566 PLANT GROWTH AND PLANT COMMUNITIES 



on the length of the circadian cycle. Thus a tropical plant cannot grow 

 in a cool climate because the internal rhythm is slowed down to a 

 much longer span than 24 hours, so that the rhythm no longer coincides 

 with the 24-hour cycle of the day. Conversely, a temperate-region plant 

 will not grow at high temperature unless the external "day" is reduced 

 to well below 24 hours. 



In all these cases of low Qio (in the neighborhood of 1.2) control 

 of growth is exerted by a diflfusion-like process in which all the mole- 

 cules involved are equally energized by temperature— which would 

 lead to a Qio of 1.18 at room temperature. Since diffusion in an open 

 system cannot be influenced by factors other than temperature, there 

 is little chance that processes with low Qio's can be stimulated beyond 

 their normal rate. Differences in concentration, for instance, have little 

 effect on the diffusion rate. Therefore there seems to be little chance of 

 increasing the growth rates of plant organs in the temperature range 

 over which they show a Qio of 1.2, which means in their optimal grow- 

 ing range. 



In the lower temperature range, and for most individual plant 

 physiological processes, Qio's of 2 or more are found, and in such cases 

 we can conclude that the growth process is controlled by a chemical 

 reaction. It is exactly in that range that the application of plant growth 

 hormones produced positive effects, which fits in nicely with the theo- 

 retical considerations just given above. 



Our analysis has tended to emphasize the importance of polarity 

 and diffusion processes in the growth of plants, and the relatively sub- 

 ordinate importance of chemical control of these processes. This does 

 not dispute the role of chemical processes in life in general, but it 

 emphasizes some aspects which have been relatively neglected, to the 

 detriment of a well-balanced view of growth and life in general. We 

 have to admit that our knowledge of biochemistry completely affirms 

 the validity of the laws of themodynamics in most partial processes in 

 the living system. It has been shown that as far as growth and assimila- 

 tory or synthetic processes in general are concerned, the second law 

 of thermodynamics loses its applicability, and therefore we should try 

 to understand the processes that oppose this law in the living organ- 

 ism. We can have a better chance of arriving at an understanding of 

 life by focusing on those processes than by affirming the processes that 

 agree with the second law. 



Reference 



Went, F. W., 1957. The Experimental Control of Plant Growth (N. Y., Ronald 

 Press ) . 



