444 PLANT GROWTH AND PLANT COMMUNITIES 



optimal temperature. It would appear, then, that we may discuss the 

 eflFects of low temperature on cosmos growth in chemical terms as 

 follows: While the rates of many of the reactions leading to the pro- 

 duction of the cosmos plant must be decreased by low temperature, 

 still the rate of thiamine production appears to be decreased even more 

 than the general average. Cosmos plants grown at suboptimal tempera- 

 tures suffer from restriction in the amount of thiamine available to 

 them, and their growth rate may be increased by artificial application 

 of this material. 



Similar things may be said about the response of plants to un- 

 favorably high temperatures. Thus Langridge and Griffing ( 1959) have 

 studied three low-temperature-requiring races of Arabidopsis— races 

 which are greatly decreased in their growth by temperatures above 

 30° C. Two of these three races were found to be specifically cured of 

 high-temperature damage by the application of biotin, while the third 

 was found to respond to the application of cytidine. Here the lesion 

 induced by unfavorable climate appears to be inability to synthesize an 

 essential metabolite in sufficient quantity at the high temperature. Sim- 

 ilar observations have been made by Davern ( 1959 ) , who has found 

 that high-temperature damage to certain races of subterranean clover 

 is due to loss of the ability to produce particular amino acids. 



It may be noted in passing that the response of higher plants to 

 temperature is similar to that of the so-called temperature mutants of 

 Neurospora, which have been studied so extensively. In the tempera- 

 tiu'e mutants of Neurospora we have organisms which grow normally 

 at one temperature and fail to grow at a higher or lower temperature. 

 Failure to grow at the higher temperature has been shown to be due to 

 a genetically induced inability to make one or another essential meta- 

 bohte at that temperature. The higher plants thus far investigated are 

 not mutants, in the sense that they have been made deliberately, but it 

 appears nonetheless that what we call the normal strains of higher 

 plant species behave just as do temperature mutants produced by 

 genetic machinations. 



Another and still more complex mode of interaction between plant 

 and temperature has come to light through the work of Went (1957) 

 and of Ketellapper (1960). The essential observation is the fact that 

 certain plants can be cured of low-temperature damage by growing 

 them in a regimen of alternating light and dark in which the daily 

 rhythm is not the 24 hours of our real world but some longer period, 

 such as 30 hours. The same species may similarly be cured of high- 

 temperature damage by growing it under a regimen in which the daily 

 cycle length is less than the 24 hours of our real world; for example, 18 

 hours. These facts can be summarized by saying that some plants, at 

 least, behave as though they possess a timing mechanism— a clock— 



