenomenon. 



In comparing the values found in the mitosis velocity-measurements 

 at different levels on the temperature scale with the two types of 

 velocity-increments which Harvey points out, the striking thing is 

 that in mitosis all of the stages measured in the present investigation 

 show a greater velocity-increment for a rise of 10° C. from 10° to 20° C. 

 than from 20° to 30° C. Thus, unlike the rate of nerve conduction in 

 Cassiopea, and the increase of length in the root-tips of the seedling 

 maize along with physiological activity generally, mitosis behaves in 

 its velocity-increments to temperature-increments like the simpler chem- 

 ical reactions. This does not mean that mitosis is a ''simple 

 chemical reaction." Far from it it is a vast complex of physical and 

 chemical activities. By chance the resultant of the actions and inter- 

 actions of these processes present, when measured as a whole, an 

 aspect resembling in this one feature a simple chemical reaction. 



Many biological curves are shaped like an elongated and slanting 

 capital letter S — thus ^y'^ ; for instance, the curve of auto-catalysis, 

 when time (abscissae) and quantity of product (ordinates) are plotted. 

 If the temperature at which the onion root-tips of the present study 

 were sampled had extended beyond the cardinal temperature points 

 for mitosis in the specimens used, we would have found ultimately 

 a breaking-point and a decrease in velocity increment in the higher 

 temperatures, such as Harvey found in the velocity of nerve conduc- 

 tion in Cassiopea at 28° C. to 38° C, and Lehenbauer in the growing 

 root-tips of maize at 32° C. to 42° C. The curves for velocity of physio- 

 logical reactions in response to temperature-changes are the shape of the 

 upper end of the elongated y^ , while the curves for mitosis and also 

 for the simpler chemical reactions take the direction of the lower half. 

 The range of temperature in the mitosis experiment (10° C. to 30° C.) 

 is somewhat lower on the temperature scale than those used by Harvey 

 (18° C. to 38° C.) and by Lehenbauer (12° C. to 43° C). In the region 

 of the medium temperatures this particular contrast between the 

 velocity-gradients of mitosis and of physiological processes generally 

 and the closer resemblance of the mitosis-gradient to that of the 

 simpler chemical reactions is undeniable. We must look for its mean- 

 ing not in position on the temperature-scale, but in a physiological 

 (physico-chemical) complex in which the many specific elementary 

 reactions to temperature-changes give a resultant in which the many 

 aberrations from the velocity-gradient characteristic of a simple 

 chemical process are mutually canceled. 



