166 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



growth above the optimal temperature and a better supply of nutrients is 

 possible only at lower temperatures. Therefore it seems that we arc deal- 

 ing here with an absolute limit in the growth rate of plants. Unless we can 

 influence this diffusion process by some means, the chances of extending 

 the presently established maximum growth rate are not very good. 



These diffusion processes are apparently localized in the growing points 

 and are either inter-cellular or intra-cellular over short distances. If they 

 are inter-cellular, a possible speed-up of this process would be by decreas- 

 ing the cell size and thus increasing the diffusion rate. In this connection, 

 it is probably significant that all fast-growing plants have relatively 

 small cells and small nuclei, whereas the largest nuclei are usually found 

 in plants which grow rather slowly. It is also significant that though 

 through polyploidy it is possible to increase the size of the individual cells, 

 usually the growth rates of these cells and the organs built out of them 

 are decreased in comparison with the diploid situation. Octoploids always 

 grow very much slower. This does not necessarily have to be explained on 

 the basis of upset gene balance, but would follow from growth control by 

 inter-cellular diffusion processes. 



There are also indications that intra-cellular diffusion processes may be 

 of significance in growth. When a number of pea seeds are segregated ac- 

 cording to their size and grown under controlled conditions we find that 

 the smallest peas develop into plants which originally had the highest 

 growth rate. The larger peas, on the other hand, grow more slowly in the 

 beginning. One to two weeks after germination this trend reverses itself 

 so that the length of three-week-old peas is the same whether the plants 

 came from small or big seeds. This phenomenon seems explainable on the 

 basis that in the larger growing points of the big seeds diffusion processes 

 leading to growth are slower than in the smaller growing points of small 

 peas which therefore can grow faster. It is conceivable that the faster 

 growing rate will result in larger growing points which then automatically 

 slow down further growth. In this way an auto-regulatory feedback mech- 

 anism would exist which could control the growth of these peas within 

 rather narrow limits. 



Other processes in plants are influenced in a very different way by tem- 

 perature. In the case of protoplasmic streaming there is a direct propor- 

 tionality between temperature and velocity of streaming (fig. 3 ) . Although 

 this has been explained on the basis of several processes with opposing 

 temperature-dependence, this explanation does not really account for the 

 remarkably straight line temperature relationship. In the case of the 

 morphological development of the pea plant we also find such direct pro- 

 portionality between temperature and the rate of node formation. Figure 

 4 gives an example of this. Also, there is no indication that we are dealing 



