564 PLANT GROWTH AND PLANT COMMUNITIES 



against radiation in general but against green light, which is specifi- 

 cally absorbed by anthocyanins. 



When we take all the effects of light into consideration, we can 

 say that the higher plants contain a remarkable number of different 

 pigments and light-absorbing systems, each tied in with a special re- 

 acting system. Therefore it is possible to influence the growth of plants 

 in many different ways by applying monochromatic light of different 

 spectral regions. 



Whereas in the case of light it is possible to add very specific 

 amounts of energy to very specific molecules, in the case of tempera- 

 ture effects the activation is far less specific, since individual wave 

 lengths are not involved. There are several ways in which temperature 

 can affect reactions and processes. In the first place, there are the proc- 

 esses in which all the molecules of a species are equally involved, such 

 as diffusion. In this case the temperature effect is relatively minor and 

 amounts to a Qio of approximately 1.2. It is also possible that we are 

 dealing with reactions in which only thermally-activated molecules are 

 involved. This is the usual case in chemical reactions in living sub- 

 stances. In such cases the Qio is very much higher and is usually more 

 than 2. The actual value of the Qio can give us a further idea about the 

 type of reaction involved, since the higher the activation energy 

 needed, the greater the Qio. 



Most of the individual growth processes, such as the growth of 

 isolated roots or of tissue cultures, have Qio's between 2 and 3 and are 

 stimulated by heat up to temperatures near the thermal deathpoint of 

 protoplasm. For intact plants, the optimal temperatures are practically 

 always very much lower. These optimal temperatures may range from 

 10° C. or even lower to 20° C. or slightly higher. Therefore the in- 

 tegrating mechanism that makes a complete plant out of a number of 

 separate organs has a very different temperature characteristic from the 

 individual organs themselves. Thus far there is no indication that this 

 temperature response is due to the hormonal mechanism, but every- 

 thing points to the involvement of the plant's translocating system. 



At the optimal growing temperatures we often find in intact plants 

 a rather wide range over which growth is hardly affected by changing 

 temperature. I would like to interpret this by assuming that a diffusion 

 process is the limiting growth factor in the plant over such a tempera- 

 ture range. There are several other reasons why we have to accept the 

 fact that physical rather than chemical processes are limiting when the 

 plant is growing at an optimal rate. 



Finally, during the last few years it has been shown that the 

 circadian cycle, which is so generally operative in the normal growth 

 of plants, has a Qio of 1.2 to 1.3, and it can also be shown that part of 

 the temperature adaptation of plants is due to the effect of temperature 



