§ 2] UPON THE DIRECTION OF GROWTH 463 



We may now gather together the results of our study of the 

 effect of heat on the rate of growth of organisms. The relation 

 between temperature and rate of growth may be expressed by 

 a curve which, starting near 0°, reaches its highest point at a 

 temperature varying with the species, and falls to a maximum 

 temperature generally not far from the lethal temperature for 

 the race. Three critical points are thus distinguishable, — the 

 minimum, the optimum, and the maximum. In both animals 

 and i^lants these points are correlated. The optimum lies 

 close to the normal temperature for the species, the minimum 

 lies usually only a little above the point of cold-rigor, and the 

 maximum only slightly below the point of heat-rigor. The 

 optimum lies nearer the maximum than the minimum, but the 

 curve is not so unsymmetrical as that of metabolism. Any 

 change in temperature leads to a change in the rate of growth, 

 but this does not take place completely at once : there is a 

 latent period of an hour or so. A sudden rise in temperature, 

 especially from 0° to the ordinary summer temperature, is 

 accompanied not only by a mechanical (elongating) effect, but 

 also by a physiological response, showing itself in accelerated 

 growth. Finally, in the acceleration of growth by heat, the 

 imbibition of water is slightly more accelerated than the other 

 growth processes. 



§ 2. Effect of Heat on the Direction of Growth — 

 Thermotropism * 



Two sorts of heat are to be here distinguished — radiant and 

 conducted. The former is a form of radiant energy and allied 

 to light, consisting, indeed, of the rays lying beyond the visible 

 red of the spectrum. The latter is due to molecular vibrations 

 of the medium. 



1. Effect of Radiant Heat. — When Phycomyces nitens or 

 seedlings of Lepidium sativum, Zea mais, etc., are reared on a 



* The first suggestion of this phenomenon was based upon an analogy with 

 the action of light and was made by van Tieghem ('82), who gave it the name 

 Thermotropism. Woetmann ('83 and '85) first published extensive critical studies 

 on the subject. Others who have contributed data are Baethelemt ('84), 

 working on the roots of bulbous plants ; Vochting ('88), upon flower buds : and 

 AF Kleeckek ('91) upon radicles. 



