886 VEGETABLE GROWTft. 



instances of plants growing in hot springs, it may be said to 

 lie at or very near 50 C. The figures obtained by Sachs for the 

 common plants he experimented upon are in general between 

 36 and 46 C. It is a curious fact that some tropical plants 

 are not capable of bearing a higher temperature than a few 

 plants of cold countries. 1 



1013. The optimum temperature for growth lies in most cases 

 between 20 and 36 C. 



1014. The following table, compiled by Pfeffer, exhibits at a 

 glance the cardinal points of temperature as they have been 

 determined by four observers : 



1 Pfeffer : Pflanzenphysiologie, ii., 1881, p. 123. 



PIG. 171. Apparatus for keeping seedlings in a constant temperature The drum at 

 d is an ordinary tlierrao-regulator by which the flow of illuminating gas can be controlled 

 within narrow limits. To insure still greater control, the more sensitive regulator, r, 

 is also employed. The cylindrical vessel, z, has double walls, the space between them 

 being filled with water. Under this vessel a very small burner is sufficient even for 

 optimum temperature. (Pfeffer.) 



