3] PHYSIOLOGICAL REACTIONS 77 



which it cannot grow normally and many even be killed. However, 

 for most plants, in the words of one specialist correspondent, 

 ' temperature requirements depend on illumination ; in low light 

 [intensities] the optimum is cooler than in high light '. In nature, 

 temperatures fluctuate more or less markedly and affect different 

 life-processes differently, so that the optimum must take into 

 consideration such natural fluctuations on one hand and the optima 

 for different life-processes on the other. Even the maxima and 

 minima, outside of which death may result, often vary with other 

 physical factors and with the recent experience as well as evolutionary 

 history of the plant in question. They may also vary with the 

 time of exposure as well as with the state of the plant structure or 

 its stage of development. Thus resting seeds and other reproductive 

 bodies are, in general, far more resistant to extremes than are adult 

 plants or, particularly, tender young parts : whereas the killing of 

 young shoots and blossoms by even the slightest frosts is an all-too- 

 common experience in temperate regions, more mature parts of the 

 selfsame plants often survive. Indeed there are numerous known 

 instances, involving all the main groups of plants, of such resistant 

 bodies as spores and seeds surviving much lower temperatures in 

 laboratories than are ever found in nature — including those of liquid 

 hydrogen or even of liquid helium near absolute zero. 



Far from all vital activity ceasing at the freezing point of water 

 (32° F. = 0° C), there are known instances of such physiological 

 functions as photosynthesis and respiration proceeding at tempera- 

 tures below this point in higher plants, while some Bacteria and 

 Fungi are capable of growth at temperatures as low as i6° F. 

 (— 8-89° C). It has even been claimed, in Russia, that flagellate 

 Algae have been observed swimming in drops of brine cooled 

 artificially to — 15° C. On the other hand, whereas most plant 

 bodies are killed by heat at much lower temperatures than the boiling 

 point of water at sea level (100° C), some bacterial spores are merely 

 stimulated to germinate by being so boiled (though of course 

 the actual germination only takes place subsequently, at lower 

 temperatures). 



The responses of plants to night temperatures have recently been 

 demonstrated to have considerable significance in connection with 

 their geographical distribution. Thus the Big Bluegrass {Poa 

 ampla) of western North America flowers equally well at day tempera- 

 tures of 20, 23, and 30° C. — but only when the night temperature 

 is below 14° C, for at i']'^ C. right temperature it does not flower 



