RELATIONS TO TEMPERATURE. 



385 



al'tcM- a considerable time. During tlie period of inactivity the 

 plant is ready to respond at once to the influence of oxygen, 

 growth being then immediatel}' resumed. 



1010. If assimilated matters and free oxygen, both essential 

 to growth, are abundantly supplied to a plant which is kept at 

 too low a temperature, growth does not occur. The minimum 

 limit for growth is different for different plants, and is not the 

 same for all organs. 



Again, it must be noted that there is a maximum limit of tem- 

 perature above which growth does not take place, and this limit 

 is also diffei'ent for different plants. Between the lower and 

 upper limits there is, for the plants which have been thus far 

 studied with respect to the effect of heat on growth, an optimum 

 of temperature at which growth is most rapid. 



1011. Relations of growth to temperature. The minimii.m tem- 

 perature required for growth is generally much higher for plants 

 of warm regions than for plants of cold 

 climates, and there are wide differences 

 even among plants belonging to the same 

 climate. A few of the earliest spring 

 plants begin their growth at or verj' near 

 the freezing-point of water : it is thought 

 b}' some observers that growth may, in 

 a few cases, take place even below this 

 point. Kjellmann states that the ma- 

 rine algae at Spitzbergen continue to de- 

 velop their thallus during the polar night 

 of three months, and that most of them 

 during this time pi'oduce theii' spores, 

 the temperature of the sea-water being 

 on the average one degree below zero, 

 Centigrade.' 



But, on the other hand, manj- of the 

 tropical plants ^ cultivated in hot-houses 

 cease growing when the temperature falls below 10° or 15° C. 



1012. The maximum temperatu7-e for growth is as wide in its 

 range for different plants as the minimum. Aside from the 



^ Comptes Eeudiis, Ixxx., 1875, p. 474. See also Falkenberg : Die Algen 

 iin weitesten Sinne, in Schenk's Botanik, 1882. 

 2 See De Candolle : Physiologie vegetale, 1832. 



Fig. 170. Double-walled metallic box for keeping microscopic objects at a given 

 temperature while under observation. (Sachs.) 



25 



