468 RELATION TO ENVIRONMENT. 



907. Heat. Heat has a great influence on plant growth and 

 distribution, as well as upon the general form of much of the 

 earth's vegetation. 



908. Vegetation at low temperatures. Many arctic and al- 

 pine plants are able to vegetate and even fruit at extremely low 

 temperatures, some of them very near to zero centigrade. Some 

 arctic algae fruit in the dark at 1.8 C., while many arctic plants 

 can exist at 50 to 70 C. Spores of some fungi, bacteria, and 

 algae, and certain dry seeds, can endure much lower tempera- 

 tures. The growth period for most vegetation begins at about 

 6C. ( = 43F.), or in the tropics at io-i2C., while often a 

 much higher temperature is necessary for reproduction of many 

 plants. Among the flowering plants annuals are largely ex- 

 cluded from polar lands, because the summer season is so short 

 that the total available heat is not sufficient for growth and repro- 

 duction. Annuals need a longer time for this than perennials. 

 The upper limit of temperature favorable for plants in general 

 is 45-5o C., while the optimum is below this. 



909. Very high temperatures, however, are injurious if not 

 fatal to most plants, though certain algae are known to vegetate 

 at 8o-90 C. Some desert plants are able to resist a tempera- 

 ture of 70 C., while some flowering plants are killed at 45. 

 Where plants or their seeds or spores contain a large percentage 

 of water, they are more easily killed by high degrees of heat or 

 low degrees of cold. But dry seeds and spores are often very 

 resistant. Some air-dry seeds will resist a temperature of 

 i2o-i3oC. Spores of certain bacteria will resist a tempera- 

 ture of 130 C. for a short time, while at a temperature of ioo- 

 110 they may live for several hours. But if allowed to germi- 

 nate and thus pass into the vegetative condition in which they 

 contain much more water, and the protoplasm is in a less resist- 

 ant condition, they are killed at 100 in a short time, or even at 

 80 continued longer. 



910. Cardinal temperature points for different plant func- 

 tions. It has been found in the case of many plants that for 

 the different life functions, or separate processes in growth and 



