GROWTH 301 



opment this optimum lies at a lower point than during the later 

 ones. This coincides with the natural march of temperature from 

 spring to early fall. A temperature too high at the beginning of 

 growth is one of the chief causes of the development of weak plants 

 yielding a poor crop. This commonly results from late sowing. 

 This rule, however, cannot be applied to thermophilous plants of 

 tropical origin. 



Certain plants may even show a pronounced need of cold tem- 

 perature. Such are the winter cereals which are usually sown in 

 autumn and, after a prolonged winter period, produce seed 

 during the following summer. If sown in the spring, they will 

 tiller throughout the summer without reaching the fruiting stage. 

 Formerly it was thought that winter plants require a rest period. 

 Gassner, however, demonstrated that if winter cereals are allowed 

 to germinate at a temperature not higher than 3 to 5° C, and after 

 that are planted in the open, production may be induced in the 

 first year without any intermission in the development of the plants. 

 Thus, the winter cereals may be considered as requiring a cold 

 environment during the early stages of growth. The same may 

 be said of several spring plants, especially certain kinds of oats 

 and vetches, which bear earlier and better fruit when allowed to 

 germinate in cold weather. 



Investigations of the temperature requirements of lower plants, 

 notably the saprophytes which do not require light, are of course 

 much easier, and hence more exact data are available. 



In this group of organisms, a much greater diversity is encoun- 

 tered than among the higher plants. Here may be found certain 

 fungi, like Fusarium nivale, frequently causing considerable damage 

 to winter cereals, and certain thermophilic bacteria, which develop 

 in damp hay and heating manure. The temperature minimum of 

 many of these thermophilic bacteria and fungi lies at 25 to 35° C. 

 At room temperature they commonly die. Their maximum tem- 

 perature may be as high at 70° C. It is interesting to note that 

 some of these organisms have a very high respiratory rate. Great 

 quantities of organic substances are oxidized, which causes con- 

 siderable self-heating of hay and manure. In this heating, the 

 consecutive work of various microorganisms may be observed. 

 The process is started by common putrefactive bacteria and molds 

 which raise the temperature to 35 to 40° C. Then, the thermo- 

 philes make their appearance driving the heat up to 60 to 70° C. 



